Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://hea-www.harvard.edu/~pgreen/Papers/Kalfountzou2014.pdf Äàòà èçìåíåíèÿ: Fri Oct 2 16:40:38 2015 Äàòà èíäåêñèðîâàíèÿ: Sun Apr 10 10:32:25 2016 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: m 2-9

MNRAS 445, 1430­1448 (2014)

doi:10.1093/mnras/stu1745

The largest X-ray-selected sample of z > 3 AGNs: C-COSMOS and ChaMP
E. Kalfountzou,
1 2 3 4

1,2<

F. Civano,

1,3

M. Elvis,1 M. Trichas4 and P. Green1

Harvard­Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, MA 02138, USA Centre for Astrophysics, Science & Technology Research Institute, University of Hertfordshire, Hatfield, Herts AL10 9AB, UK Yale Center for Astronomy and Astrophysics, 260 Whitney Ave, New Haven, CT 06520-8121, USA Airbus Defence & Space, Gunnels Wood Road, Stevenage, Hertfordshire SG1 2AS, UK

Accepted 2014 August 25. Received 2014 August 22; in original form 2014 May 29

ABSTRACT

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We present results from an analysis of the largest high-redshift (z > 3) X-ray-selected active galactic nucleus (AGN) sample to date, combining the Chandra Cosmological Evolution Survey and Chandra Multi-wavelength Project surveys and doubling the previous samples. The sample comprises 209 X-ray-detected AGNs, over a wide range of rest-frame 2­10 keV luminosities log LX = 43.3­46.0 erg s-1 . X-ray hardness rates show that 39 per cent of the sources are highly obscured, NH > 1022 cm-2 , in agreement with the 37 per cent of type-2 AGNs found in our sample based on their optical classification. For 26 per cent of objects have mismatched optical and X-ray classifications. Utilizing the 1/Vmax method, we confirm that the comoving space density of all luminosity ranges of AGNs decreases with redshift above z > 3 and up to z 7. With a significant sample of AGNs (N = 27) at z > 4, it is found that both source number counts in the 0.5­2 keV band and comoving space density are consistent with the expectation of a luminosity-dependent density evolution (LDDE) model at all redshifts, while they exclude the luminosity and density evolution (LADE) model. The measured comoving space density of type-1 and type-2 AGNs shows a constant ratio between the two types at z > 3. Our results for both AGN types at these redshifts are consistent with the expectations of LDDE model. Key words: surveys ­ galaxies: active ­ X-rays: galaxies.

1 I NTR O DUCTION Active galactic nucleus (AGN) evolution at high redshifts, before their density peak, illuminates the role of AGN in the formation and co-evolution of galaxies and their central supermassive black holes (SMBHs) during the time of rapid SMBH growth. The so-called downsizing evolution has been revealed for both AGN (e.g. Ueda et al. 2003; Hasinger, Miyaji & Schmidt 2005; Aird et al. 2010) and galaxies (e.g. Cowie et al. 1996; Kodama et al. 2004; Draper et al. 2009). Supporting this idea, X-ray surveys have shown that the number density of luminous AGN peaks at higher redshifts than less luminous ones (e.g. Ueda et al. 2003; Aird et al. 2010). This sort of cosmological co-evolution scenario is inferred from the tight correlation exists locally between SMBH mass and galactic bulge properties (e.g. Magorrian et al. 1998; Ferrarese & Merritt 2000; Gebhardt et al. 2000; McConnell & Ma 2013). To elucidate the co-evolution of SMBH and galaxies (e.g. Granato et al. 2001, 2004; Croton et al. 2006; Hopkins et al. 2006; Menci et al. 2008; Trichas et al. 2009, 2010; Kalfountzou et al. 2011, 2012, 2014), the accretion activity in the Universe has to be studied
E-mail: el.kalfountzou@gmail.com

both at high redshifts and for low luminosities. This requires large samples of AGNs spanning wide ranges of properties. While many optical surveys have investigated the space density of high-redshift AGNs (e.g. Richards et al. 2006; Jiang et al. 2009; Willott et al. 2010; Glikman et al. 2011; Ikeda et al. 2011; Ross et al. 2013), the results are still controversial due to their inevitable incompleteness, especially at the faint luminosity end due to the host contamination, and the bias against obscured sources. As compared with optical surveys, X-ray observations are less contaminated by the host galaxy emission and include AGN populations with a wide range of neutral hydrogen column density. For the investigation of absorption evolution (e.g. Ueda et al. 2003; Hasinger 2008; Draper & Ballantyne 2010), X-ray selected samples include all types of AGN (e.g. type-1/unobscured and type2/obscured) and provide reduced obscuration bias in comparison with optically selected AGN. Although X-ray surveys have inferred the existence of an anticorrelation between the obscured AGN fraction and the luminosity, several of these studies have suggested that this fraction increases toward higher redshift from z = 0 to z 2 with limited samples at z > 3 (e.g. La Franca et al. 2005; Ballantyne, Everett & Murray 2006; Treister & Urry 2006; Ballantyne 2008; Hiroi et al. 2012).

C 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society


The largest X-ray-selected sample of z > 3AGNs
However, the evolution of AGN is still rife with uncertainty. On the basis of hard X-ray surveys, many studies agreed that the X-ray Luminosity Function (XLF) of AGN is best described by a luminosity-dependent density evolution (LDDE) model (e.g. Ueda et al. 2003; Gilli, Comastri & Hasinger 2007;Silvermanetal. 2008; Ueda et al. 2014). Aird et al. (2010) preferred instead a luminosity and density evolution model (LADE). In LADE, the shift in the redshift peak of the AGN space density versus X-ray luminosity is much weaker than in LDDE models, yet gives a similarly good fit to their data. While the z < 2 downsizing behaviour is common to both models, quite different numbers of AGNs are predicted at higher redshifts (z 3). X-ray surveys (2­10 keV) are now sensitive enough to sample the bulk of the z > 3 AGN population. Two studies have been performed on high-redshift AGN exploiting the deep X-ray surveys in the Cosmological Evolution Survey (COSMOS) field carried out with XMM­Newton (NAGN = 40; Brusa et al. 2009) and Chandra (NAGN = 81; Civano et al. 2011), limited to 2­10 keV luminosities L2-10 keV > 1044.2 and 1043.5 erg s-1 , respectively. A more recent study based on the 4 Ms Chandra Deep Field South (CDF-S; Xue et al. 2011) was able to investigate the evolution of z> 3 AGN down to LX 1043 erg s-1 (NAGN = 34; Vito et al. 2013). These results are consistent with a decline of the AGN space density at z> 3, but the shape of this decline remains highly uncertain at z> 4. To overcome these limitations, in this work we combined the two largest samples of z > 3 X-ray-selected AGNs with spectroscopic redshifts, both derived from Chandra X-ray Observatory (Weisskopf et al. 2002) surveys: the wide but shallow ChaMP survey (Kim et al. 2007; Green et al. 2009), and the deeper but narrower C-COSMOS survey (Elvis et al. 2009). This combination results in the largest X-ray AGN sample with NAGN = 211 at z > 3 and NAGN = 27 at z > 4. At the same time, by combining two surveys with different flux limits, we are able to determine the density evolution of both lowluminosity (LX < 1044 erg s-1 ) and high-luminosity AGNs. Our sample includes both obscured and unobscured AGNs, and their separate evolution has been determined. The paper is structured as follows. In Section 2, we discuss the data sets used in this work and the selection of the high-z sample. In Section 3, we present the optical and X-ray properties of the selected high-z AGN sample, and we explain the AGN type classification using X-ray or optical data. In Sections 4 and 5, the number counts and space density of the sample are compared with model predictions. Section 6 summarizes the conclusions. A cosmological model with o = 0.3, o = 0.7, and a Hubble constant of 70 km s-1 Mpc-1 is used throughout (Spergel et al. 2003). Errors are quoted at the 1 level. 2 S AMPLE S ELECTION The high-redshift AGN sample used in this work has been selected from the C-COSMOS X-ray catalogue, combining the spectroscopic and photometric information available from the identification catalogue of X-ray C-COSMOS sources (Civano et al. 2011, 2012) and the ChaMP (Chandra Multi-wavelength Project) X-ray catalogue using only the 323 ChaMP obsids overlapping with Sloan Digital Sky Survey (SDSS; Richards et al. 2006) DR5 imaging. In Fig. 1, we show the sky coverage (the area of a survey that is sensitive to sources above a given X-ray flux) using the observed soft band (0.5­2 keV) source detections for the two surveys, and their sum. This corresponds to 2­8 keV rest frame for z > 3. A schematic diagram of the sample selection with the detailed number of sources for each step is presented in Fig. 2.

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Figure 1. Sky area versus X-ray flux sensitivity curves for the C-COSMOS (blue solid line) and ChaMP/SDSS (red solid line) samples and the total area (black dashed line). The vertical blue dashed line indicates the flux corresponding to 10 per cent of the total C-COSMOS area (see Section 4). The vertical red dashed line indicates the ChaMP X-ray flux limit with >75 per cent completeness from SDSS/UKIDSS/WISE (see Section 2.2). The total area, after the applied cuts, used for this work is represented by the shadowed grey area.

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2.1 The C-COSMOS sample The Chandra-COSMOS survey (C-COSMOS; Elvis et al. 2009; Civano et al. 2012) covers the central 0.9 deg2 of the COSMOS field up to a depth of 200 ks in the inner 0.5 deg2 , with the ACIS-I CCD imager (Garmire et al. 2003) on board Chandra. The C-COSMOS X-ray source catalogue comprises 1761 point-like X-ray sources detected down to a maximum likelihood threshold detml = 10.8 in at least one band. This likelihood threshold corresponds to a probability of 5 â 10-5 that a catalogue source is instead a background fluctuation (Puccetti et al. 2009). Given this likelihood threshold, the flux limit reached in the survey is 5.7 â 10-16 erg cm-2 s-1 in the full band (0.5­10 keV), 1.9 â 10-16 erg cm-2 s-1 in the soft band (0.5­2 keV) and 7.3 â 10-16 erg cm-2 s-1 in the hard band (2­10 keV). The z > 3 C-COSMOS sample, as presented by Civano et al. (2011), comprises 107 X-ray-detected sources with available spectroscopic (32) and photometric (45) redshifts plus 30 sources with a formal zphot < 3 but with a broad photometric redshift probability distribution, such that zphot + 1 phot > 3. All of the spectroscopic C-COSMOS sources have a quality flag 3 (two sources) or four corresponding, respectively, to a secure redshift with two or more emission or absorption lines and a secure redshift with two or more emission or absorption lines with a good-quality, high S/N spectrum (see Lilly et al. 2007, 2009 for thorough explanation of quality flags). Tuned photometric redshifts for the C-COSMOS sources have been computed and presented in Salvato et al. (2011). Due to the large number of photometric bands and the sizeable spectroscopic training sample spanning a large range in redshift and luminosity the estimated photometric redshifts are expected to be quite robust at z> 2.5 even at the fainter magnitudes (iAB > 22.5). The COSMOS photometric redshifts for X-ray-selected sources have an accuracy of z/(1+zspec ) = 0.015 with a small fraction of outliers (<6 per cent), considering the sample as a whole at i < 22.5. At fainter magnitudes, the dispersion increases to z/(1+zspec ) = 0.035 with 15 per cent outliers, still remarkably good for an AGN sample. For the z > 3 C-COSMOS sample, an accuracy of z/(1+zspec ) = 0.014 is MNRAS 445, 1430­1448 (2014)


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Figure 2. Schematic flow diagram of the high-z sample selection.

achieved with only three catastrophic outliers (<9 per cent). The spectral energy distributions (SEDs) of the sources with photometric redshift larger than 3 have been visually inspected together with the photometric fitting and the probability distribution of all the possible solutions. There are 91 sources selected in the 0.5­2 keV band, 14 in the 2­10 keV, and 4 in the 0.5­10 keV bands. There are 15 C-COSMOS sources without a counterpart in the optical bands, but with a K-band and IRAC (7), only IRAC (6) or no infrared detection (2). Given

the small number of bands in which these objects are detected, no photometric redshift is available for them. In X-ray-selected samples, non-detection in the optical band has been often assumed to be a proxy for high redshift (e.g. Koekemoer et al. 2004), or for high obscuration, or a combination of both. 4 of the 15 sources have no detection in the soft band suggesting high obscuration, possibly combined with high redshift. More details about the sample selection can be found in Civano et al. (2011) and are also presented in Fig. 2.

MNRAS 445, 1430­1448 (2014)


The largest X-ray-selected sample of z > 3AGNs
2.2 The ChaMP sample The ChaMP is a wide-area non-continuous X-ray survey based on archival X-ray images of the high Galactic latitude (|b| > 20 deg) sky observed with ACIS on Chandra. The flux levels (in erg cm-2 s-1 ) reached in the survey are 9.4 â 10-16 ­5.9 â 10-11 in the full (0.5­8 keV), 3.7 â 10-16 ­2.5 â 10-11 (0.5­2 keV) in the soft and 1.7 â 10-15 ­6.7 â 10-11 (2­8 keV) in the hard band, respectively. The ChaMP survey includes a total of 392 fields, omitting pointings from dedicated serendipitous surveys like C-COSMOS, the Chandra Deep Fields, as well as fields with extended (>3 arcmin) bright optical or X-ray sources. The list of Chandra pointings avoids any overlapping observations by eliminating the observation with the shorter exposure time. The survey has detected a total of >19 000 X-ray sources (Kim et al. 2007; Green et al. 2009) over 33 deg2 with 15 350 X-ray sources positionally matched to SDSS optical counterparts (Green et al. 2009). The study of the X-ray-detected AGN properties requires accurate estimation of redshifts, luminosities and source classification thus, good quality spectra or, when not available, multiband photometry. Hence for our X-ray analysis we chose only the 323 fields overlapping with SDSS DR5 imaging for which the sensitivity curve is given by Green et al. (2009), to determine accurate number counts. Optical spectroscopy of ChaMP X-ray sources was described by Trichas et al. (2012), where redshifts and classifications for a total of 1569 Chandra sources are presented. Since the ChaMP is a Chandra archival survey, most ChaMP fields contain targeted sources selected by the target's PI, and those targets are likely to be biased towards special X-ray populations such as bright AGN. Of the targeted sources 90 per cent have a secure spectroscopic redshift with 33 of them having at z> 3 and 29 at z> 4 (see Trichas et al. 2012). The high rate of high-redshift-detected sources clearly shows the strong selection biases that could affect our analysis if we included the targeted sources. Therefore, we exclude all targeted sources (153) to reduce bias in sample properties and source number counts. For SDSS point sources with i < 21 and without available spectroscopy, efficient photometric selection of quasars is possible using a nonparametric Bayesian classification based on kernel density estimation as described in Richards et al. (2009). To select high-z candidates without available spectroscopic or photometric redshift, SDSS detection is required in at least the i and z bands, to detect Lyman dropouts (e.g. Steidel et al. 1996). Searching the ChaMP catalogue for X-ray sources within 4 arcsec of the optical SDSS quasar coordinate (95 per cent of the matched sample has an X-ray/optical position difference of less than 3 arcsec; see Green et al. 2009), yields 9727 unique matches (63 per cent of the total ChaMP X-ray-selected sample). We additionally searched for cross-matches in the Wide-field Infrared Survey Explorer (WISE; Wright et al. 2010) and UKIRT (UK Infrared Telescope) Infrared Deep Sky Survey (UKIDSS; Warren, Hewett & Foltz 2000; Hewett et al. 2006; Maddox et al. 2008).1 For a source to be included in the WISE All Sky Source Catalog (Wright et al. 2010), an SNR > 5 detection was required for one of the four photometric bands, W1, W2, W3, or W4, with central wavelengths of roughly 3.4, 4.6, 12, and 22 µm, and angular resolutions of 6.1, 6.4, 6.5, and 12.0 arcsec. Because of the different
1 The UKIDSS project is defined in Lawrence et al. (2007). UKIDSS uses the UKIRT Wide Field Camera (WFCAM; Casali et al. 2007) and a photometric system described in Hewett et al. (2006). The pipeline processing and science archive are described in Hambly et al. (2008).

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Figure 3. Optical and/or infrared ChaMP survey completeness as a function of 0.5­2 keV X-ray flux. The red dotted line indicates the ChaMP X-ray flux cut with >75 per cent completeness from SDSS/UKIDSS/WISE.

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spatial resolutions, 6.0 arcsec (WISE; W1) and 1­2 arcsec (SDSS), we use 6 arcsec as the matching radius for WISE counterparts (Wu et al. 2012). Similarly, we searched the UKIDSS Large Area Survey (LAS; Lawrence et al. 2007) Data Release 10 for NIR counterparts to ChaMP X-ray sources. The photometric system is described in Hewett et al. (2006), and the calibration is described in Hodgkin et al. (2009). We used the LAS YJHK source table, which contains only fields with coverage in every filter and merges the data from multiple detections of the same object. The X-ray source catalogues were then matched within 3 arcsec of the X-ray position separately to each UKIDSS band: Y (0.97­1.07 µm), J (1.17­1.33 µm), H (1.49­1.78 µm), and K (2.03­2.37 µm) recovering also the areas with coverage in a single UKIDSS band. The individual band lists were then combined. For objects not detected in a UKIDSS band, we use the 5 detection limits provided in Dye et al. (2006) of Y = 20.23, J = 19.52, H = 18.73, and K = 18.06. Matching the ChaMP catalogue to WISE and UKIDSS, we find 1103 additional WISE and/or UKIDSS counterparts which do not have a SDSS counterpart (the detailed numbers are reported in Fig. 2). In summary, 70 per cent of the total ChaMP X-ray sample have SDSS and UKIDSS/WISE photometry (9727 SDSS and/or WISE and/or UKIDSS and 1103 WISE and/or UKIDSS). The limited fraction of optical matches shows how optical counterparts of faint X-ray sources are fainter than the SDSS magnitude limit (i = 21.0). SDSS quasars were identified to i < 19.1 for spectroscopy by their UV-excess colours, with an extension for z > 3 quasars to i = 20.2 using ugri colour criteria (Richards et al. 2002). Based on the X-ray limits, the identification completeness of ChaMP X-ray sources falls rapidly for objects with fainter optical counterparts. Fig. 3 shows the optical (SDSS i-band counterparts) and infrared (WISE and UKIDSS counterparts) completeness of the X-ray-selected sample as a function of the soft (0.5­2 keV) X-ray flux. This incompleteness can severely bias determination of the number counts and space density, particularly at high redshifts (e.g. Barger & Cowie 2005). To address this issue, we set a relatively high X-ray flux limit in ChaMP, where spectroscopic completeness is higher, and photometric coverage allows good photometric redshifts. We use a soft flux limit for ChaMP at S0.5­2 keV > 3 â 10-15 erg s-1 cm-2 as at these brighter fluxes the completeness is higher than 75 per cent (see Fig. 3). The completeness fraction as a function of flux has MNRAS 445, 1430­1448 (2014)


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Table 1. Photometric redshift reliability defined by Wu et al. (2012) and number of sources for ChaMP sources without spectroscopic or SDSS photometric redshifts. Surveysa Reliability (per cent) N
obj b

been taken into account for the estimation of the number counts and comoving space density (see Sections 4 and 5). For sources not detected in the soft band, the 0.5­2 keV flux has been computed by converting the 2­10 keV flux using = 1.8 (see Section 3.2). One of the main advantages of our compilation is that we do not miss the faint high-redshift population, since this is recovered by the C-COSMOS survey. In this way, the ChaMP sample is used for the determination of the bright end of the luminosity function at high redshifts.

Nobj

-lim

c

2.2.1 Spectroscopic redshifts We compiled secure spectroscopic redshifts for a total of 1547 sources. We have used 1056 sources (excluding target sources) from existing ChaMP spectroscopy (Trichas et al. 2012) for the selected ChaMP fields. Additional spectroscopic redshifts are given in the SDSS-III (N = 91; Noterdaeme et al. 2012) and SDSS-DR10 ^ quasar catalogues (N = 145; Paris et al. 2014). We also searched the literature by cross-correlating optical positions with the NASA Extragalactic Database (NED), using a 2 arcsec match radius where we found 255 more sources with spectroscopic redshift. The high-redshift spectroscopic sample consists of 44 sources with z> 3. All of these sources have a soft band X-ray detection, and only three sources lack a hard-band detection. Among them, there are seven sources with z> 4 and one source with z = 6.016 ± 0.005 (Jiang et al. 2007). All but six of them have SDSS optical spectra with mean S/N > 4.5 (none of them has S/N < 2.0) with at least two broad emission lines (Ly and C IV) significantly detected. For five of the remaining sources, redshifts have been obtained by Trichas et al. (2012) while for the source with the highest spectroscopic redshift (z = 6.016) we have used the estimate from Jiang et al. (2007). For 30 sources of the ChaMP spectroscopic sample, there are available photometric redshifts derived by Richards et al. (2009, see Section 2.2.2) with an accuracy of z/(1+zspec ) = 0.013 and only one catastrophic outlier.

W U U+W S S+W S+U S+U+W Total S = SDSS; Number of c Number of with S0.5­2 ke
a b

67.4 70.4 77.2 84.8 87.0

955 27 19 637 733 71 238 2680

359 19 16 367 526 44 177 1508

W = WISE;U = UKIDSS. point-like objects in each combination of surveys. point-like objects in each combination of surveys -15 erg s-1 cm-2 . V > 3 â 10

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between some minimum and maximum value, which is crucial for dealing with catastrophic failures. The redshift probability distribution for each source is taken into account for the estimation of the number counts and comoving space density (see Sections 4 and 5). As for C-COSMOS selection of high-z sources, we also included 13 sources having zphoto + 1 zphoto > 3 and zphoto < 3. This adds another 10 objects to the main sample, all of them detected in both soft and hard bands. 2.2.3 High-z candidate selection and photometric redshift estimation For the remaining 7759 without a spectroscopic or photometric SDSS redshift, we selected the high-redshift AGN candidates using their optical and/or their infrared colours. Most of these sources (70 per cent), despite being included in SDSS DR6 catalogue, were rejected from Richards et al. (2009) selection criteria. The remaining sources come from later SDSS data releases. Following the same morphological criteria as Richards et al. (2009), a candidate is required to be unresolved in images taken through the two redder filters (e.g. g and r for z 3 selection). This minimizes contamination from low-z galaxies since even type-2 AGN at z> 3 appear point like. However, we avoid using any faint flux cut in order to ensure that we do not miss faint high-z candidates since non-detection can imply high-z dropouts. We reject sources with flags indicating that their photometry may be problematic (e.g. blending of close pairs of objects, objects too close to the edge of the frame, objects affected by a cosmic ray hit). Overall, we reject 5079 non-point like sources or with problematic photometry. This number (65 per cent) is in good agreement with the rejected number of sources by Richards et al. (2009) using the same criteria which explain the lack of a photometric redshift for these sources. Photometric redshift criteria must strike a quantifiable balance between completeness and efficiency, i.e. a probability can be assigned both to the classification and the redshift. Using the SDSS, UKIDSS, and WISE2 photometric data can help us to select quasar candidates more efficiently than using each survey individually (see Table 1). The photometric redshift reliability, defined by Wu et al.

2.2.2 SDSS photometric redshifts For the sources without spectroscopic redshifts, we derived photometric redshifts. The criteria used in SDSS DR6 have now been refined to include objects redder than (u - g) = 1.0 which may well be high-z quasars. The resulting catalogue of 1 million photometrically identified quasars and their photometric redshifts from SDSS Data Release 6 (DR6) is described in Richards et al. (2009). Only point sources (type = 6) with i-band magnitudes between 14.5 and (de-reddened) 21.3 (psfmag i > 14.5 and psfmag i - extinction i < 21.3; where psfmag are the pointspread-function magnitudes). They estimate the overall efficiency of the catalogue to be better than 72 per cent, with subsamples (e.g. X-ray-detected objects) being as efficient as 97 per cent. At the faint limit of the catalogue some additional galaxy contamination is expected. There are 1611 sources with SDSS high-quality photometric redshifts and no spectroscopic redshifts in ChaMP (i.e. those with good 0.0, where good is the quality flag; 6 = most robust; -6 = least robust; Richards et al. 2009). Among them there are 14 sources with zphot > 3 and one with zphot > 4, above the adopted ChaMP flux limit. All of these sources are detected in both soft and hard band. The SDSS photo-z code also gives a probability of an object being in a given redshift range. In this way, we have not only the most likely redshift but also the probability that the redshift is MNRAS 445, 1430­1448 (2014)

2 We use the colours related to WISE W3and W4 magnitudes only for sources lacking SDSS and/or UKIDSS detections because WISE uncertainties are substantially larger (Wu, Zhang & Zhou 2004).


The largest X-ray-selected sample of z > 3AGNs
(2012) as the fraction of the sources with the difference between the photometric and spectroscopic redshifts smaller than 0.2 is given in Table 1. The highest reliability can be reached only in the UKIDSS surveyed area, which is much smaller (4000 deg2 ) than the sky coverage of both SDSS and WISE surveys. Richards et al. (2002) used a 3D multicolour space to select highredshift QSO candidates in SDSS: griz (g - r, r - i, i - z) for candidates with z > 3.0. Following the SDSS group, we search for high-z candidates in three redshift intervals (z 3.0­3.5, z 3.5­4.5, z 4.5). The details of the selection criteria are given in the appendix. Our selection criteria require that our sources lie outside of a 2 region surrounding the stellar locus. We still expect the sample to be contaminated by stars and low-z galaxies. For this reason, we use some additional criteria described by Richards et al. (2002) to exclude objects in colour regions containing predominantly white dwarfs, A stars and unresolved red­blue star pairs. During the colour selection process, no specific line is drawn between optically selected quasars (type-1 AGN) and type-2 AGN. Taking into account that both type-1 and type-2 AGNs are unresolved in optical images at z > 3 and type-2 AGNs should lie outside the stellar locus due to their red optical colours, we expect that the above criteria efficiently select both high-redshift AGN populations. We found 53 SDSS-detected high-z candidates. To increase the reliability of the photometric estimation, we also combine the SDSS selection with the redder baselines from UKIDSS and WISE, where the contamination of the stellar locus and low-redshift galaxies is lower. We used the combination of UKIDSS and SDSS colours in the Y - K versus g - z colour­ color diagram suggested by Wu & Jia (2010) to efficiently separate quasars with redshift z < 4 from stars. Similarly, Wu et al. (2012) suggested that z - W1and g - z colours could be used to separate stars from quasars. Based on these criteria, we have rejected 10 sources associated with stars based on both SDSS­UKIDSS and SDSS­WISE colour­color diagrams. For sources detected only by UKIDSS, we used the i = 21.3 upper limit and a Y - K versus i - Y colour­color diagram to separate stars and low-z galaxies from high-z candidates. We found four high-z candidates. In the case of sources detected only by WISE, there is no efficient way detailed in the literature to separate high-z quasars from stars. Photometric redshifts have been estimated for the high-z candidates by comparing the observed colours with theoretical colour­ redshift relations derived from samples with known redshifts (Richards et al. 2002; Wu & Jia 2010; Wu et al. 2012). A standard 2 minimization method is used to estimate the most probable photometric redshifts. Here, the 2 is defined as (see Wu et al. 2004) 2 =
ij

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suggesting that even in the case of type-2 AGNs the photometric redshifts are reliably estimated. Overall, we found eight sources with z > 3 at greater than 1 significance, four sources with z > 3 but lower than 1 significance, and two sources with zphot + 1 phot > 3. 2.2.4 The ChaMP high-z sample The total z > 3 ChaMP sample includes 87 sources with z > 3. Among them there are 44 sources with secure spectroscopic redshift, 15 sources with SDSS zphot > 3 and 13 sources with SDSS zphot + 1 phot > 3 available from Richards et al. (2009), and 15 sources with estimated photometric redshifts based on optical/ infrared colour­redshift relations (13 with zphot > 3 and 2 with zphot + 1 phot > 3). 3 T HE C-COSMOS AND CHAMP z > 3 A GN SAMPLE In summary, we have assembled a sample of X-ray-selected AGNs at z > 3 in the C-COSMOS and ChaMP on the basis of both spectroscopic and photometric redshifts. The total sample includes 209 sources with z> 3. Of these, 45 are selected to be at z> 3 from their broad P(z). There are also 15 C-COSMOS sources considered to be at z > 3 on the basis of their optical non-detection these are included only in the derivation of the upper boundary of the log N­log S curve. The properties of the sample members are given in Table A1 (Appendix A) and the detailed numbers are given in Fig. 2. Fig. 4 shows the optical and near-infrared (i, K, and 3.6 µm) observed magnitude distributions for the total high-z population and for sources with spectroscopic and photometric redshifts, separately. Sources selected as i-dropouts are also presented. The hard (2­10 keV rest-frame) X-ray luminosity versus plane is showninFig. 5 together with the flux limit of the C-COSMOS and ChaMP surveys (dashed line) and the applied flux cut for ChaMP (dotted line). Luminosities were computed from in every case assuming an intrinsic = 1.8. The C-COSMOS and ChaMP high-z sample is a factor of 4­ 5 larger than all the previous individual X-ray-selected samples at z > 3 (e.g. Brusa et al. 2009; Hiroi et al. 2012; Vito et al. 2013). Most importantly, this is the first time that a significant sample of 29 X-ray-selected AGNs at z > 4 is assembled. At these redshifts previous studies had a maximum of nine sources. The z > 3 X-ray-selected AGN sample also covers more than a factor of 2 of soft (2­10 keV rest-frame) X-ray luminosity, and includes a significant number of both broad-line and non-broad-line AGNs. To discuss the obscured AGN fraction requires each object in our sample to be classified as obscured or unobscured. There are two commonly adopted methods for classification: one is based on the optical emission line widths (`optical type') or, if a spectrum is not available, by the type of template that best fits the optical­ infrared SEDs of the sources. The other is based on the column densities, NH , in the X-ray spectra (`X-ray type') or, if an X-ray spectrum is unavailable, by the hardness ratio (HR; e.g. Hasinger et al. 2001). X-ray absorption should typically correlate with optical AGN type. In the unified scheme (e.g. Lawrence & Elvis 1982; Antonucci 1993; Urry & Padovani 1995) as the narrow emission line AGNs are viewed through the dusty torus, and hence have higher absorption column densities than broad emission line AGNs. In fact, evidence has been mounting over the years that the optical- and X-ray-based classifications often give contrasting results (Lawrence & Elvis 2010; Lanzuisi et al. 2013; Merloni et al. 2014). MNRAS 445, 1430­1448 (2014)

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[(mi,cz - mj,cz ) - (mi,ob - mj,ob )]2 , 2 2 mi,ob + mj,ob

(1)

where the sum is obtained for all four SDSS colours and/or WISE and/or UKIDSS colours, mi, cz - mj, cz is the colour in the colour­ redshift relations, mi,ob - mj,ob is the observed colour of a quasar, and mi,ob and mj,ob are the uncertainties of observed magnitudes in two bands. The uncertainty in the measurement was obtained by mapping the 2 error. Since the above studies are dominated by optically selected quasars, we would expect that the photometric redshifts uncertainties in type-1 AGNs are smaller. However, since the Ly break enters the g band at z 3.5, the g - r colours quickly redden with redshift for both populations. Alexandroff et al. (2013) found that g - r colours are indistinguishable at an 84 per cent confidence level between type-1 and type-2 quasars at z > 2


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Figure 5. The hard X-ray luminosity (computed with = 1.8) redshift plane for the objects in our sample. Blue squares = C-COSMOS sample. Red circles = ChaMP sample. Filled = spectroscopic redshift. Open = photometric redshift. The dashed lines represent the 2­10 keV luminosity limit of the surveys computed from the 0.5­2 keV limiting flux. The dotted red line represents the completeness flux cut we have adopted at 3 â 10-15 erg cm-2 s-1 . The dotted black lines correspond to the flux limits we imposed for the computation of the space density and their associated areas, purple (43.4 < log LX < 44.0), green (44.0 < log LX < 44.7), orange (log LX > 44.7).

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Figure 4. Observed AB magnitude distribution of all the i-band, K-band, and 3.6 µm band (from top to bottom) high-z objects. Black solid, blue dot­dashed, red dashed, and green solid lines represent the total, spectroscopic, photometric redshift, and i-dropout samples, respectively. The i-band dropouts are not included in the i-band histogram.

3.1 Optical types The optical type of the sources has been determined by the measured full width at half-maximum (FWHM) of the permitted emission lines. Those objects with emission lines having FWHM >1000 km s-1 (e.g. Stern & Laor 2012) are classified as `optical broad-line' (BLAGN), and all others as `optical non-broad-line' (non-BLAGN), i.e. they show narrow emission lines or absorption lines only, following Civano et al. (2011, 2012). In the C-COSMOS spectroscopic z> 3 sample, 21 of 32 sources are classified as BLAGN. These are mainly associated with the brighter optical sources (iAB 22­23) of the spectroscopic sample (see Fig. 7). At fainter optical magnitudes (iAB > 23), equal MNRAS 445, 1430­1448 (2014)

numbers of broad-line and non-broad-line AGNs are found. The classification for the 75 AGNs in C-COSMOS with photometric redshifts is obtained by the Salvato et al. (2011) photometric fitting method fitting the SED via 2 minimization with code LEPHARE (Arnouts & Ilbert 2011).3 More details on the fitting can be found in Salvato et al. (2011). Briefly, two libraries of templates were used, depending on morphology, optical variability, and X-ray flux of the source. The first library (defined in Salvato et al. 2009, table 2) consists of AGN templates, hybrid (host + AGN) templates, and a few normal galaxies and was used for all the point-like optical sources and for the extended sources with an X-ray flux brighter than 8 â 10-15 erg cm-2 s-1 . The second library (as defined in Ilbert et al. 2009) includes only normal galaxy templates and it was used for the remaining sources (i.e. extended and with X-ray flux <8 â 10-15 erg cm-2 s-1 ). The flowchart in fig. 6 of Salvato et al. (2011) summarizes the procedure. Civano et al. (2012), according to this fitting, divide the sources into obscured AGN, galaxies and unobscured AGN. About 40 per cent (28 sources) of the photometric sample is best fitted with an unobscured quasar template, and 47 sources with an obscured quasar template. For 29 AGNs with spectroscopic identification, the photometric and spectroscopic types match. Given the mismatch rate of 9 per cent, we estimate that 7 out of the 75 AGNs could have been assigned the wrong SED classification. In the ChaMP z > 3 spectroscopic sample, as expected at these fluxes (e.g. Brusa et al. 2009), and due to the predominantly SDSS spectroscopic target selection, only 2/44 sources are non-BLAGN. The characterization of these sources based on their SED fittings has been obtained by Trichas et al. (2012). In order to be in agreement with the spectroscopic ChaMP sample, we followed the same SED fitting method for the characterization of the 43 sources without a spectroscopic classification. According to this fitting, 11 of
3

http://www.cfht.hawaii.edu/arnouts/LEPHARE/lephare.html


The largest X-ray-selected sample of z > 3AGNs
43 sources are best fitted with an obscured quasar template (nonBLAGN). More details on the fitting can be found in Trichas et al. (2012) and Ruiz et al. (2010). Briefly, a total of 16 templates has been used including QSO, Seyfert-2 galaxies, starburst galaxies, absorption line galaxies and composite templates that are known to harbour both an AGN and a starburst. The Ruiz et al. (2010) model has been adopted, which fits all SEDs using a 2 minimization technique within the fitting tool SHERPA (Freeman, Doe & Siemiginowska 2001). The fitting allows for two additive components, one associated with the AGN emission and the other associated with the starburst emission. The fit with the lowest reduced 2 has been chosen as the best-fitting model. A general problem of relying on the optical type is that the classification may depend on the quality of the available optical spectra, since good signal-to-noise ratio is required to detect less-luminous broad-emission lines above the stellar continuum emission of host galaxies. Also, at z > 3, the H emission line moves into the infrared and so, until recently, was difficult to observe. Intermediate AGN types (1.8, 1.9; Osterbrock & Koski 1976) rapidly lose their broad H emission, and without H these may be misclassified as type-2. Nevertheless, such effects are not expected to be significant in our sample, as t consists predominantly of luminous AGNs for which contamination from the host galaxies is negligible.

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Figure 6. HR versus redshift. Blue squares = C-COSMOS sample. Red circles = ChaMP sample. Filled = spectroscopic redshift. Open = photometric redshift. Sources with no hard band or soft band detection are shown with arrows. Four curves of constant NH (1020 , 1022 , 5 â 1022 , and 1023 cm-2 ) are reported for = 1.8 (dashed lines) and = 1.4 (solid lines).

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3.2 X-ray types Most sources in our sample have a low number of detected counts (median 25 in the 0.5­8.0 keV full band). In this count regime, spectral fit results are not reliable, especially if more than one free parameter is fitted; even if the fit converges the uncertainties on the parameters are large. For these reasons, we use the Bayesian Estimation of Hardness Ratios (BEHR) method (Park et al. 2006) to derive X-ray spectral type. Hardness count ratios (HR), defined as HR = (CHB - CSB )/(CHB + CSB ), where CSB and CHB are the counts in the soft band and hard band, respectively. BEHR is particularly powerful in the low-count Poisson regime, because it computes a realistic uncertainty for the HR, regardless of whether the X-ray source is detected in both energy bands. Sources with unconstrained upper or lower limits due to non-detections (14 hard-only and 49 soft-only detections) have been computed by converting the 3 flux upper limit in the undetected band into counts. To estimate the column density, curves of constant NH as a function of redshift have been derived for two spectral slope values, = 1.4 and = 1.8. The flatter spectral slope has been chosen to be consistent with the assumptions adopted in producing the original X-ray catalogues (Kim et al. 2007; Puccetti et al. 2009). The steeper value is more representative of the intrinsic value if the spectrum is not affected by obscuration (Nandra & Pounds 1994). The relationship between HR and redshift of our C-COSMOS and ChaMP AGN samples is shown in Fig. 6.Curvesof NH = 1020 ,1022 , 5 â 1022 , and 1023 cm-2 are reported for = 1.8 (dashed lines) and = 1.4 (solid lines). We observe that C-COSMOS sample tend to be more obscured as expected due to the fainter X-ray sensitivity limit, than the ChaMP sample (Lawrence & Elvis 1982; Ueda et al. 2003; Hasinger 2008; Brusa et al. 2010; Burlon et al. 2011). Though the two samples (C-COSMOS and ChaMP) of z > 3 AGNs show different trends regarding their obscuration, the large HR errors and the similarity in this redshift range of the curves with widely different NH values for the same spectral slope, do not allow an accurate estimate of the column density for each source

to be made. Using the CIAO4 spectral analysis package, SHERPA,5 we have simulated X-ray spectra for AGN populations at 3 < z < 7 in order to quantify the evolution of X-ray spectral slopes due to the k-correction of the observed AGN spectra towards high-z. Based on these simulations, we find that the HR distribution for the ChaMP sample peaks at 1.8­2.0 while the HR distribution for C-COSMOS sample peaks at 1.4­1.9. Hereafter, to better constrain the column density and for the purpose of comparison with previous studies, we fixed the photon index to = 1.8 and converted all source fluxes. In this work, we adopt NH = 1022 cm-2 as the dividing criterion; AGNs with NH < 1022 cm-2 and >1022 cm-2 are classified as X-ray unobscured and obscured, respectively. This criterion is adopted by many authors, and is known to be generally in good agreement with the optical type (see e.g. Ueda et al. 2003; Hiroi et al. 2012). 3.3 X-ray/optical flux ratio The X-ray/optical (X/O) flux ratio is a redshift dependent quantity for obscured AGN, given that the k-correction is negative in the optical band and positive for the X-rays (Comastri et al. 2003;Fiore et al. 2003; Brusa et al. 2010). As a result, obscured sources have higher X/O at high redshift. On the other hand, unobscured sources have similar k-corrections in the two bands, and the distribution in X/O is not correlated with the redshift (Civano et al. 2012). Usually, the r- or i-band flux is used (e.g. Brandt & Hasinger 2005) while a soft X-ray flux was used originally used for this relation with the majority of luminous spectroscopically identified AGNs in the Einstein and ASCA surveys characterized by X/O = 0 ± 1 (e.g. Stocke et al. 1991; Schmidt et al. 1998; Lehmann et al. 2001). The same relation has been used also in the hard band, without really accounting for the X-ray band used or the change in spectral slope (e.g. Alexander et al. 2001; Brusa et al. 2003; Fiore et al. 2003; Civano, Comastri & Brusa 2005; Laird et al. 2009; Xue et al. 2011). Fig. 7 shows the distribution of X-ray soft (left) and hard (right) flux versus optical magnitude to illustrate the parameter space
4 5

http://cxc.harvard.edu/ciao/ http://cxc.harvard.edu/sherpa/

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Figure 7. X-ray flux (soft-left, hard-right) versus the i-band magnitude for all the X-ray sources with an i-band counterpart. The grey shaded region represents the locus of AGNs along the correlation X/O = 0 ± 1. Sources with secure spectroscopic redshifts are represented by filled symbols and sources with a photometric redshift by open symbols. Orange circles and black squares represent non-BLAGN and BLAGN, respectively. Green upper limits represent i-band dropouts and black left pointing arrows represent soft and hard X-ray flux upper limits for undetected sources in each band. The C-COSMOS sample is represented by the open big blue circles.

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spanned by the broad-line and non-broad-line populations. The X/O ratio (Maccacaro et al. 1988) is defined as X/O = log10 (fX /fopt ) = log10 (fX ) + C + mopt /2.5, (2)

where fX is the X-ray flux in a given energy range, mopt is the magnitude at the chosen optical wavelength, and C is a constant which depends on the specific filter used in the optical observations. For both X-ray bands, the X/O = ±1 locus (grey area) has been defined using as C(i) = 5.91 (Civano et al. 2012), which was computed taking into account the width of the i-band filters in Subaru, CFHT (Canada­France­Hawaii Telescope), or for bright sources SDSS. In the hard band, the locus is plotted taking into account the band width and the spectral slope used to compute the X-ray fluxes ( = 1.8). The majority of BLAGNs with a secure spectroscopic redshift, follow the trend of -1 < log10 (fX /fi ) < 1. However, given the variation in OX with luminosity (e.g. Vignali, Brandt & Schneider 2003; Young, Elvis & Risaliti 2010; Trichas et al. 2013), there can be some shift in the locations of QSOs with luminosity within the so-called BLAGN region. This shift is consistent with the X/O relation being originally calibrated on soft-X-ray-selected sources, bright in the optical and also in the X-rays. This might explain the mild shift between the ChaMP and C-COSMOS BLAGNs. Apart from the AGN population found in the BLAGN region, there is also a significant population that lie at log10 (fX /fi ) > 1 suggesting obscured nuclei. The main characteristics of this sample are as follows: (1) lack of spectroscopic redshifts (open symbols), (2) non-BLAGN optical classification (green symbols), and (3) low X-ray luminosities (1043 erg s-1 < L2­10 keV < 1044 erg s-1 ) with NH > 1022 cm-2 for 65 per cent of them, which is consistent with previous studies finding that mild obscuration is common at these luminosities (e.g. Silverman et al. 2010). Furthermore, nearly 75 per cent of all the sources with X/O > 1 are obscured, thus confirming that selections based on high X/O ratio are efficient in finding samples of obscured AGNs. 3.4 Comparison of optical and X-ray types X-ray absorption is an alternative good indicator of AGN type. In order to compare our optical classification to the expected obscuration of BLAGNs and non-broad-line AGNs based on the unified MNRAS 445, 1430­1448 (2014)

scheme, we have separated our total sample into broad-line and non-broad-line AGNs (as described in Section 3.1). In the case of the BLAGNs, the X-ray classification criterion (NH = 1022 cm-2 ) gives 28/124 X-ray obscured sources for = 1.8. Half of these sources have a spectroscopic redshift and all but three come from C-COSMOS sample. If we also take into account the HR errors, then for the lower HR limits, 11 BLAGNs are classified as X-ray obscured sources and 41 are classified X-ray obscured sources for the upper HR limits. In the non-BLAGN subset, the above criterion gives 49/71 X-ray obscured sources (detected in both soft and hard bands) for = 1.8. The 27 soft band sources in non-BLAGN sample with no detection in the hard band (reported as downward arrows in Fig. 6) have very high upper limits on the HR, due to the conservative flux upper limit computed by Puccetti et al. (2009), but most of them do not thereby satisfy the NH > 1022 cm-2 criterion. For the total sample, we find agreement between the optical and X-ray classification for 74 per cent: 77 per cent for the BLAGNs and 69 per cent for the non-BLAGNs. These rates are consistent with recent studies (e.g. Lanzuisi et al. 2013; Merloni et al. 2014). Possible explanation can be a misclassification of faint type-1 with strong optical/IR contamination from host galaxy light. To improve the statistics and gain information on the average properties of the two subclasses, we compared their mean HR values as a function of redshift (Fig. 8). Despite the 30 per cent misclassification for the individual sources (see Table 2), the mean properties of the BLAGNs and non-BLAGNs seem to agree with the NH 1022 cm-2 division. These results does not change even if we use only sources with spectroscopic redshifts. The upper and lower limits detected only in the soft or the hard band were used to compute the upper and lower boundary of the shaded area. We discuss the results in Section 5. 4 T H E log N - log S OF THE z > 3 A G N We derived the soft band number counts of the z > 3 and z > 4 samples by folding the observed flux distribution through the sky coverage area versus flux curve of the C-COSMOS survey (Puccetti et al. 2009) and the ChaMP's 323 fields (Green et al. 2009). Additionally, we have corrected the number counts for ChaMP


The largest X-ray-selected sample of z > 3AGNs

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Figure 8. The mean HR as a function of redshift for BLAGN (black squares) and non-BLAGN (orange circles) z > 3 AGN subsamples. The error bars represent the 68 per cent dispersion. Only sources with both soft and hard band detections are taken into account for the estimation of the mean HR in each bin. Undetected sources in one of these bands are included only for estimation of the upper and lower limits (dashed areas). The zphot = 6.88 source with an upper limit HRup = 0.74 has been shifted down to HR = 0.2 in order to be included in the figure. Table 2. Comparison of optical and X-ray types. The upper and lower limits have been estimated taking into account only the error ranges in HR. Number of sources BLAGN Non-BLAGN Unobscured NH < 1022 cm- 96+ - 2
16 14 +13 2-9

2

Obscured NH 1022 cm 28 49
+ - + - 14 16 9 13

-2

incompleteness in the spectroscopic/photometric coverage as a function of X-ray flux (i.e. Fig. 3). To minimize the error associated with the most uncertain part of the sensitivity curve, we truncate the C-COSMOS sample at the flux corresponding to 10 per cent of the total area (blue dashed line in Fig. 1). All the sources with a 0.5­2 keV flux above 3 â 10-16 erg cm-2 s-1 have been considered (73 objects out of the 81 soft band detected). The flux limit applied to the sample is consistent with the signal-to-noise ratio thresholds chosen by Puccetti et al. (2009), on the basis of extensive simulations, to avoid the Eddington bias in the computation of the number counts of the entire C-COSMOS sample. Thus, by applying a flux limit cut, we also reduce the Eddington bias affecting our sample. For ChaMP this would be at S0.5-2keV > 2 â 10-15 erg cm-2 s-1 ,below theflux limit already applied. The binned log N­log S relations for two redshift ranges (z > 3; orange points and z > 4; blue points, with associated errors) are plotted in Fig. 9 (top panel). In integral form, the cumulative source distribution is represented by
NS

N (> S ) =
i =1

1
i

,

(3)

Figure 9. Top: the binned log N­log S relation (with associated errors) of the z > 3 (orange circles) and z > 4 (blue squares) QSOs population. The grey shaded area represents the maximum and minimum number counts under the assumptions described in Section 4. The blue and orange curves correspond to the prediction based on the LDDE+exp (thick solid), LDDE (dotted), LADE (dashed), and CT AGNs (dash cross) models for each redshift range, respectively. The small open circles represents the number counts estimated by Brusa et al. (2009), the small open squares are from Civano et al. (2011), and small filled triangles from Vito et al. (2013). Middle: the ratio of the observed number counts for z > 3 relative to the LDDE+exp model (thick solid line at the top panel). The thick solid line represents the LDDE+exp model N/Nexp = 1, the dotted line represents the LDDE relative to the LDDE+exp model, the dashed lines the LADE relative to the LDDE+exp model, and the dash cross lines the CT AGNs model relative to the LDDE+exp model. Bottom: the ratio of the observed number counts for z > 4 relative to the LDDE+exp model. Symbols are similar to the medium panel. The ratio of the LDDE relative to the LDDE+exp model is N/Nexp > 8.0 and is not presented.

where N(>S) is the number of sources with a flux greater than S and i , is the limiting sky coverage associated with the ith source. MNRAS 445, 1430­1448 (2014)


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NS

E. Kalfountzou et al.
2

The associated error is the variance: 2 =
i =1

1
i

(4)

The grey shaded area represents an estimate of the maximum and minimum number counts relation at z > 3 obtained by considering three different effects: (1) the 1 uncertainty in the sky coverage area for each source using the sky coverage as a function of flux (see Fig. 1)and the1 uncertainty in the flux; (2) the 14 sources from C-COSMOS with no-optical detection (seen in the soft band); (3) the sources with photometric redshift zphoto < 3 but zphoto + zphoto > 3. To compute the upper boundary of the shaded area, we included all the sources in the main sample plus the sources with no optical detection at their flux+1 error. For the lower boundary, we used the flux-1 error only for the sources with zspec > 3. Under these assumptions, the lower limit corresponds to the (very unlikely) hypothesis that all the photometric redshifts are overestimated, while the upper limit corresponds to the assumption that non-detection in the optical band is a 100 per cent reliable proxy of high redshift in X-ray-selected samples. We have compared our number counts with previous X-ray surveys that span the range from deep, small area (CDF-S, at 464.5 arcmin2 with a soft band flux limit of 9.1 â 10-18 erg cm-2 s-1 ; Xue et al. 2011), to moderate area and moderate depth (Chandra-COSMOS at 0.9 deg2 with a 0.5­ 2 keV flux limit of 1.9-16 erg cm-2 s-1 ; Elvis et al. 2009), and finally to moderate area and shallower depth (XMM-COSMOS, at 2 deg2 and a soft band flux limit of 1.7 â 10-15 erg cm-2 s-1 ; Cappelluti et al. 2009). The binned log N­log S relations are plotted in Fig. 9 (top panel), together with the XMM-COSMOS (Brusa et al. 2009, open circles), C-COSMOS (Civano et al. 2011, open squares), and 4 Ms CDF-S number counts (Vito et al. 2013, filled triangles). Good agreement exists among the comparison surveys presented here. At z > 3 and fainter X-ray fluxes (S0.5-2keV < 2 â 10-15 erg cm-2 s-1 ) our points confirm the agreement with the model predictions, previously found by Brusa et al. (2009), Civano et al. (2011), and Vito et al. (2013). At the same redshift range but brighter X-ray fluxes, where only XMM-COSMOS sample have available points (Brusa et al. 2009) based on four sources, we reduce the uncertainties by factor of 4 using a sample of 66 sources with (S0.5-2keV > 2 â 10-15 erg cm-2 s-1 ). Notably, it is the first time that data points at the bright end at z> 4 are included. At redshift z > 4, where the XMM-COSMOS sample had only 4 sources, the 4 Ms CDF-S 9 sources and the C-COSMOS 14 sources, the C-COSMOS and ChaMP sample has 27 sources (two to five times larger), making it possible to compare the slope of the counts with models. A comparison with the AGN number counts from three different phenomenological model predictions is also presented in Fig. 9 with the different types of curves (orange colour for z> 3 and blue colour for z > 4). (1) The thick solid lines correspond to the predictions of the XRB synthesis model of Gilli et al. (2007), based on the X-ray luminosity function observed at low redshift (e.g. Hasinger et al. 2005), parametrized with an LDDE and a high-redshift exponential decline with the same functional form adopted by Schmidt, Schneider MNRAS 445, 1430­1448 (2014)

& Gunn (1995, (z) = (z0 ) â 10-0.43(z-z0 ) and z0 = 2.7) to fit the optical luminosity function between z 2.5and6(Fanetal. 2001), corresponding to one e-folding per unit redshift (hereafter referred to as LDDE+exp). (2) The dotted curves correspond to the predictions of the LDDE model without the high-z decline (Gilli et al. 2007), obtained extrapolating to high-z the best-fitting parameters of Hasinger et al. (2005). (3) The dashed line is the LADE model (Aird et al. 2010)which fits the hard X-ray luminosity function derived by Aird et al. (2010) using the 2Ms Chandra Deep Fields and the AEGIS-X (200 ks) survey to probe the faint end (log LX < 43 erg s-1 ) and the high-z (z 3) range. (4) The dash-crossed lines correspond to the Treister, Urry & Virani (2009) X-ray background population synthesis predictions (CT AGNs).6 While at z> 3 the two model predictions are very close, at z> 4, where the models have different slopes, the errors on the data of the previous studies do not allow a firm preference of one of the two models, highlighting the advantage of our sample with respect to previous surveys. In this work, we find that the LDDE model (Gilli et al. 2007) without decline clearly overestimates the observed counts even in the most optimistic scenario (upper boundary) in both the z> 3and z> 4 redshift ranges. Our results for the z> 3 sample (orange colour) are in good agreement with both LDDE+exp (thick solid line) and LADE (dashed line) model predictions but only up to flux 2 â 10-15 erg cm-2 s-1 , where the difference of the two models is <20 per cent. However, the main advantage of our sample becomes clear at brighter fluxes; our results strongly exclude the LADE model. This is in contrast to previous studies which could not distinguish between the two models due to their large uncertainties. At z > 4 (blue colour), our results are in good agreement with LDDE+exp predictions. We cannot clearly exclude the LADE model if we take into account the upper and lower boundaries. However, we can point out for first time that there is no sign of the expected decline to higher fluxes. LDDE is fully ruled out at z > 4. While fainter samples would be also useful for a better description of the model, considering only the z > 4 subsample by Vito et al. (2013, 4 Ms CDF-S, filled triangles), the data lie between the LDDE+exp and LADE models prediction. Vito et al. (2013) have included the presence of 3 sources at 5 < z < 7.7, whose redshifts are determined on the basis of relatively uncertain photometric information. If these sources were placed at 3 4 sample would be consisted only by two sources.
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5 2­10 keV COMO VING S P A CE DENSITY To investigate the cosmological evolution of AGN at z > 3, we calculate the comoving space density from our sample utilizing the 1/Vmax method (Schmidt 1968). This method takes into account the fact that more luminous objects are detectable over a larger volume and is readily adapted to the case in which the survey area depends on flux.

6 Model predictions from the work of Treister et al. (2009) for a range of input values are publicly available at http://agn.astroudec.cl/j_agn/main.html


The largest X-ray-selected sample of z > 3AGNs
The maximum available volume, over which each source can be detected, was computed by using the formula: V
max

1441

=

zmax zmin

(f (LX ,z,NH ))

dV dz, dz

where (f(LX , z, NH )) is the sky coverage at the flux f(LX , z) corresponding to a source with absorption column density NH and observed luminosity LX ,and zmax is the maximum redshift at which the source can be observed at the flux limit of the survey. If zmax > zup, bin , where zup, bin is the maximum redshift of the redshift bin, then the zmax is the upper boundary of the redshift bin used for computing Vmax . In the case of the ChaMP sample, zmax is estimated using both the X-ray and optical survey limits and is selected to be the minimum of the two estimates so the source can be observed at the flux limit of both surveys. We computed the space density using the luminosities derived with = 1.8. The contribution of sources with photometric redshift to the space density is weighted for the fraction of their P(z) at z > 3. After calculating the Vmax for each source, we sum the reciprocal values in each redshift bin:
zmin
=
i =1

1 V
max,i

,

(5)

where is the comoving space density in the range z = zmin zmax , and i is the index of the sample AGN falling into the redshift bin. The 1 uncertainty is estimated as
zmin
=
i =1

1 V
max,i

2

Figure 10. The comoving space density in three different 2­10 keV X-ray luminosity ranges. The solid lines corresponds to the X-ray-selected AGN space density computed for the same luminosity limit from the Gilli et al. (2007) LDDE+exp model. The dashed curve corresponds to the space density derived from the LADE model of Aird et al. (2010). The colours respond to the shaded areas in Fig. 5 and the shaded area represents the maximum and minimum space density under the assumptions described in the text. When only one source is included in the bin it has been plotted as an upper limit (at 3 ). The small black symbols and dotted lines correspond to the comoving space density data points and model derived from Ueda et al. (2014) for similar X-ray luminosity ranges.

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.

(6) space density boundaries. However, we computed the space density assuming that all the 15 sources were at the redshift corresponding to the first bin, then to the second bin and so on. The space density values computed in this case, in the first three bins, are within the shaded areas. The space density in the three luminosity ranges is compared with the predictions, at the same luminosity threshold, from the same three models discussed in Section 4. The LDDE+exp model (Gilli et al. 2007) used for the log N­log S (solid lines), including in the model all the sources up to a column density of 1025 cm-2 . We also compare with the LADE model (Aird et al. 2010, dashed line). The LDDE model is fully ruled so it is not included in the following comparisons. In agreement with the results obtained from the number counts, the LDDE+exp model provides an excellent representation of the observed data, although the LADE model cannot be rejected taking into account the upper and lower boundaries. We confirm that the shape of the space density evolution of X-rayselected luminous AGN is consistent with that derived from optical quasar surveys within current uncertainties. The results from the Ueda et al. (2014) for AGNs with the same X-ray luminosity ranges are also plotted for comparison (small black symbols). As can be seen, our results are consistent with those of Ueda et al. (2014) within the statistical errors, indicating a significant decline in the AGN space density from z = 3 to higher redshifts. To take into account the observed decline in their LDDE model (Fig. 10, black dotted lines), Ueda et al. (2014)introduced another (luminosity-dependent) cutoff redshift above which the model declines. Their model indicates an `up-sizing' evolution instead of the global `downsizing' evolution, where more luminous AGNs have their number density peak at higher redshifts compared with less luminous ones. In the cases of lower and intermediate X-ray luminosities, the Ueda et al. (2014) LDDE model slightly overestimates our results but it is within the upper boundaries. In MNRAS 445, 1430­1448 (2014)

Including soft, hard, and full band detected sources allows us to compute a space density which takes into account both unobscured sources, which emit more at softer energies, and obscured sources, which emit more at harder energies, without having to introduce any further correction or assumption. The resulting comoving space densities are shown in Fig. 10. To reduce the effects of incompleteness and to have a complete sample over a given redshift range, we divided the sample in three luminosity intervals (see Fig. 5, shaded areas): (1) at low luminosities (purple shading), we computed the space density in three redshift bins (z = 3­4.3) at 43.3 < log (LX /erg s-1 ) 44.0; (2) at intermediate luminosities (green shading), we computed the space density in three redshift bins (z = 3­5.4) at 44.0 < log (LX /erg s-1 ) 44.7; (3) at high luminosities (orange shading), we computed the space density in five redshift bins (z = 3­5.0) at log (LX /erg s-1 ) > 44.7. We also estimated space density upper and lower boundaries by taking into account the X-ray flux errors. If a source has been excluded from the main sample because its flux is lower than the flux limit applied, the same source could be included in the upper boundary sample if its flux+1 error exceeds the flux limit. Likewise, if a source has been included in the main sample because its flux is higher than the flux limit applied, the same source could be excluded by the lower boundary sample if its flux-1 error is lower than the flux limit. The shaded areas include the above uncertainties affecting the computation of the space density, i.e. the flux errors and thus errors on the maximum volume associated with each source. As explained in Section 2.1, the 15 sources with no optical band detection from C-COSMOS survey have not been included in the


1442

E. Kalfountzou et al.

Figure 11. The comoving space density in three different 2­10 keV X-ray luminosity ranges for type-1 (left) and type-2 corresponds to the X-ray-selected AGN space density computed for the same luminosity limit from the Gilli et al. (2007) in the case of type-1 AGNs and NH > 1022 cm2 for type-2 AGNs. The colours respond to the shaded areas in Fig. 5 and maximum and minimum space density under the assumptions described in Section 5. The small black symbols represent the three different 2­10 keV X-ray luminosity ranges for sources classified as unobscured (left) and obscured (right) AGNs based the NH = 1022 cm2 limit.

(right) AGNs. The solid lines model and for NH 1022 cm2 the shaded area represents the comoving space density in the on the X-ray classification and

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the case of the higher X-ray luminosities, where our data significantly reduce the uncertainties of the Ueda et al. (2014) sample, their model underestimates our result while our data may indicate a flatting up to redshift z 4.

5.1 Type-1 versus Type-2 AGNs Comparing the high-redshift evolution of optical- and X-rayselected AGN samples, the same decline profile has been revealed. Considering that X-ray-selected samples provide reduced obscuration bias in comparison with optically selected AGN, this similarity suggests no significant cosmological evolution of the obscured AGN fraction at least at higher redshifts. Supporting these result, previous studies have concluded that the obscured AGN fraction increases with redshift z = 1to z = 2 (Ballantyne et al. 2006) but decreases at higher redshifts (Hasinger 2008). To investigate further the cosmological evolution of type-1 and type-2 AGNs and their fraction with in the redshift range of z = 3­7, we calculate the comoving space density for the two sub-classes of AGNs in our sample following the same method as described in Section 5. The comoving space density is shown in Fig. 11. The upper and lower boundaries are estimated similarly to Section 5 taking into account the X-ray flux errors and the 15 C-COSMOS source with no optical band detection. We have used both optical (large symbols) and X-ray (small symbols) AGN classification and they seem to be in good agreement. To estimate the upper and lower boundaries, we also take into account the 29 per cent of mismatches into the optical and the X-ray classification of AGN types, in order to estimate the upper and lower boundaries. Specifically, in the case of optically classified type-1 AGNs, for the upper boundary we also include in each bin the sources classified as unobscured (NH < 1022 cm-2 ) even if they are defined as type-2 AGNs based on their optical classification. For the lower boundary, we exclude from each bin the type-1 AGNs which have NH > 1022 cm-2 . The same method is also applied for the type-2 AGNs. So, the upper and lower boundaries include also the uncertainties due to the misclassification of sources. The space density in the three luminosity ranges is compared with the predictions, at the same luminosity threshold, from the same LDDE+exp Gilli et al. (2007) model, including in the model all the sources with NH 1022 cm-2 for the case of type-1 AGNs MNRAS 445, 1430­1448 (2014)

and all the sources with NH = 1022 ­1024 cm-2 for the case of type2 AGNs. In agreement with our previous results, the LDDE+exp model provides an excellent representation of both type-1 and type-2 AGNs. Since both type-1 and type-2 AGNs follow the same decline profile it suggests that there is no significant cosmological evolution for their fraction above z> 3. The same results are obtained even if we follow the X-ray classification of the AGN sample into obscured and unobscured sources based (Fig. 11, small black symbols). The LDDE+exp model for these three luminosity ranges and the NH = 1022 cm-2 division predicts a fraction of objects classified as type-2 over the type-1 AGN sources of 0.63, 0.5, and 0.48 at low luminosities 43.3 < log (LX /erg s-1 ) 44.0, intermediate luminosities 44.0 < log (LX /erg s-1 ) 44.7, and high luminosities log (LX /erg s-1 ) > 44.7, respectively. For the same luminosity bins, we have calculated the mean comoving space density ratio for the two types type2 / type1 = 0.59 ± 0.02, 0.48 ± 0.04, and 0.31 ± 0.18 from low to high luminosities. These are in good agreement with the LDDE+exp model predictions. Recently, Hiroi et al. (2012) estimated the X-ray type-2 AGN fraction, classified based on the NH > 1022 cm-2 criterion, to be 0.54+0..17 at z = 3.0­5.0 and in the luminosity range of -0 19 log LX = 44.0­45.0 while for their optical selection of type-2 AGNs they found a fraction of 0.59 ± 0.09. Their estimates are somewhat larger than our result of 0.48 ± 0.04, although they agree within the errors. This difference could be easily explained from the fact that their sample is significantly smaller (30 sources) than ours and contains only four sources with z> 4 from which the two classified as type-2 AGN have only photometric redshifts. 6 S UMMAR Y We have presented the results of the largest X-ray-selected sample of z> 3 AGNs to date, compiled from the C-COSMOS and ChaMP surveys. The large body of C-COSMOS and ChaMP data and their combination allowed us to devise a robust method to build a sizeable sample of X-ray-selected AGNs and control the selection effects including both type-1 (unobscured) and type-2 (obscured) AGNs, at z > 3. Our sample consists of 209 detections in the soft and/or hard and/or full band. We find: (1) The average HR of the type-1 and type-2 AGN samples is consistent with the NH < 1022 cm-2 criterion for the


The largest X-ray-selected sample of z > 3AGNs
classification of the X-ray unobscured AGNs and the NH > 1022 cm-2 criterion for the classification of the X-ray obscured AGNs, respectively. (2) For the individual sources, there is a mismatch of 26 per cent between optical and X-ray classification. The contribution from starburst emission in the soft band, or misclassification of faint type-1 with strong optical/IR contamination from host galaxy light could possibly explain the differences between the two classifications. (3) The number counts derived in this work (Fig. 9) are consistent with previous determinations from the literature, yet significantly reduce the uncertainties especially at bright fluxes and at high redshifts (z > 4). The number counts of our combined C-COSMOS/ChaMP sources are consistent with the trend that the space density significantly declines at higher redshifts, similarly to XMM (Brusa et al. 2009)and Chandra (Civano et al. 2011) COSMOS results at similar fluxes at least within their errors and CDF-S (Vito et al. 2013) at fainter fluxes, and are better described by the LDDE+exp model (Gilli et al. 2007). In contrast to the previous studies, and due mainly to the large sample and wide flux coverage, our results exclude the Aird et al. (2010) LADE model, at the brighter fluxes. At fluxes 2 â 10-16 F0.5­2 keV 2 â 10-15 erg cm-2 s-1 , the predictions of this model are very similar to the Gilli et al. (2007) LDDE+exp model, but at fainter and brighter fluxes the two models deviate significantly. The Vito et al. (2013) results trace the Gilli et al. (2007) LDDE+exp model well at fainter fluxes, but only for z > 3, while our sample give the same results at the brighter fluxes where (due to the low expected counts) a large sample is required. (4) In agreement with the number counts, the space density is well described with the LDDE+exp model at all X-ray luminosity bins and redshifts, while the LADE model fails to fit the data. These results confirm the declining space density as observed in the optical wavelengths. (5) Taking into account both optical and X-ray classifications, we derived the space density for type-1 and type-2 (obscured and unobscured) AGNs separately. In both cases, the results are in agreement with the LDDE+exp model suggesting that the high-redshift evolution of obscured AGNs is similar to that of unobscured AGNs. For each luminosity bin, we derived the type-2 AGN fraction among the total AGN sample to be 0.59 ± 0.02, 0.48 ± 0.04, and 0.31 ± 0.18 at z > 3 in the luminosity ranges of LX = 1043.3­44.0 ,1044.0­44.7 , and1044.7­46.0 erg s-1 . The last result should have a significant impact on our understanding of the galaxy and black hole co-evolution. Either or both line-of-sight orientation or evolutionary phase (e.g. covering factor of high column obscuration) can affect the apparent obscuration (and therefore classification) of AGNs. Given that orientation does not evolve preferentially towards us, this work allows us to say that the obscured and unobscured evolutionary phases do evolve similarly. REFERENCES

1443

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APPENDIX A QSO candidates at redshift r u - - - - - < > > < < > < 3.0­3.5 satisfied the following cuts: AND AND AND AND AND AND

u g r i g

g r i z r

0.13 20.6 1.5 1.2 0.3 -1.0 0.44(u - g ) - 0.76. < > > > < 0. 1. 0. 2. 0.

(A1)

For the redshift range (A) (B) (C) (D) (E) u g g i r - - - - g r r z

3.5­4.5, this selection becomes

2 5 OR u > 20.6 7 8 OR r - i < 0.44(g - r ) - 0.558 25 AND i - z > -1.0,

(A2)

in the combination A AND B AND C AND D AND E. For the redshifts above 4.5, we use u g r- i- i- > > i> z> z< 21.5 21.0 0.6 -1.0 0.52(r - i ) - 0.762. AND AND AND AND

(A3)

MNRAS 445, 1430­1448 (2014)


The largest X-ray-selected sample of z > 3AGNs
Table A1. Properties of the high-redshift AGN sample. Survey
a

1445

RA (deg) 1.682 5.826 18.1925 29.8140 29.886 32.680 38.906 116.8620 119.1940 119.218 120.2877 125.4118 125.4123 127.837 130.2606 132.169 133.446 134.4131 137.6868 137.786 138.0425 139.086 141.0521 143.3646 149.425 149.5942 150.7009 155.4521 156.5952 162.590 162.938 163.275 168.5505 168.7335 169.4807 169.607 170.0871 170.5414 171.748 175.151 178.8811 182.037 183.2733 183.366 188.1259 190.4594 192.551 192.9299 195.0080 195.445 196.534 196.6521 197.4348 198.2170 199.322 201.424 201.8283 202.9168 203.5933 203.6122 206.078 207.7399

Dec. (deg) -0.200 -1.048 -1.2188 0.4529 0.482 -0.3051 -0.722 27.6253 41.1201 45.044 36.1514 12.2922 12.2917 19.151 13.2202 44.959 57.987 9.0255 17.7420 54.298 5.7952 29.522 31.2774 55.4032 33.251 7.7968 32.7668 13.1647 47.3187 58.625 57.468 57.5735 40.5661 40.6372 48.0722 7.7262 43.4292 24.2938 56.772 66.221 -1.770 0.129 2.8584 2.960 47.6153 9.6307 33.855 0.1240 1.3067 -0.112 3.940 46.4961 57.6389 42.4716 1.1538 11.477 29.1482 11.2840 -1.4221 -1.4816 -0.511 60.1377

z

spec

z

phot

zphot b lower ­ 4.3 3.35 3.51 4.3 4.05 3.72 2.58 2.88 ­ 3.59 ­ 2.78 4.2 2.76 ­ 3.4 2.76 3.35 ­ 3.09 ­ 2.21 2.78 ­ 2.8 3.11 2.55 4.77 3.47 ­ 2.52 3.09 0.9 2.54 3.45 2.92 2.73 3.5 ­ 3.16 2.4 2.76 2.4 2.88 2.66 ­ 2.69 3.88 ­ ­ 2.76 2.67 2.75 3.275 4.3 3.6 2.21 3.01 2.57 ­ 3.05

zphot c upper ­ 4.7 4.37 4.05 4.9 4.98 4.58 3.25 4.29 ­ 4.06 ­ 3.23 5.0 3.29 ­ 4.8 3.39 4.47 ­ 3.63 ­ 3.03 3.33 ­ 3.47 4.37 3.26 5.1 3.83 ­ 3.1 4.35 1.52 3.23 4.05 4.12 3.39 3.7 ­ 4.21 4.6 3.83 4.6 3.83 3.37 ­ 3.23 4.69 ­ ­ 3.23 3.25 3.3 3.525 4.9 4.08 3.03 4.33 3.26 ­ 3.95

Optical typeh 1 1 1 1 2 1 2 1 1 1 1 1 1 2 1 1 2 1 1 2 1 1 1 1 1 1 1 1 1 2 1 1 1 2 1 1 1 1 1 1 1 2 1 1 1 2 1 1 1 1 1 2 1 1 1 2 1 1 1 1 1 1

X-ray typei 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 1 1 1 1 1 2 1 1 1 2 1 1 1 2 1 1 1 1 1 2 1 1 1 1 1 1

Ssoft

d

logLxe

HR

f

HR lowj -0.77 ­ -0.98 ­ ­ -0.85 -0.76 -0.70 -0.66 -0.98 -0.69 -0.76 -0.51 -0.73 -0.66 -0.41 -0.55 -0.89 -0.67 -0.55 -0.70 -0.57 -0.79 -0.43 -0.93 ­ -0.61 -0.67 -0.90 -0.82 -1.0 -0.87 -0.6 -0.73 -0.77 -0.41 -0.61 -0.66 -0.96 -0.74 -0.47 -0.44 -0.71 -0.65 -0.99 -0.44 -0.83 -0.68 -0.73 -0.41 -0.75 -0.53 ­ -0.82 -0.94 -0.33 -0.99 -0.68 -0.90 -0.94 -0.84 -0.75

HR upk - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 0.12 0.38 0.57 0.84 0.53 0.56 0.41 0.09 0.14 0.62 0.56 0.47 0.42 0.42 0.44 0.22 0.19 0.05 0.35 0.35 0.29 0.19 0.48 0.20 0.49 0.88 0.34 0.21 0.24 0.22 0.73 0.59 0.32 0.64 0.57 0.09 0.32 0.38 0.53 0.61 0.21 0.08 0.53 0.24 0.76 0.04 0.35 0.27 0.02 0.09 0.61 0.06 0.64 0.52 0.20 0.4 0.53 0.40 0.34 0.54 0.51 0.63

N

H

g

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

3.109 ­ 3.592 ­ ­ 4.733 ­ 3.152 3.734 3.185 ­ 3.112 3.114 ­ ­ 3.093 ­ 3.130 4.098 3.234 3.246 3.098 ­ ­ 3.001 3.220 ­ 3.055 4.941 ­ 3.409 ­ 3.597 4.913 ­ ­ 3.552 3.336 ­ 3.337 3.202 ­ ­ ­ 3.041 ­ 3.6 ­ 4.612 3.7 6.016 ­ ­ 3.181 ­ ­ ­ ­ 3.827 ­ 3.070 ­

­ 4.5 3.545 3.675 4.6 4.565 4.15 2.905 3.135 ­ 3.675 ­ 2.995 4.6 2.945 ­ 4.1 3.065 4.105 ­ 3.345 ­ 2.795 3.045 ­ 3.145 3.405 2.905 4.895 3.65 ­ 2.755 3.495 1.305 2.905 3.75 3.665 3.045 3.6 ­ 3.495 3.5 3.145 3.5 3.205 3.045 ­ 2.905 4.485 ­ ­ 3.065 2.905 2.945 3.4 4.6 3.675 2.755 3.775 2.905 ­ 3.285

0.071 0.094 0.090 0.039 0.037 0.277 0.203 0.072 0.130 0.105 0.156 0.053 0.443 0.065 0.078 0.030 0.043 0.076 0.044 0.036 0.065 0.054 0.367 0.086 0.128 0.086 0.085 0.075 0.102 0.077 0.051 0.045 0.053 0.253 0.101 0.035 0.073 0.054 0.033 0.065 0.055 0.056 0.165 0.090 0.175 0.057 0.074 0.220 0.454 0.160 0.031 0.055 0.068 0.072 0.090 0.071 0.030 0.057 0.032 0.074 0.060 0.110

44.65 45.13 44.90 44.56 44.75 45.65 45.39 44.67 45.09 44.85 45.16 44.53 45.45 44.99 44.66 44.28 44.70 44.69 44.71 44.40 44.64 44.53 45.34 44.72 44.87 44.77 44.82 44.66 45.25 44.84 44.60 44.44 44.67 45.64 44.84 44.53 44.79 44.61 44.46 44.68 44.57 44.66 45.11 44.87 45.02 44.54 44.81 45.38 45.84 45.17 44.93 44.53 44.69 44.81 44.84 45.03 44.45 44.53 44.51 44.69 44.57 44.89

-0.50 ­ -0.84 -0.92 -0.70 -0.67 -0.58 -0.42 -0.41 -0.85 -0.64 -0.64 -0.48 -0.57 -0.54 -0.31 -0.36 -0.57 -0.50 -0.44 -0.50 -0.40 -0.65 -0.31 -0.77 ­ -0.45 -0.47 -0.44 -0.56 -0.99 -0.75 -0.44 -0.69 -0.68 -0.24 -0.46 -0.51 -0.77 -0.67 -0.35 -0.17 -0.64 -0.46 -0.99 -0.18 -0.54 -0.5 -0.37 -0.16 -0.69 -0.26 ­ -0.66 -0.59 0.01 -0.97 -0.56 -0.51 -0.66 -0.64 -0.68

0.1 <1.0 <0.1 <0.1 <0.1 <0.1 <0.1 0.1 0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 4.0 4.0 0.1 0.1 0.1 0.1 0.1 <0.1 4.0 <0.1 <0.1 0.1 0.1 0.1 0.1 <0.1 <0.1 0.1 <0.1 <0.1 13.0 0.1 0.1 <0.1 <0.1 2.0 17.0 <0.1 0.1 <0.1 13.0 0.1 0.1 4.0 20.0 <0.1 8.0 <0.1 <0.1 0.1 50.0 <0.1 <0.1 0.1 <0.1 <0.1 <0.11

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1446

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Table A1 ­ continued Survey
a

RA (deg) 208.2111 212.7671 214.2243 214.4241 214.8523 215.1504 215.7710 216.0719 216.447 218.114 219.5024 219.633 219.6653 220.8577 221.3084 225.0836 230.1945 230.8973 240.6529 244.0288 258.2694 258.956 260.0728 340.9458 359.3544 149.5438 149.6242 149.6908 149.6696 149.7362 149.7425 149.7617 149.7710 149.7821 149.7837 149.7975 149.8035 149.8046 149.8079 149.8085 149.8123 149.8144 149.8233 149.8458 149.8515 149.8517 149.8585 149.8605 149.8697 149.8744 149.8792 149.8825 149.8861 149.8894 149.8942 149.8977 149.9084 149.9093 149.9108 149.9111 149.9137 149.9195 149.9249

Dec. (deg) 33.4822 52.2988 44.6294 53.0886 53.5323 47.2415 24.0856 22.8406 35.455 -1.160 3.6388 3.641 3.6288 58.8995 -0.4237 22.7196 52.9896 28.6609 42.5518 47.2661 61.3793 63.246 26.5628 -9.6855 0.6643 1.9537 1.8854 2.2641 2.1677 2.1799 2.5354 2.4358 2.3658 2.4713 2.4521 2.2897 2.1417 2.1189 1.8105 2.3148 2.2830 2.7348 2.5398 2.4817 2.2764 2.4269 2.4093 2.3876 2.2941 2.3615 2.2258 2.5051 2.2759 1.9662 2.4330 2.3244 2.5723 2.6199 1.8996 1.8427 2.2465 2.3454 1.8441

z

spec

z

phot

zphot b lower 2.32 2.66 2.52 3.15 3.41 2.71 3.36 3.58 ­ 3.0 2.92 3.05 2.88 2.68 2.68 3.18 3.15 2.44 3.27 2.92 2.76 3.5 2.77 3.36 2.98 2.91 2.1 >3.0 2.75 4.23 2.9 3.64 3.35 3.23 1.9 >3.0 2.24 1.59 2.33 3.41 3.24 >3.0 >3.0 3.36 3.3 3.16 3.68 2.33 3.39 6.88 3.64 3.02 3.26 2.2 3.38 2.9 >3.0 2.13 2.585 2.81 >3.0 3.03 1.79

zphot c upper 3.07 3.13 3.12 4.37 3.9 3.26 4.54 4.06 ­ 3.6 4.08 3.5 3.94 3.22 3.36 4.21 4.35 3.02 4.46 3.97 3.33 3.7 3.5 4.56 4.19 3.01 3.86 ­ 3.21 4.26 3.01 3.66 3.53 3.26 1.98 ­ 3.09 4.58 3.260 3.79 3.35 ­ ­ 3.39 3.33 3.58 4.29 3.97 3.41 7.0 3.66 3.19 3.3 3.12 3.41 3.16 ­ 4.23 3.194 3.45 ­ 3.05 3.05

Optical typeh 1 1 1 1 1 1 1 1 1 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 1 ­ 2 1 2 1 2 1 2 ­ 1 2 1 2 1 ­ ­ 1 1 2 2 1 1 2 1 2 2 2 1 2 ­ 1 2 2 ­ 1 2

X-ray typei 1 1 1 1 1 1 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 1 2 2 1 2 2 2 2 2 2 2 2 2 1 1 2 2 1 1 2 2 2 2 2 1 2 2 1 2 2 2 1 2 ­ 2 1 2

Ssoft

d

logLxe

HR

f

HR lowj -0.57 -0.48 -0.66 -0.85 -0.95 -0.90 -0.54 -0.64 -0.66 -0.56 -0.68 -0.89 -0.77 -0.96 -0.54 -0.97 -0.83 -0.72 -1.0 -0.95 ­ -0.86 -0.95 -0.88 ­ 0.32 ­ -0.17 -0.18 -0.59 0.31 -0.39 ­ -0.05 ­ 0.69 -0.58 ­ -0.65 -0.46 ­ ­ 0.52 ­ -0.70 0.15 -0.34 -0.13 -0.02 ­ -0.54 -0.29 0.29 ­ -0.70 ­ ­ ­ ­ ­ 0.23 ­ ­ - - - - - - - - - - - - - - - - - - - - - - - - -

HR upk 0.30 0.36 0.43 0.61 0.47 0.58 0.01 0.56 0.41 0.28 0.48 0.72 0.56 0.35 0.0 0.41 0.34 0.48 0.71 0.01 0.89 0.27 0.71 0.48 0.75 0.79 0.34 0.53 0.10 0.23 ­ 0.11 0.73 ­ 0.11 ­ 0.12 0.60 0.20 0.23 0.69 0.51 ­ 0.498 0.35 0.52 0.21 0.57 0.15 0.74 0.28 0.16 ­ 0.32 0.31 0.45 0.88 0.29 0.62 ­ 0.52 0.69 0.21

N

H

g

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2

­ ­ ­ ­ ­ 3.237 4.112 ­ 3.53 ­ 3.306 ­ 3.194 ­ 3.142 3.268 3.385 ­ 3.889 ­ 3.150 ­ 3.057 ­ ­ ­ ­ ­ 3.089 4.255 ­ ­ ­ ­ 5.07 ­ ­ ­ ­ ­ ­ ­ ­ ­ 3.371 ­ ­ ­ 3.345 ­ 3.65 ­ 3.335 ­ 3.382 ­ ­ ­ ­ ­ ­ 3.021 ­

2.755 2.815 2.795 3.495 3.535 2.905 4.165 3.675 ­ 3.3 3.285 3.275 3.205 2.905 3.065 3.475 3.495 2.715 4.125 3.205 3.005 3.6 3.145 4.125 3.285 2.951 2.264 ­ 3.094 4.245 2.942 3.647 3.447 3.246 1.939 ­ 2.838 3.791 2.382 3.471 3.297 ­ ­ 3.375 3.317 3.35 4.108 2.949 3.4 6.88 3.647 3.1 3.277 3.053 3.393 2.998 ­ 2.569 3.063 3.295 ­ 3.043 2.936

- - -

-

-

-

0.056 0.097 0.134 0.037 0.185 0.300 0.032 0.734 0.053 0.079 0.058 0.070 0.040 0.030 0.058 0.051 0.060 0.062 0.045 0.058 0.126 0.139 0.053 0.078 0.039 0.024 0.007 0.031 0.018 0.014 0.015 0.023 0.002 0.014 0.007 0.002 0.009 0.004 0.006 0.025 0.009 0.021 0.010 0.007 0.012 0.004 0.049 0.002 0.024 0.002 0.018 0.046 0.004 0.004 0.003 0.006 0.004 0.006 0.002 0.003 0.010 0.009 0.004

44.54 44.99 44.94 44.49 45.20 45.32 44.58 45.83 44.65 44.76 44.62 44.70 44.44 44.32 44.58 44.56 44.66 44.56 44.72 44.60 44.91 45.09 44.51 44.97 44.45 44.28 43.98 45.46 44.21 44.40 44.30 44.46 43.38 44.334 44.31 44.33 43.87 43.68 43.79 44.44 43.95 45.29 44.97 43.87 44.09 43.69 44.91 43.51 44.42 44.10 44.35 44.60 43.83 43.47 43.43 43.72 43.64 44.02 43.24 43.50 44.02 43.85 43.55

-0.462 -0.42 -0.54 -0.75 -0.74 -0.75 -0.26 -0.60 -0.56 -0.41 -0.58 -0.81 -0.68 -0.84 -0.24 -0.79 -0.63 -0.61 -0.82 -0.58 ­ -0.61 -0.85 -0.71 ­ 0.49 ­ 0.22 -0.04 -0.41 ­ -0.16 ­ ­ ­ ­ -0.37 ­ -0.22 -0.34 ­ ­ ­ ­ -0.53 0.33 -0.27 0.22 0.06 ­ -0.41 -0.22 ­ ­ -0.19 ­ ­ ­ ­ ­ 0.46 ­ ­

- - - - - -

- - -

-

- -

0.1 0.1 <0.1 <0.1 <0.1 <0.1 14.0 <0.1 <0.1 0.1 <0.1 <0.1 <0.1 0.1 10.0 <0.1 <0.1 <0.1 <0.1 0.1 <0.1 <0.1 <0.1 <0.1 <0.1 80.0 <3.0 50.0 22.0 0.1 55.0 22.0 <1000.0 25.0 <75.0 <1000.0 1.5 <300.0 11.0 4.0 0.1 <85 >85 <0.1 0.1 68.0 11.0 65.0 37.0 <1000.0 0.1 10.0 >65.0 <5.0 14.0 <75.0 <1000.0 <6.0 <200.0 ­ 71.0 <0.1 <13.0

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The largest X-ray-selected sample of z > 3AGNs
Table A1 ­ continued Survey
a

1447

RA (deg) 149.9316 149.9445 149.9522 149.9666 149.9692 149.9728 149.9816 149.9905 149.9986 150.0044 150.0070 150.0078 150.0094 150.0172 150.0258 150.0427 150.0465 150.0483 150.0621 150.0635 150.0637 150.0647 150.0673 150.0862 150.0910 150.0969 150.0999 150.1010 150.1074 150.1336 150.1522 150.1640 150.1763 150.1807 150.1926 150.1997 150.2050 150.2053 150.2089 150.2090 150.2108 150.2142 150.2146 150.2259 150.2478 150.2595 150.2608 150.2618 150.2674 150.2684 150.2716 150.2848 150.285 150.2881 150.2973 150.2994 150.3007 150.3048 150.3060 150.3100 150.3158 150.3179 150.3187

Dec. (deg) 2.3519 1.7404 2.6514 2.4325 2.3048 1.9415 2.3150 2.2973 1.9745 2.0389 1.9180 2.1489 1.8526 2.4979 2.0038 1.8722 2.3674 2.4816 1.7226 2.4219 1.7774 2.1910 2.0843 2.139 1.9292 2.0215 2.1527 2.4194 1.7592 2.4574 2.3079 2.1793 2.5697 2.0760 2.2199 1.7312 1.7360 2.5029 2.4819 2.4385 2.3915 2.4750 2.5827 1.7999 2.4422 2.3761 2.1180 1.7316 1.7009 1.7891 1.6139 1.8183 2.309 1.6511 2.1489 1.6878 2.3007 1.8242 1.7616 2.5045 2.3369 2.0050 2.2477

z

spec

z

phot

zphot b lower >3.0 2.0 3.07 2.23 3.076 2.63 2.97 2.93 2.88 3.49 >3.0 2.77 4.02 2.5 2.67 3.32 1.94 >3.0 2.91 3.02 1.21 1.53 2.99 4.73 2.84 3.33 2.6 4.53 3.93 3.08 3.08 2.75 3.09 3.0 3.07 2.66 2.4 3.06 3.37 3.55 3.07 3.06 5.15 3.12 2.97 2.66 >3.0 3.08 3.33 >3.0 3.44 >3.0 2.42 3.13 3.45 2.67 3.41 2.49 3.24 >3.0 3.85 3.38 >3.0

zphot c upper ­ 3.66 3.09 3.3 3.13 3.06 3.03 3.05 3.01 3.51 ­ 3.43 4.05 3.26 3.17 3.35 3.1 ­ 3.41 3.25 3.84 3.06 3.03 5.14 3.23 3.39 3.06 4.56 3.96 3.15 3.24 3.15 3.24 3.03 3.09 3.45 4.19 3.09 3.39 3.57 3.1 3.09 5.28 3.46 3.02 2.68 ­ 3.55 3.46 ­ 3.5 ­ 3.0 3.95 3.47 3.14 3.46 3.35 3.29 ­ 4.44 3.49 ­

Optical typeh ­ 1 2 2 1 2 2 2 2 1 ­ 1 2 2 2 1 2 ­ 2 2 2 1 1 1 1 2 2 1 1 2 2 1 2 1 1 2 1 1 1 1 1 1 2 1 1 1 ­ 2 1 ­ 2 ­ 2 1 1 1 2 1 2 ­ 2 2 ­

X-ray typei 2 1 2 2 2 1 2 1 1 1 2 2 1 1 2 2 1 2 2 2 2 2 1 2 2 1 1 2 2 1 2 2 2 1 2 2 2 2 1 1 2 2 2 2 2 1 ­ 2 1 2 2 2 1 1 2 1 2 1 2 2 2 2 2

Ssoft

d

logLxe

HR

f

HR lowj 0.87 ­ 0.411 0.753 -0.07 -0.41 -0.15 ­ ­ -0.54 0.33 -0.03 ­ ­ 0.48 -0.58 ­ ­ -0.21 0.48 0.32 0.03 ­ 0.01 0.54 ­ ­ -0.49 -0.28 ­ 0.24 0.407 ­ ­ -0.39 -0.10 -0.22 -0.28 -0.62 ­ -0.35 -0.17 ­ 0.16 0.02 ­ ­ ­ ­ 0.48 0.0 -0.18 -0.56 ­ -0.42 ­ ­ ­ ­ 0.09 ­ ­ -0.13

HR upk ­ -0.28 ­ ­ 0.19 -0.12 0.04 -0.72 -0.78 -0.28 0.78 0.37 -0.63 -0.71 ­ -0.06 -0.39 0.86 0.33 ­ ­ 0.19 -0.57 0.49 ­ -0.59 -0.67 -0.05 0.3 -0.494 0.60 ­ 0.65 -0.77 0.1 0.47 0.30 0.12 -0.35 -0.717 0.02 0.03 0.36 0.55 0.18 -0.68 ­ 0.51 -0.45 0.87 0.49 0.25 -0.20 -0.48 0.20 -0.24 0.70 -0.47 0.77 0.48 0.76 0.03 0.18

N

H

g

2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2

­ ­ 3.08 ­ 3.155 ­ ­ 3.026 ­ 3.515 ­ ­ ­ ­ ­ 3.371 ­ ­ ­ ­ ­ ­ ­ ­ ­ 3.546 ­ 4.66 4.16 3.189 3.175 ­ ­ 3.01 3.09 ­ ­ ­ 3.333 3.715 3.095 ­ 5.3 ­ 3.029 3.717 ­ ­ ­ ­ ­ ­ ­ ­ 3.328 ­ 3.498 ­ ­ ­ ­ ­ ­

­ 2.044 3.083 2.893 3.099 2.843 3.003 2.97 2.911 3.5 ­ 2.851 4.032 2.914 3.124 3.338 2.313 ­ 3.033 3.087 3.426 2.887 3.01 5.045 2.864 3.362 2.844 4.545 3.949 3.111 3.163 2.82 3.144 3.015 3.077 2.821 2.929 3.072 3.378 3.563 3.085 3.075 5.201 3.264 2.992 2.673 ­ 3.126 3.412 ­ 3.466 ­ 2.897 3.871 3.459 2.976 3.434 2.949 3.265 ­ 4.216 3.428 ­

- -

-

- -

-

-

- -

0.004 0.006 0.011 0.013 0.014 0.013 0.023 0.009 0.013 0.018 0.006 0.005 0.008 0.009 0.006 0.008 0.004 0.004 0.008 0.006 0.009 0.025 0.006 0.003 0.007 0.006 0.008 0.009 0.006 0.006 0.003 0.004 0.005 0.011 0.007 0.004 0.006 0.014 0.024 0.010 0.009 0.020 0.009 0.004 0.023 0.008 0.009 0.005 0.014 0.010 0.011 0.011 0.011 0.018 0.005 0.008 0.003 0.006 0.003 0.005 0.002 0.007 0.008

43.62 43.88 44.16 44.33 44.12 44.02 44.28 43.87 44.02 44.31 43.76 43.78 44.11 43.95 43.93 43.94 43.50 43.64 43.84 43.93 44.17 44.35 43.66 43.99 44.04 43.81 43.81 44.33 44.01 43.71 43.55 43.57 43.61 43.94 43.76 43.68 44.02 44.07 44.37 44.09 43.88 44.24 44.42 43.63 44.31 44.03 43.97 43.67 44.17 44.02 44.14 44.06 43.94 44.40 43.74 43.83 43.55 43.79 43.46 43.75 43.47 43.86 43.88

­ ­ ­ ­ 0.06 -0.26 -0.06 ­ ­ -0.41 0.60 0.17 ­ ­ ­ -0.32 ­ ­ 0.06 ­ ­ 0.11 ­ 0.25 ­ ­ ­ -0.27 0.01 ­ 0.42 ­ ­ ­ -0.15 0.18 0.04 -0.08 -0.49 ­ -0.17 -0.07 ­ 0.36 0.10 ­ ­ ­ ­ 0.72 0.24 -0.033 -0.38 ­ -0.11 ­ ­ ­ ­ 0.33 ­ ­ 0.06

1000.0 <6.0 >70.0 >1000.0 32.0 7.0 22.0 <0.1 <0.1 0.1 150.0 48.0 <0.1 <0.1 1000.0 5.0 0.1 1000.0 32.0 >80.0 >50.0 35.0 <0.1 100.0 >90.0 <0.1 <0.1 14.0 43.0 <0.1 75.0 27.0 <5000.0 <0.1 15.0 50.0 48.0 20.0 0.1 <0.1 13.0 20.0 <500.0 68.0 35.0 <0.1 ­ >65.0 <0.1 1000.0 60.0 12.0 1.2 <0.1 20.0 <9.0 <1000.0 <0.1 <1000.0 68.0 <1000.0 <30.0 11.0

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Table A1 ­ continued Survey
a

RA (deg) 150.3332 150.3344 150.3443 150.3451 150.3566 150.3598 150.3647 150.3790 150.3809 150.3836 150.3989 150.4153 150.4155 150.4251 150.4550 150.4679 150.4851 150.4862 150.5467 150.5512 150.6475

Dec. (deg) 2.4415 1.7882 1.6361 1.9579 2.2242 2.0737 2.1438 1.8761 2.0995 2.0748 2.2709 1.9342 2.3648 2.3120 1.9674 2.5315 2.4135 2.4281 2.2243 2.1400 2.4182

z

spec

z

phot

zphot b lower 2.12 2.34 3.74 2.98 2.51 1.96 3.35 3.02 4.43 3.83 >3.0 1.28 2.24 2.98 3.46 4.43 2.57 2.95 3.35 2.97 2.6

zphot c upper 3.07 3.02 3.85 3.17 3.09 2.01 3.37 3.44 4.56 3.88 ­ 3.75 3.29 3.29 3.52 4.47 3.51 3.61 3.66 3.07 3.32

Optical typeh 2 1 2 2 2 2 1 2 2 2 ­ 1 1 2 1 2 2 2 2 2 1

X-ray typei 1 1 2 2 2 2 1 2 2 2 2 1 1 2 1 1 2 2 2 2 2

Ssoft

d

logLxe

HR

f

HR lowj -0.76 ­ ­ -0.07 0.56 ­ -0.38 -0.44 0.514 0.13 0.24 ­ ­ -0.05 ­ ­ 0.14 0.07 -0.21 0.67 0.69

HR upk -0.33 -0.24 -0.08 0.25 ­ 0.27 -0.14 0.06 ­ 0.68 0.61 -0.54 -0.29 0.48 -0.46 -0.563 ­ ­ 0.03 ­ ­

N

H

g

2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
a b

­ ­ ­ ­ ­ 4.917 3.328 ­ ­ ­ ­ ­ ­ ­ 3.485 ­ ­ ­ ­ ­ ­

2.481 2.683 3.805 3.065 2.701 1.991 3.361 3.33 4.498 3.852 ­ 3.681 2.423 3.092 3.493 4.45 2.949 3.464 3.506 3.028 3.034

-

-

- - - -

0.015 0.005 0.019 0.010 0.008 0.008 0.025 0.009 0.008 0.003 0.005 0.011 0.003 0.003 0.007 0.014 0.006 0.005 0.029 0.024 0.052

44.09 43.60 44.41 43.93 44.02 44.30 44.40 43.96 44.39 43.62 43.72 44.15 43.47 43.42 43.89 44.46 44.04 43.77 44.53 44.50 44.84

-0.54 ­ ­ 0.18 ­ ­ -0.26 -0.18 ­ 0.40 0.46 ­ ­ 0.21 ­ ­ ­ ­ -0.08 ­ ­

0.1 <8.0 <30.0 45.0 >75.0 <70.0 8.0 14.0 >200.0 90.0 200.0 <0.1 <6.0 48.0 <0.1 <0.1 >25.0 32.0 25.0 1000.0 1000.0

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1 = ChaMP; 2 = C-COSMOS. Photometric redshift lower limit. c Photometric redshift upper limit. d The 0.5­2 keV flux in units of 10-13 erg cm-2 s-1 converted to = 1.8. For sources undetected in soft band a negative symbol is given and the flux estimated by the hard or full band detection. e The 2­10 keV luminosity in units of erg s-1 . f Hardness ratio defined as HR = (H + S)/(H - S), S: 0.5­2.0 keV count rate and H: 2.0­10 keV count rate. g The absorption column density in units of 1022 cm-2 . h 1: optical type-1 AGNs, and 2: optical type-2 AGNs. i 1: unobscured AGNs, and 2: obscured AGNs; based on N = 1022 cm-2 limit. H j HR lower limit. k HR upper limit.
A This paper has been typeset from a TEX/L TEX file prepared by the author.

MNRAS 445, 1430­1448 (2014)