Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.2dfquasar.org/Papers/mnr7619_1397-1418.pdf Äàòà èçìåíåíèÿ: Thu Apr 29 13:19:22 2004 Äàòà èíäåêñèðîâàíèÿ: Mon Oct 1 19:35:26 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: galaxy cluster

Mon. Not. R. Astron. Soc. 349, 1397­1418 (2004)

doi:10.1111/j.1365-2966.2004.07619.x

The 2dF QSO Redshift Survey ­ XII. The spectroscopic catalogue and luminosity function
S. M. Croom,1 R. J. Smith,2 B. J. Boyle,1 T. Shanks,3 L. Miller,4 P. J. Outram3 and N. S. Loaring4,5
1 2 3 4 5

Anglo-Australian Observatory, PO Box 296, Epping, NSW 1710, Australia Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD Department of Physics, University of Durham, South Road, Durham DH1 3LE Department of Physics, Oxford University, 1 Keble Road, Oxford OX1 3RH Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey RH5 6NT

Accepted 2004 January 9. Received 2004 January 8; in original form 2003 May 23

ABSTRACT

We present the final catalogue of the 2dF QSO Redshift Survey (2QZ), based on AngloAustralian Telescope 2dF spectroscopic observations of 44 576 colour-selected (ub Jr ) objects with 18.25 < b J < 20.85 selected from automated plate measurement scans of UK Schmidt Telescope (UKST) photographic plates. The 2QZ comprises 23 338 quasi-stellar objects (QSOs), 12 292 galactic stars (including 2071 white dwarfs) and 4558 compact narrow emission-line galaxies. We obtained a reliable spectroscopic identification for 86 per cent of objects observed with 2dF. We also report on the 6dF QSO Redshift Survey (6QZ), based on UKST 6dF observations of 1564 brighter (16 < b J < 18.25) sources selected from the same photographic input catalogue. In total, we identified 322 QSOs spectroscopically in the 6QZ. The completed 2QZ is, by more than a factor of 50, the largest homogeneous QSO catalogue ever constructed at these faint limits (b J < 20.85) and high QSO surface densities (35 QSOs deg-2 ). As such, it represents an important resource in the study of the Universe at moderateto-high redshifts. As an example of the results possible with the 2QZ, we also present our most recent analysis of the optical QSO luminosity function and its cosmological evolution with redshift. For a flat, m = 0.3 and = 0.7, universe, we find that a double power law with luminosity evolution that is exponential in look-back time, , of the form L J (z ) e6.15 , b equivalent to an e-folding time of 2 Gyr, provides an acceptable fit to the redshift dependence of the QSO LF over the range 0.4 < z < 2.1 and MbJ < -22.5. Evolution described by a 2 quadratic in redshift is also an acceptable fit, with L J (z ) 101.39z -0.29z . b Key words: catalogues ­ surveys ­ white dwarfs ­ galaxies: active ­ quasars: general ­ galaxies: Seyfert.

1 INTR ODUCTION The new generation of quasi-stellar object (QSO) surveys provide us with an unparalleled data base with which to study the properties of the QSO population. In this paper we report on the completion of the 2-degree Field (2dF) QSO Redshift Survey (2QZ) and the associated 6-degree Field (6dF) QSO Redshift Survey (6QZ). These surveys provide almost 25 000 QSOs in a single homogeneous data base, covering almost five magnitudes (16 < b J < 20.85). When combined with QSOs from complementary spectroscopic observations carried out as part of the Sloan Digital Sky Survey (SDSS; e.g. York et al.

E-mail: scroom@aaoepp.aao.gov.au
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2000), the number of known QSOs has increased more than fivefold in the past 5 yr. Furthermore, the vast majority of known QSOs are now in a few homogeneous samples with well-defined selection criteria, rather than a highly heterogeneous assemblage of small surveys and serendipitous discoveries. The properties of the QSO population may now be determined with a new level of statistical precision. The 2QZ provides a valuable resource to study the large-scale structure of the Universe on the largest scales over a wide range of redshifts. It can also be used to directly probe the mass distribution via lensing studies. In addition, the 2QZ also contains significant new catalogues of other classes of astronomical objects, for example, white dwarfs (WDs; Vennes et al. 2002) and cataclysmic variables (CVs) (Marsh et al. 2002).

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2QZ sample) and the 6dF instrument at the UKST (the 6QZ sample). The 2dF instrument is a multifibre spectrograph which can obtain simultaneous spectra of up to 400 objects at once over a 2 diameter field of view, and is positioned at the prime focus of the AAT. Fibres are robotically positioned within the field of view and are fed to two identical spectrographs (200 fibres each). Two field plates, and a tumbling system allow one field to be observed while a second is being configured, reducing down-time between fields to a minimum. The spectrographs each contain a Tektronix 1024 â 1024 CCD with 24-µm pixels. Details of the 2dF instrument can be found in Lewis et al. (2002). The 6dF instrument is a multifibre spectroscopic instrument on the UKST with the capability to simultaneously observe up to 150 sources over a 6 diameter field of view. Fibres are positioned robotically on to a field plate, which is then installed at the focus of the UKST. Fibres are fed to a bench mounted spectrograph containing an EEV 1032 â 1027 CCD with 13-µm pixels. Further details can be found in Watson et al. (2000). Based on the need to restrict the magnitude range of objects observed by 2dF (to reduce scattered light) and the observed surface density of candidates, objects in the input catalogue with 18.25 < bJ 20.85 were observed with 2dF and those with 16.0 < b J 18.25 were observed with 6dF. Although the 6QZ sample is approximately 100 times smaller than the 2QZ, it is important in extending the coverage of the QSO OLF to almost a factor of 100 in luminosity based on a single homogeneous input catalogue. Full details of the 2dF spectroscopic observations and subsequent catalogue were provided by Croom et al. (2001b) at the time of the initial 10-k data release. We therefore only present a brief description in this paper, updating information where appropriate. 2dF spectroscopic observations began in January 1997 and were completed in April 2002. The spectral dispersion was 4.3 å pixel-1 , giving an instrumental resolution of 9å. The spectra covered the wavelength range 3700­7900 å. Each 2dF field in the survey was observed for 3300­3600 s, giving a median signal-to-noise ratio (S/N) of 5.0 per pixel in the B band (4000­4900 å), see Fig. 1(a). The data were taken in a wide variety of conditions. Under conditions of poor seeing (>2 arcsec) or transparency, exposure times were increased to compensate for the lower signal rates. In the event that conditions prevented any field reaching its pre-determined completeness level, it was scheduled for re-observation (see below). The 2QZ input catalogue was merged with that of the 2dF Galaxy Redshift Survey (2dFGRS; Colless et al. 2001) and a complex tiling algorithm was applied to the resultant joint catalogue in order to maximize the efficiency with which the two declinations strips could be covered with the minimum number of circular 2dF fields. The 2QZ survey area is an exact subset of the 2dFGRS area. Where possible, when conditions were unsuitable for the more exacting requirements of the 2QZ program (e.g. poor seeing, significant moonlight), observations of the 2dFGRS only fields were carried out. Six-degree Field observations were performed over the period 2001 March­2002 September. The spectral range covered was 3900­7600 å using a low-dispersion 250B grating which provided a dispersion of 286 å mm-1 (3.6 å pixel-1 ) and a spectral resolution of 11.3 å. Typical observation times were between 1.5 and 2 h per field. An average of 50 6QZ objects were observed in each 6dF pointing. A median S/N = 16 per pixel was obtained in the continuum of 6dF spectra (see Fig. 1b). This higher S/N value was set by our requirement to effectively identify the much larger numbers of contaminating stars in the 6QZ. A number of 6QZ targets where also observed in preliminary survey work carried out between 1996 September and 1999 December with the Fibre-Linked Array-Image
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The 2QZ sample presented in this paper has already provided new information on the power spectrum of QSOs (Outram et al. 2003), the QSO correlation function (Croom et al. 2001a) and QSO spectral properties (Croom et al. 2002; Corbett et al. 2003). It has also been used to search for rare/unusual objects (Londish et al. 2002), carry out lensing studies (Meyers et al. 2003; Miller et al. 2003) and place limits on cosmological parameters (Outram et al. 2001; Hoyle et al. 2002). We also provide an updated estimate of the QSO optical luminosity function (OLF) and its evolution with redshift based on the 2QZ sample in this paper. In this we include a new sample of QSOs obtained from observations obtained with the 6dF facility on the UK Schmidt Telescope (UKST). The 6dF sample is based on the same input catalogue as the 2QZ, but extends the coverage of the luminosity function (LF) at any given redshift to almost a factor of 100 in luminosity and over 1000 in space density. This will aid tests of the claimed departure from pure luminosity evolution at higher luminosities (Hewett, Foltz & Chaffee 1993; La Franca & Cristiani 1997). We describe the 2QZ and 6QZ surveys in Section 2 and then discuss their selection effects and completeness in Section 3. In Section 4 we present our analysis of the QSO LF. 2 D ATA 2.1 Input catalogue The selection of the QSO candidates for the 2QZ and 6QZ surveys was based on broadband ub J r colours from automated plate measurement (APM) of UKST photographic plates. Full details are given by Smith et al. (2004), and so only a brief overview will be given here. The survey area comprises 30 UKST fields, arranged in two 75 â 5 declination strips, one passing across the South Galactic Cap centred on = - 30 (the SGP strip) and the other across the North Galactic Cap centred on = 0 (referred to in this paper as the equatorial strip, but also know as the NGP strip). The SGP strip extends from = 21h 40 to = 3h 15 and the equatorial strip from = 9h 50 to = 14h 50 (B1950). Note that the survey was originally defined in the B1950 coordinate system, although the publically available catalogue is presented with J2000 positions. The total survey area is 721.6 deg2 , when allowance is made for regions of sky excised around bright stars. In each UKST field, measurements of one b J plate, one r plate and up to four u plates/films were used to generate a catalogue of stellar objects with 16 < b J < 20.85. A sophisticated procedure was devised to ensure catalogue homogeneity. Corrections were made for vignetting and field effects owing to variable desensitization in the UKST plates, these effects being particularly noticeable at the edges of plates (Croom 1997; Smith 1998; Smith et al. 2004). Candidates were selected for the 2QZ based on fulfilling at least one of the following colour criteria: u - b J -0.36; u - b J < 0.12 - 0.8(b J - r ); b J - r < 0.05. The u - b J limit was tightened to u - b J -0.50 for the 6QZ sample (b J 18.25) to reduce the high fraction of contamination by galactic stars. With these criteria, the 2QZ input catalogue comprised 47 768 candidates and the 6QZ comprised 1657 candidates (SGP strip only, see the following text). 2.2 Spectroscopic observations Spectroscopic observations of the input catalogue were made with the 2dF instrument at the Anglo-Australian Telescope (AAT; the

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Figure 1. The distribution of measured spectral S/N per pixel in the B band for the best observation of each object in the (a) 2QZ and (b) 6QZ catalogues. Note that a small number of sources have S/N < 0 owing to residual errors in sky subtraction of faint sources.

0% 2 1 0 -1 -2 10 0% -28 -29 -30 -31 -32 22

100%

Dec.

11 100%

12 RA

13

14

Dec.

23

0 RA

1

2

3

Figure 2. The coverage map for the 2QZ catalogue for the equatorial (top) and SGP (bottom) regions. The grey-scale indicates the percentage of 2QZ candidates observed over the two survey strips. Regions removed owing to bright stars or plate features/defects can be seen, as can the unobserved fields at the 14h end of the equatorial strip.

Reformatter (FLAIR) spectrograph (Watson & Parker 1994) on the UKST prior to the 6dF commissioning. These FLAIR spectra cover the range 3800­7300 å using the same 250B grating with 11 å resolution, but are generally lower S/N values than the 6dF data. All but a few (28) QSOs found with FLAIR have been re-observed with 6dF. FLAIR spectra are not available as part of the publically available data base. Observations were obtained for candidates in both declination strips. However, the 6dF observations of the equatorial strip candidates are quite incomplete (less than 60 per cent of the candidates were observed), and so only the SGP strip is included in the final catalogue. In total, 25 overlapping 6dF pointings were used to cover the SGP strip. 2.3 Catalogue composition 2.3.1 The 2QZ catalogue Two-degree Field data were reduced at the telescope using the 2dF data-reduction pipeline, 2DFDR (Bailey & Glazebrook 1999; Bailey et al. 2003) and information on the spectroscopic completeness achieved on each field was immediately fed back into the catalogue. 2dF fields which failed to achieve a pre-determined comC

pleteness level (either 70 per cent for the 2QZ or 90 per cent for the 2dFGRS) were scheduled for re-observation. In the end 556 out of 573 fields in the two 2QZ declination strips were observed. These included 10 fields that were kindly added to the equatorial strip when observations of 2QZ (and 2dFGRS) sources were made using spare fibres from spectroscopic follow-up of the Millennium Galaxy Catalogue (Lemon, Driver & Cross 2001). The coverage map for the 2dF survey is given in Fig. 2, showing the percentage of 2QZ targets that we obtained spectra for. We failed to obtain observations of a few 2dF fields in the most northerly declination band of the equatorial strip at right ascension values greater than 14h and a few fields at the very extreme edges/corners of both strips. In total, spectra were obtained for 44 576 objects (93.3 per cent of the input catalogue) in the 2QZ, corresponding to an effective area of 673.4 deg2 . A small number of objects in the catalogue were observed not by 2dF, but by other instruments and/or telescopes. These are also included in our final catalogue. A total of 111 equatorial sources with NRAO VLA Sky Survey (NVSS; Condon et al. 1998) radio detections and b J < 20 were observed with the low-resolution imaging spectrograph on the Keck II Telescope (Brotherton et al. 1998). In addition, 69 targets were observed with either the double-beam spectrograph (DBS) on the Siding Spring

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Observatory 2.3-m Telescope, or the Royal Greenwich Observatory (RGO) Spectrograph on the AAT, as part of the follow-up of close (<20 arcsec) QSO candidate pairs (Smith 1998). DBS spectra of 20 objects in the 6QZ were also obtained. Objects without spectra in the 2QZ lie either in the fields that were not observed by 2dF or in regions of high 2QZ/2dFGRS candidate surface density where the tiling algorithm was unable to configure a fibre observation efficiently given the minimum fibre-to-fibre spacing restriction (30 arcsec). However, we note that 2QZ targets were given higher priority in the tiling algorithm than 2dFGRS targets, in order not to imprint the stronger angular clustering pattern of the galaxies on to the QSO distribution. Due to the significant amount of overlap between 2dF fields and the repeat observations of low-completeness fields, 10 528 objects (23.6 per cent) in the 2QZ have more than one spectroscopic observation. Once reduced, 2QZ spectra were classified using the AUTOZ program (see Croom et al. 2001b) which uses a 2 -minimization technique to fit each spectrum to a number of QSOs [including broad absorption line (BAL) QSOs], galaxy and stellar templates and measure a redshift for all extragalactic identifications. The QSO template was based on the composite spectrum of Francis et al. (1991). AUTOZ produces a single identification based on the best-fitting template in one of six categories based on the following spectral criteria: QSO : Broad (>1000 km s-1 ) emission lines. NELG : Narrow (< 1000 km s-1 ) emission lines only. gal : Redshifted galaxy absorption features. star : Stellar absorption features at rest. cont : No emission or absorption features (high S/N). ?? : No emission or absorption features (low S/N). All AUTOZ identifications were independently checked visually by at least two members of the 2QZ team to correct any identifications that were clearly in error. Approximately 5 per cent of the AUTOZ identifications were changed in this fashion. As part of the classification process a quality flag was attached to each identification and redshift measurement as follows: Quality 1 : High - quality identification or redshift. Quality 2 : Poor - quality identification or redshift. Quality 3 : No identification or redshift assignment. The quality flag was determined independently for the identification and redshift of an object. For example, a quality 1 QSO identification could have a quality 1 or 2 redshift. Table 1 gives the breakdown between the numbers identified in each class in the 2QZ. Fig. 3 shows the number­redshift relationship, n(z), for the QSOs and narrow emission line galaxies (NELGs)
Figure 3. Number­redshift histograms for the 2QZ (upper panel) and 6QZ (lower panel) showing both QSOs (solid lines) and NELGS (dashed lines). The unshaded solid line includes all quality 1 and 2 QSOs, while the shaded histograms denote quality 2 QSO identifications only. Bin widths are dz = 0.025 and dz = 0.25 for the 2QZ and 6QZ, respectively.

in the full 2QZ. The n(z) histograms for both classes of objects are relatively smooth, indicating that there are no strong biases towards the selection of QSOs/NELGs over specific redshift intervals. The decline in QSO numbers at z < 0.4 and z > 2.2 are largely due to incompleteness in the colour and morphological selection process (see Section 3). We show the number counts, n (b J ), for each identification class (omitting the small numbers of `cont' and `gal' identifications for clarity) in Fig. 4. The surface densities are based on the observed counts divided by effective area of the full survey, with no further correction for incompleteness. At magnitudes fainter than b J 18.5 mag the QSOs are the largest population, while at brighter magnitudes, galactic stars dominate. The `cont' identification relates to potential BL Lac sources, however this class of object could be contaminated by other weak-lined objects (e.g. DC WDs). To indicate the uncertainty inherent in this classification, in our final catalogue all `cont' IDs have quality 2. A

Table 1. 2QZ/6QZ catalogue composition as a function of minimum sector completeness level (see text). The 70, 80 and 90 per cent columns give the properties of survey subsamples which have those spectroscopic completenesses, respectively. We list the number of objects in each of the different quality classes (Q1, Q2 and Q3). The final row gives the fraction (compared to the total number of observed sources) of IDs in each of the specified subsamples and quality classes. 2QZ Class. QSO NELG gal star cont ?? Total ID Frac Q1 All Q2 Q3 70 per cent Q1 Q2 Q3 80 per cent Q1 Q2 Q3 90 per cent Q1 Q2 Q3 Q1 6QZ All Q2

Q3

22655 683 ­ 20905 480 ­ 18068 312 ­ 11046 86 ­ 317 5 ­ 4484 74 ­ 4086 50 ­ 3458 31 ­ 2005 10 ­ 50 0 ­ 79 16 ­ 73 11 ­ 59 6 ­ 34 1 ­ 1 0 ­ 10904 1388 ­ 9965 1007 ­ 8466 676 ­ 4996 168 ­ 1148 4 ­ 102 52 ­ 84 41 ­ 68 28 ­ 24 12 ­ 3 1 ­ ­ ­ 4139 ­ ­ 2749 ­ ­ 1669 ­ ­ 405 ­ ­ 35 38224 2213 4139 35113 1589 2749 30119 1053 1669 18105 277 405 1519 10 35 0.858 0.050 0.093 0.890 0.040 0.070 0.917 0.032 0.051 0.964 0.015 0.022 0.971 0.006 0.022 2004 RAS, MNRAS 349, 1397­1418

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Figure 4. Number­magnitude counts for QSO (filled circle), NELG (filled triangle), star (open triangle) and ?? (open circle) classifications in the 2QZ and 6QZ. Poisson error bars are shown.

more detailed investigation of possible BL Lacs in the survey has been carried out by Londish et al. (2002). Of the 44 576 objects with spectroscopic observations in the full 2QZ, 38 224 received quality 1 IDs (85.8 per cent) 2213 quality 2 IDs (5.0 per cent) and 4139 quality 3 IDs (9.3 per cent). We obtained a quantitative assessment of the quality flags from the objects with repeat observations. Based on the 5073 objects with quality 1 identifications and redshifts in two observations; 186 (3.7 per cent) had a different identification and different redshift. A further 38 (0.75 per cent) objects had a different identification, but the same redshift (to within z = 0.015); arising as a result of a classification change from NELG to QSO or vice versa. A total of 2697 objects with quality 1 IDs were classified as QSOs in both observations, of these 2589 (96.0 per cent) had the same redshifts. Amongst the 615 objects classified as NELGs in both observations, 609 (99.0 per cent) were assigned the same redshift. Of the 1026 objects with a quality 1 identification and redshift, but also a quality 2 identification, 334 (32.6 per cent) had a different identification and redshift. A total of 21 (2.0 per cent) had a different identification but the same redshift. Of the 265 (14) objects that were classified as QSOs (NELGs) in both observations 185 (12) had the same redshift, or 70 (86) per cent of the respective populations. Overall, we therefore conclude that the quality 1 identifications and redshifts are more than 95 per cent reliable, while the quality 2 identifications and redshifts are only reliable at the 70 per cent level. We also obtained an estimate of the likely composition of the quality 3 object catalogue by looking at the 2735 quality 3 objects that were subsequently re-observed and given a quality 1 identification and redshift. Of these objects, 1611 (58.9 per cent) were classified as QSOs, 902 (33.0 per cent) as stars, 212 (7.8 per cent) as NELGs and 10 (0.4 per cent) as gals. This is close to the distribution of classifications in the full 2QZ (excluding ?? objects, see
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Table 1), with a slightly higher fraction (4 per cent) of stars and a lower fraction of NELGs amongst the unidentified objects. QSOs remain at an approximately constant 59 per cent of sources amongst both the identified and unidentified populations. We can also increase the mean completeness of the 2QZ catalogue by limiting the sectors formed from the overlap of individual 2dF observations included in the catalogue to only those which meet a specified spectroscopic completeness threshold (see Section 3). Table 1 lists the composition of the catalogue based on a number of spectroscopic completeness thresholds set at 70, 80 and 90 per cent (based on the percentage of quality 1 identifications in a given sector). By spectroscopic completeness we mean the fraction of spectroscopically observed sources for which we obtained a quality 1 identification. In most analysis that we have carried out with the 2QZ, we have used the 70 per cent sector spectroscopic completeness threshold to define the sample. Furthermore, we have only used objects with a quality 1 identification. These criteria were chosen to give the best compromise between maximizing the sample size (over 20 000 QSOs) and minimizing spectroscopic incompleteness (11 per cent) in the 2QZ. The repeat observations also provide a useful method to determine redshift accuracy. The rms pairwise dispersion between redshift measurements for the 2589 QSOs with quality 1 redshifts and z < 0.015 (excluding the objects with an incorrect redshift owing to line misidentification) is ( z ) = 0.0038; giving a mean redshift error of (z ) = 0.0027 for an individual measurement. We note, however, that the dispersion increases as a function of redshift, from (z , z < 1) = 0.0022 to (z , z > 2) = 0.0044. A more z-independent estimate of the redshift error is given by the fractional error in the redshift (z )/z = 0.0027, which is actually identical to the mean of (z ). A plot of the redshifts for the repeat observations with quality 1 identifications and redshifts is shown in Fig. 5. From this we can also see that the most common mis-identification was between the C IV 1549 and Mg II 2798 lines. For NELGs, the rms dispersion in the redshifts ( z ) = 0.0005, corresponding to a mean error in an individual redshift measurement error of (z ) = 0.00035.

2.3.2 The 6QZ catalogue An identical procedure was followed to reduce the 6dF data and carry out the spectroscopic identification of the 6QZ. In total, 6dF spectra were obtained for 1564 of the 1657 colour-selected candidates (94.4 per cent) in the SGP strip, giving an effective area of 333.0 deg2 . The composition of the 6QZ catalogue is given in Table 1. The 6QZ contains a much higher fraction of galactic stars (73.7 per cent of observed sources) and a correspondingly lower fraction of QSOs (20.6 per cent) given its brighter magnitude limit. The number­ redshift relationship for the QSOs and NELGs in the 6QZ is shown in Fig. 3. The n (b J ) relationship is shown in Fig. 4. We note that the QSO n (b J ) relationship fits smoothly on to that obtained for the 2QZ at b J = 18.25, while the stellar n (b J ) relationship obtained for the 6QZ lies approximately a factor of 2 lower than expected from an extrapolation of the 2QZ stellar counts. This reflects the more restrictive colour cut, (u - b J ) -0.50, used to select the 6QZ candidate list. At this cut, the vast majority of the stars identified have been scattered into this sample by photometric errors. This explains the increase in the stellar surface density at the very brightest magnitudes (b J < 17mag), where the photometric errors on the photographic magnitudes increase.

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Figure 5. Redshift comparison for extragalactic emission-line objects in the 2QZ. (a) z 1 versus z 2 for QSOs. The dashed lines indicate the redshift differences caused by the confusion of specific QSO emission lines. (b) Relative difference versus mean z for QSOs. (c) z 1 versus z 2 for NELGs. (d) Relative difference versus mean z for NELGs. The quantization seen in (b) and (d) at low redshift is due to the redshifts being determined to a precision of 4 decimal places (30 km s-1 ). In all cases the determined redshift errors are considerably larger than this.

The 6QZ also has a much higher spectroscopic completeness level (over 97 per cent for quality 1 identifications) than the 2QZ and so we choose to make no further cuts on the basis of sector completeness as we did for the 2QZ. The higher completeness levels are directly attributable to the much higher mean S/N value obtained for the 6dF observations. Repeat observations were obtained for 475 objects in the 6QZ. Based on a similar comparison of the quality 1 identifications and redshifts to that carried out for the 2QZ sample, we conclude that the quality 1 identifications in the 6QZ are 99 per cent accurate (only 2 out of 254 repeat quality 1 identifications changed between observations) and the quality 1 redshifts are over 96 per cent accurate (only 6 out of 177 QSO redshift measurements differed by more than z = 0.015, and of these, only one QSO had z > 0.03). We obtain a redshift error of (z ) = 0.0026 for the 6QZ QSOs, identical to that obtained for the 2QZ, based on a comparison of the redshifts obtained for the 171 QSOs in the 6QZ with repeated quality 1 observations with z < 0.015.

2.4 Data products The 2QZ and 6QZ catalogues are available as ASCII files from the 2QZ World Wide Web (WWW) site http://www.2dfquasar.org, and on a CD-ROM release. The catalogue format is identical for both

surveys and is given in Table 2. Note that the catalogue format has been extended to include more information since the preliminary data release (Croom et al. 2001b). The first part of a catalogue entry contains details from the input catalogue, such as position and magnitude. We note that the object names may in some cases not correspond exactly to the source position in and as we have improved the astrometry of the catalogue (Smith et al. 2004) since the object names were defined. The names have not been changed subsequently, to avoid confusion with previously published lists. We now include internal catalogue names and numbers, although objects should generally be referenced by their main IAU format name. A new entry is included for the name of the sector inhabited by the object. A sector is defined as the intersection of overlapping 2dF fields (see Section 3). The format of these is, for example, S 200 201 247, where the S denotes the SGP strip and the numbers indicate that the sector is formed by the overlap of fields 200, 201 and 247. There are no sectors defined for the 6QZ. Coordinates are also supplied in the B1950 system, as the survey was constructed in this system and the production of completeness masks etc. is more straight forward in this system. We also note that sources which had only upper limits (i.e. non-detections) on the r plates are also included in the catalogue and have a listed b J - r colour. In this case the colour term is (b J - r lim ) - 10, the -10 being used to differentiate upper limits from normal colours (objects with real rband detections have colours in the range -1.4 < b J - r < 3.4, while upper limits have b J - r < -9.8).
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Table 2. Format for the 2QZ catalogue. The format entries are based on the standard Field Name RA Dec. Catalogue number Catalogue name Sector RA Dec. UKST field XAPM YAPM RA Dec. bJ u - bJ bJ - r N obs Observation # 1 z1 q1 ID 1 date 1 fld 1 fibre 1 S/N 1 Observation # 2 z2 q2 ID 2 date 2 fld 2 fibre 2 S/N 2 z prev radio x-ray dust comment 1 comment 2 Format a20 i2 i2 f5.2 a1i2 i2 f4.1 i5 a10 a25 i2 i2 f5.2 a1i2 i2 f4.1 i3 f9.2 f9.2 f11.8 f11.8 f6.3 f7.3 f7.3 i1 f6.4 i2 a10 a8 i4 i3 f6.2 f6.4 i2 a10 a8 i4 i3 f6.2 f5.3 f6.1 f7.4 f7.5 a20 a20
FORTRAN

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format descriptors.

Description IAU format object name RA J2000 (hh mm ss.ss) Dec. J2000 (±dd mm ss.s) Internal catalogue object number Internal catalogue object name Name of the sector this object inhabits RA B1950 (hh mm ss.ss) Dec. B1950 (±dd mm ss.s) UKST survey field number APM scan X position (8 - µm pixels) APM scan Y position (8 - µm pixels) RA B1950 (radians) Dec. B1950 (radians) b J magnitude u - b J colour b J - r colour [including r upper limits as: (b J - r lim ) - 10.0] Number of observations Redshift Identification quality â 10 + redshift quality Identification Observation date 2dF field number â 10 + spectrograph number 2dF fibre number (in spectrograph) S/N in 4000­5000 å band Redshift Identification quality â 10 + redshift quality Identification Observation date 2dF field number â 10 + spectrograph number 2dF fibre number (in spectrograph) S/N in the 4000­4900 å band Previously known redshift (Veron-Cetty & Veron 2000) ´ ´ 1.4-GHz Radio flux, mJy (NVSS) X-ray flux, â10-13 erg s-1 cm-2 (RASS) E ( B - V ) (Schlegel et al. 1998) Specific comments on observation 1 Specific comments on observation 2

The input catalogue information is followed by detai