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The Astrophysical Journal, 569:L1­L4, 2002 April 10
2002. The American Astronomical Society. All rights reser ved. Printed in U.S.A.

DISCOVERY OF A z p 4.93, X-RAY­SELECTED QUASAR BY THE CHANDRA MULTIWAVELENGTH PROJECT (ChaMP) John D. Silverman,1,2,3 Paul J. Green,1 Dong-Woo Kim,1 Belinda J. Wilkes,1 Robert A. Cameron,1 David Morris, Anil Dosaj,1,3 Chris Smith,4 Leopoldo Infante,5,6 Paul S. Smith,7 Buell T. Jannuzi,8 and Smita Mathur9
Received 2002 January 22; accepted 2002 February 28; published 2002 March 11
1

ABSTRACT We present X-ray and optical obser vations of CXOMP J213945.0 234655, a high-redshift (z p 4.93) quasar discovered through the Chandra Multiwavelength Project (ChaMP). This object is the most distant X-ray­selected quasar published, with a rest-frame X-ray luminosity of L X p 5.9 # 10 44 ergs s 1 (measured in the 0.3­2.5 keV band and corrected for Galactic absorption). CXOMP J213945.0 234655 is a g dropout object (126.2), with r p 22.87 and i p 21.36. The rest-frame X-ray­to­optical flux ratio is similar to quasars at lower redshifts and slightly X-ray bright relative to z 1 4 optically selected quasars obser ved with Chandra. The ChaMP is beginning to acquire significant numbers of high-redshift quasars to investigate the X-ray luminosity function out to z 5. Subject headings: galaxies: active -- galaxies: nuclei -- quasars: general -- quasars: individual (CXOMP J213945.0 234655) -- X-rays: general
1. INTRODUCTION

The obser ved characteristics of known quasars are remarkably similar over a broad range of redshift. For example, Xray studies utilizing the ROSAT database (Green et al. 1995; Kaspi, Brandt, & Schneider 2000) show little variation of the ratio of X-ray­to­optical flux for optically selected quasars. Also, the rest-frame UV spectra of quasars, including the broad Lya, N v, and C iv emission lines, are nearly identical for a large range of redshift and present no evidence for subsolar metallicities even up to a z 6 (Fan et al. 2001). Even though the individual properties of quasars are similar, the comoving space density of quasars changes drastically with redshift. At high redshift (z 1 4), a significant drop-off in the comoving space density of quasars seen in optical (e.g., Schmidt, Schneider, & Gunn 1995; Warren, Hewett, & Osmer 1994; Osmer 1982) and radio (Shaver et al. 1996) sur veys hints at either the detection of the onset of accretion onto supermassive black holes or a missed high-redshift population, possibly due to obscuration. X-ray­selected quasars from ROSAT have been used to support the latter interpretation based on evidence for constant space densities beyond a redshift of 2 (Miyaji, Hasinger, & Schmidt 2000). Unfortunately, the ROSAT sample size is small, with only eight quasars beyond a redshift of 3. Significant numbers of quasars with z 1 4 are being amassed to investigate both their intrinsic properties and the evolutionar y
1 Har vard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; jds@head-cfa.har vard.edu. 2 Astronomy Department, University of Virginia, P.O. Box 3818, Charlottesville, VA 22903-0818. 3 Visiting Astronomer, Cerro Tololo Inter-American Obser vator y, National Optical Astronomy Obser vator y, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under cooperative agreement with the National Science Foundation. 4 Cerro Tololo Inter-American Obser vator y, National Optical Astronomy Obser vator y, Casilla 603, La Serena, Chile. 5 Departamento de Astronomia y Astrofisica, P. Universidad Catolica, Cas´ ´ ´ illa 306, Santiago, Chile. 6 Visiting Astronomer, ESO New Technology Telescope. 7 Steward Obser vator y, University of Arizona, 933 North Cherr y Avenue, Tucson, AZ 85721. 8 National Optical Astronomy Obser vator y, P.O. Box 26732, Tucson, AZ 85726-6732. 9 Astronomy Department, Ohio State University, 140 West 18th Avenue, Columbus, OH 43210.

behavior of the quasar population. The Sloan Digital Sky Sur vey (SDSS) reports 123 optically selected quasars with z 1 4 (Schneider et al. 2002; Anderson et al. 2001). However, optical sur veys suffer from selection effects due to intrinsic obscuration and the inter vening Lya forest. Current X-ray sur veys with Chandra and XMM do not have a strong selection effect based on redshift and can detect emission up to 10 keV (obser ved frame) to reveal hidden populations of active galactic nuclei (AGNs) including heavily obscured quasars (Norman et al. 2001; Stern et al. 2002). High-z objects can be detected through a larger intrinsic absorbing column of gas and dust because the obser ved-frame Xray bandpass corresponds to higher energy, more penetrating Xrays at the source.10 Therefore, optical and X-ray sur veys will complement each other, providing a fair census of mass accretion onto black holes at high redshift. Larger samples of X-ray obser vations of z 1 4 quasars are needed since there are currently only 24 (Vignali et al. 2001), of which only three are X-ray­selected quasars. Chandra and XMM-Newton are beginning to probe faint flux levels for the first time to detect the high-z quasar population. Initial Chandra and XMM-Newton obser vations of optically selected quasars have shown a systematically lower X-ray flux relative to the optical at high redshift (Vignali et al. 2001; Brandt et al. 2001a). In this Letter, we present the X-ray and optical properties of a newly discovered, X-ray­selected z p 4.93 quasar with the Chandra X-Ray Observatory. This quasar is the highest redshift object published11 from an X-ray sur vey. These results are a component of the Chandra Multiwavelength Project (ChaMP; Wilkes et al. 2001). A primar y aim of the ChaMP is to measure the intrinsic luminosity function of quasars and lower luminosity AGNs out to z 5. The sur vey will provide a medium-depth, wide-area sample of serendipitous X-ray sources from archival Chandra fields in Cycles 1 and 2 covering 14 deg2. The broadband sensitivity between 0.3 and 8.0 keV enables the selection to be far less affected by absorption than previous optical, UV, or soft X-ray sur veys. Chandra's small point-spread function (1 resolution on-axis) and low background allow sources to be detected to fainter
10 eff The obser ved-frame, effective absorbing column is NH NH/(1 z)2.6 (Wilman & Fabian 1999). 11 A z 5.2, X-ray­selected quasar detected in the Chandra Deep Field­ North was presented at the 199th AAS meeting (Brandt et al. 2001c).

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Fig. 1.--Optical (i ) and X-ray (0.3­2.5 keV) imaging. To improve the visual clarity, in this figure we have smoothed the Chandra image with a Gaussian function (j p 1 .5). The spatial distribution of the 17 X-ray counts at 9 . 1 off-axis is as expected from a point source. The circles show the region containing 50% of the encircled energy (radius p 7 .3) of the Chandra counts. The plus signs mark the centroid of the X-ray emission in both images.

flux levels, while the 1 X-ray astrometr y greatly facilitates unambiguous optical identification of X-ray counterparts. The project will effectively bridge the gap between flux limits achieved with the Chandra Deep Field obser vations and those of past ROSAT sur veys. Throughout this Letter, we assume H0 p 50 km s 1 Mpc 1 and a flat cosmology with q0 p 0.5.
2. OBSERVATIONS AND DATA ANALYSIS

2.1. X-Ray The X-ray source CXOMP J213945.0 234655 (sequence 800104) was obser ved on 1999 November 18 by Chandra (Weisskopf et al. 2000) with the Advanced CCD Imaging Spectrometer (ACIS-I; Nousek et al. 1998) in the field of the Xray cluster MS 2137.3 2353 (PI: M. Wise). We have used data reprocessed (in 2001 April) at the Chandra X-ray Center
TABLE 1 Properties of CXOMP J213945.0 234655 Parameter aJ2000.0 ................................ dJ2000.0a ................................ z ...................................... g ..................................... r ..................................... i ..................................... X-ray countsb ........................ fX (#10 15 ergs s 1 cm 2)b, c ........ LX (#1044 ergs s 1)c, d ............... Hardness ratioe ....................... aox .................................... AB1450(1 z)f ...........................
a b a h

(CXC).12 We then ran a detection algorithm, XPIPE (D.-W. Kim et al. 2002, in preparation), which was specifically designed for the ChaMP to produce a uniform and high-quality source catalog. CXOMP J213945.0 234655 is one of 72 sources detected using CIAO/WAVDETECT (Freeman et al. 2002) within the ACIS configuration (Fig. 1). The 41 ks obser vation yielded a net 16.7 7.5 counts within the soft bandpass (0.3­2.5 keV) and no counts in the hard bandpass (2.5­8.0 keV). This corresponds to a Galactic absorption­corrected, obser ved frame Xray flux of f (0.3­ 2.5 keV) p (2.82 1.26) # 10 15 ergs cm 2 s 1 (Table 1). The source-naming convention of the ChaMP (CXOMP Jhhmmss.s ddmmss) is given with a prefix CXOMP (Chandra X-ray Observatory Multiwavelength Project) and affixed with the truncated J2000.0 position of the X-ray source after a mean field offset correction is applied, derived from the crosscorrelation of optical and X-ray sources in each field. 2.2. Optical Imaging and Source Matching We obtained optical imaging of the field in three NOAO/ Cerro Tololo Inter-American Obser vator y (CTIO) SDSS filters (g , r , and i ; Fukugita et al. 1996) with the CTIO 4 m/MOSAIC on 2000 October 29 as part of the ChaMP optical identification program (P. J. Green et al. 2002, in preparation). Integration time in each band ranged from 12 to 15 minutes during seeing of 1 . 3­1 . 8 FWHM. Image reduction was performed with the IRAF(v2.11)/MSCRED package. We used SExtractor (Bertin & Arnouts 1996) to detect sources and measure (pixel) positions and magnitudes. Landolt standard stars were transformed to the SDSS photometric system (Fukugita et al. 1996) and used to calibrate the photometric solution. Following the convention of the early data release of the SDSS quasar catalog (Schneider et al. 2002), we present the optical photometr y here as g, r , and i since the SDSS photometr y system is not yet finalized and the CTIO filters are not a perfect match to the SDSS filters. The limiting magnitudes for a point source are given as the mean of 3 j detections: g p 26.18, r p 25.54, i p 25.11.
12 CXCDS version R4CU5UPD14.1, along with ACIS calibration data from the Chandra CALDB 2.0b.

Value 21 39 44.99 23 46 56 .6 4.930 0.004 126.2 22.87 0.07 21.36 0.10 16.7 7.5 2.82 1.26 5.89 2.63 ! 0.54 1.52 0.08 0.05 21.62
m s

Error !0 .5. Obser ved frame; 0.3­2.5 keV. c Based on an assumed X-ray power-law spectrum (SE a E ; a p 1.0); Galactic absorption­corrected (NH p 3.55 # 1020 cm 2; Dickey & Lockman 1990). d Rest frame; 0.3­2.5 keV. e (H S)/(H S). Soft band (S): 0.3­2.5 keV; hard band (H): 2.5­8.0 keV. f Obser ved monochromatic, Galactic absorption­corrected, ° AB1450(1 z) magnitude (Fukugita et al. 1996) emitted at 1450 A in the quasar's rest frame; based on an assumed optical powerlaw spectrum (Sn n a; a p 0.5).


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As evident from Figure 1, there are three optical sources detected down to a limiting i magnitude of 25.1 within the 50% encircled energy radius of the X-ray centroid. The two primar y candidates, based on optical brightness, have offsets between the optical and X-ray positions of 1 . 87 and 4 . 94. To determine whether either of these sources are the likely counterpart to the X-ray detection, we have determined errors associated with the X-ray astrometric solution. D.-W. Kim et al. (2002, in preparation) have carried out extensive simulations of point sources generated using the SAOSAC ray-trace program13 and detected using CIAO/ WAVDETECT. For weak sources of 20 counts between 8 and 10 , off-axis from the aim point, the reported X-ray centroid position is correct within 1 . 8, corresponding to a 1 j confidence contour. Therefore, the nearby optical source (Dr p 1 .9 ) is the likely counterpart to the X-ray detection. The (J2000.0) position of the optical counterpart as measured from the r image refs erenced to the Guide Star Catalog II14 is a p 21h 39 m 44.99, d p 23 46 56 .6. 2.3. Optical Spectroscopy We obtained a low-resolution optical spectrum of CXOMP J213945.0 234655 (Fig. 2) with the CTIO 4 m/HYDRA multifiber spectrograph on 2001 October 15. Spectra of 17 of 22 optical counterparts to X-ray sources with a magnitude r ! 21 were acquired in a 3 hr integration within the Chandra field. The spectrograph has 2 diameter fibers and was configured ° with a 527 line mm 1 grating that provided 2800 A of spectral ° coverage with a resolution of 4 A. The sky background was measured using 81 fibers not assigned to the Chandra X-ray detections within the 1 field spectrograph. We processed the data using the IRAF(v2.11)/HYDRA reduction package. An additional spectrum of the high-redshift quasar (Fig. 2) and the optically brighter source 4 . 9 west of the Chandra X-ray position were obtained on the following evening with the ESO New Technology Telescope (NTT) 3.5 m to verify the intriguing Hydra spectrum and obtain greater wavelength coverage. A 300 line mm 1 grating was implemented with a wavelength coverage ° ° of 4000 A and a resolution of 11 A. Because of poor weather conditions at the end of the evening, flux calibration was done using the standard star LTT 2415 obser ved the following night. From the NTT spectrum, we classify the brighter object as an M3 dwarf with no sign of emission lines, confirming the quasar as the optical counterpart of the X-ray source. We measured a mean redshift z p 4.930 0.00 4 from the Lyb O vi, C ii, Si iv O iv], and C iv emission lines in the NTT spectrum. Using this redshift, the Lya line centroid is ° shifted by 4 A redward from the expected rest wavelength, probably as a result of significant H i absorption. This is similar to the mean shift of Lya obser ved in a sample of 33 highredshift quasars by Schneider, Schmidt, & Gunn (1991). The spectrum obtained at the NTT was used to measure ° the rest-frame equivalent widths of Lyb/O vi (30 7 A), ° ° Lya N v (73 5 A), and C iv (40 8 A). For comparison, we also measured Lya N v for 10 high-redshift quasars in the range 4.8 ! z ! 5.1 from the SDSS spectra of Anderson et al. (2001). This subsample has a similar mean redshift (4.91), but with an average i p 19.7 is 4.5 times more optically luminous than CXOMP J213945.0 234655. Nevertheless, the mean restSee http://hea-www.har vard.edu/MST. The Guide Star Catalog II is a joint project of the Space Telescope Science Institute and the Osser vatorio Astronomico di Torino.
14 13

Fig. 2.--Optical spectroscopy of CXOMP J213945.0 234655. The top spectrum is the discover y obser vation taken with the CTIO 4 m/HYDRA on 2001 October 15. The spectrum has been binned to produce a resolution of ° 16.4 A. The bottom panel is a follow-up obser vation with the NTT on the ° next evening to detect spectral features redward of Lya (11 A resolution). Dashed lines indicate the expected positions of emission lines at a redshift of 4.93. Shaded regions mark the uncorrected telluric O2 absorption band regions.

frame equivalent width of Lya N v in the SDSS subsample is ° ° consistent at 79 A, with an rms dispersion of 27 A. The poor signal-to-noise ratio of the SDSS spectra and the strong Lya forest prevent meaningful comparison of other line strengths.
3. RESULTS

To compare the broadband spectral energy distribution of CXOMP J213945.0 234655 to other X-ray­detected quasars, we have calculated aox (Tananbaum et al. 1979), the slope of a hypothetical power law between the X-ray and optical flux. The rest-frame, monochromatic luminosity at 2 keV corresponding to the derived X-ray flux is log l 2 keV p 26.76 ergs s 1 Hz 1. Assuming a p 0.5 for the optical continuum power-law slope, we derive the rest-frame, monochromatic optical luminosity at ° 2500 A from the i magnitude to be log l2500 A p 30.73 ergs s 1 ° 1 0.0 Hz . We thus find a ox p 1.52 0.08. Table 1 lists the measured 5 X-ray and optical properties of CXOMP J213945.0 234655. We compare the X-ray­to­optical flux ratio of CXOMP J213945.0 234655 to other z 1 4 quasars by plotting the obser ved-frame, Galactic absorption­corrected 0.5­2.0 keV Xray flux versus the AB1450(1 z) magnitude (Fig. 3). The plotted lines represent the locus of points for a hypothetical quasar with a wide range of luminosities and an a ox p 1.6 0.15 (Green et al. 1995), representative of the mean for quasars selected from the Large Bright Quasar Sur vey and detected in the ROSAT All-Sky Sur vey. The aox of CXOMP J213945.0 234655 is comparable with low-redshift quasars in contrast to the X-ray faint Chandra detections of optically selected quasars at z 1 4 (Vignali et al. 2001). The X-ray weakness of the latter may be due to intrinsic absorption by large amounts of gas in the quasars' host galaxies. X-ray and optical obser vations of CXOMP J213945.0 234655 show no direct evidence of significant obscuration. The optical color (r i p 1.51 0.12) is consistent with optically selected quasars. We measured the mean color r i from 15 SDSS quasars (Anderson et al. 2001) with 4.7 ! z ! 5.2 to be 1.69 with rms dispersion of 0.30. The upper limit to the X-ray hardness ratio (! 0.54) hints at an unobscured X-ray spectrum,


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Mucket 1998), more high-redshift X-ray­selected quasars are ¨ needed to test possible biases caused by absorption. CXOMP J213945.0 234655 exemplifies the potential for the ChaMP project to detect quasars with fluxes at the faint end of the fX-fopt parameter space (Fig. 3). This will allow the ChaMP to acquire significant numbers of high-redshift quasars. From the first year of spectroscopic follow-up of Chandra Xray sources to i 21, we currently have 22 newly identified quasars with z 1 2 and eight with z 1 3, approximately two to three such objects per field. Nearly 5% of ChaMP sources identified to date are z 1 3 quasars.
4. CONCLUSION

Fig. 3.--X-ray­to­optical flux correlation for z 1 4 AGNs (adapted from Vignali et al. 2001). The primar y symbols represent the X-ray obser vator y used. Squares mark X-ray­selected AGNs. The faintest source shown is a radio-selected Seyfert galaxy at z p 4.424 (Brandt et al. 2001b). The dashed lines display the relation for AGNs with aox p 1.6 0.15 at z p 4.9 (Green et al. 1995).

although a moderately absorbed component, if present, would be redshifted out of the Chandra bandpass. X-ray­selected samples may be less biased against absorbers (both intrinsic and line of sight) than are optically selected samples, an advantage expected to be especially important at high redshifts. From our flux-calibrated NTT spectrum, we measure DA p ( fcont fobs )/fcont, the flux decrement caused by the Lya forest (Oke & Kor ycansky 1982) relative to an extrapolated power-law continuum15 in the region between rest-frame limits ° 1050 1170 A. The value we measure of DA p 0.79 0.02 is ¯ between the z 4 measurement of 0.54 from Rauch et al. (1997) and the z 6 measurements of DA 0.9 from four SDSS quasars in Becker et al. (2001). While CXOMP J213945.0 234655 thus appears consistent with the handful of bracketing measurements of optically selected quasars (see also Riediger, Petitjean, &
15 As in Fan et al. (2001), we measure the obser ved flux fobs relative to a fl l 1.5 power-law continuum normalized to the obser ved flux in the region ° 1270 10 A in the rest frame. We derive uncertainties by measuring against continua with slopes in the range 0.5 ! a 1.5.

We present the discover y of CXOMP J213945.0 234655, at z p 4.93 the most distant X-ray­selected object published to date. With a measured optical­to­X-ray flux ratio a ox p 1.52, CXOMP J213945.0 234655 is similar to low-redshift quasars, in contrast to several optically selected z 1 4 quasars previously detected by Chandra. This detection highlights the importance of wide-area, intermediate-depth sur veys like the ChaMP for studies of the high-redshift quasar population (z 3­ 5). The ChaMP16 has begun to amass a sample of high-redshift, X-ray­selected quasars with the goal of measuring the cosmic evolution of accretion-powered sources relatively unhampered by the absorption and reddening that affects optical sur veys. We gratefully acknowledge support for this Chandra archival research from NASA grant AR1-2003X. R. A. C., A. D., P. J. G., D.-W. K., D. M., and B. J. W. also acknowledge support through NASA contract NAS8-39073 (CXC). L. I. is grateful to "Proyecto Puente PUC" and the Center for Astrophysics FONDAP for partial financial support. B. T. J. acknowledges research support from the National Science Foundation, through their cooperative agreement with AURA, Inc., for the operation of the NOAO. We are thankful to Sam Barden and Tom Ingerson (NOAO) for building and commissioning Hydra/ CTIO. We greatly appreciate the obser ving support from Knut Olsen (NOAO) and constructive comments by Har vey Tananbaum and Dan Harris.
16

See http://hea-www.har vard.edu/CHAMP.

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