Second Data Release of the 6Df Galaxy Survey

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arXiv:astro-ph/0505068v1 4 May 2005 htmti nu aaousaesacal through database associated searchable online and are an data catalogues input redshift 2004 photometric March Released Releases and 2002 Data December respectively. First in place and taken Early having with intervals, yearly Funda- velocities. yield peculiar to and 2000) distances al. Plane mental et near (Jarrett (2MASS) Survey imaging All-Sky infrared Micron veloc- Two param- 6dF from brightest photometric eters their with the combining measurements dispersion and taking ity galaxies is E/S0 survey 000 15 velocity over. peculiar sky the The allocations fibre of fill-out surveys, near-infrared number radio a optical, and from these, defined to subsamples addition smaller In 16.75). 15.60, 13.75, hc r isdtwrsyugr trfriggalax- star-forming samples, younger, majority optically-selected towards the biased to in are Compared which mass spectral stellar stars the galaxies. galaxy old of of the by most up of dominated make is peak that which the distribution, to redshift energy sensitive other selection Near-infrared from most apart is surveys. it velocity set peculiar 6dFGS and the of lection summarise and survey the attributes. of main its (DR2) Incremen- Release Second the Data for tal available 6dFGS making the are present we we data paper this In Edinburgh. vatory 5 0 esit cosaprn antd limited ( magnitude to apparent complete secure across samples to aims redshifts survey regions redshift 000 150 The obscured sky. heavily southern most the en- of the survey but velocity all peculiar and compassing redshift combined a is h d aaySurvey Galaxy 6dF The Overview Survey 1 2 1 h dG aaaemd ulca approximately at public made are data 6dFGS The h iesycvrg n erifae agtse- target near-infrared and coverage sky wide The niedtbs:http://www-wfau.roe.ac.uk/6dFGS/ database: Online http://www.aao.gov.au/local/www/6df/ home: 6dFGS te rmglxe vrtesuhr k at sky southern the over galaxies from ities ulci eebr20 n ac 04 oa f8 1 sourc 014 83 of Keywords: total A on available 2004. describ http://www-wfau.roe.ac.uk/6dFGS/. photometry we March at s optical Here the and and of near-infrared 2002 magnitude. update shifts, December of an provide order in and an public (DR2) than Release more Data by Incremental kind its of Abstract: eodDt ees fte6FGlx Survey Galaxy 6dF the of Release Data Second D C B Australia 2611, ACT A .HahJones Heath D. nl-utainOsraoy ..Bx26 pig S 21 NSW Epping, 296, Box P.O. Observatory, Anglo-Australian nttt o srnm,RylOsraoy lcfr Hil Blackford Observatory, Royal Astronomy, for Institute eerhSho fAtooy&Atohsc,TeAustralia The Astrophysics, & Astronomy of School Research mi:[email protected] Email: h d aaySre smauigaon 5 0 esit a redshifts 000 150 around measuring is Survey Galaxy 6dF The aais ttsis uioiyfnto—omlg:la function—cosmology: luminosity statistics, galaxies: 2 anandb h oa Obser- Royal the by maintained ,H ,r J, H, K, 1 6FS oe ta.2004a) al. et Jones (6dFGS; A , D ilSaunders Will , F b , J 1.5 13.05, (12.75, = ) B | b ieRead Mike , | > 1 10 ◦ d aaySre,icuigteSxDge il in- Field Six-Degree the including Survey, Galaxy 6dF near-infrared- the sample. in selected are two-thirds objects 133 which 167 of are allocated, there lower by total for In reduced particularly programmes. be priority practice, will in which rates values fibre-breakage frac- theoretical coverage are These tions allocation. each field for that expected given coverage survey allocation maximum target the the and in process, weighting sample relative each the to is given shown (priority) Also the list. to target contributing master catalogues source of breakdown the velocity and mass galaxy both yields together. that survey a 6dF by as universe, addressed such readily local more the are questions of many knowledge our advanced greatly uvy aemda esit obeta fteme- the deeper of these that ¯ of double dian Both redshifts SDSS. median have as 2dFGRS surveys many as will as 6dFGS redshifts one-fifth the galaxy size, and many sample as of two-thirds terms twiceyield In and will SDSS. 2dFGRS Survey of of Galaxy area that the 6dF 2000). times the Sky 8 al. coverage, cover et Digital ultimately sky York Sloan of 2001; terms the al. In et and (2dF- Blanton 2001) Survey (SDSS; al. Redshift Survey et Galaxy Colless 2dF GRS; the to compared plane. Galactic the around o motn,adte6Fsre ed a cover a can from fields apart survey sky 6dF southern entire is the galaxies the and target of important, orientation not the the Galaxy. own consequence, to our a internal of plane As both the through dust, and to galaxies target due af- extinction minimally by is fected determinations. selection distance near-infrared tar- Plane Furthermore, best Fundamental favours the for 6dFGS are gets the that galaxies of bulge-dominated selection older, near-infrared the ies, hncmlt,i ilb h ags survey largest the be will it complete, When . ieadsacal hog an through searchable and line oe ta.(04)dsrb h anfct fthe of facets main the describe (2004a) al. et Jones summarises and (2004a) al. et Jones from is 1 Table iue1sosterdhf oeaeo h 6dFGS the of coverage redshift the shows 1 Figure z ,Eibrh H H,Uie Kingdom United 3HJ, EH9 Edinburgh, l, C ≈ ate Colless Matthew , 0 re.Ti olw ale aamade data earlier follows This urvey. ainlUiest,Wso Creek, Weston University, National n . 1 Australia 21, 5fr6FS hl hs uvy have surveys these While 6dFGS. for 05 h hrceitc fteSecond the of characteristics the e g-cl structure rge-scale snwhv hi pcr,red- spectra, their have now es d1 0 euirveloc- peculiar 000 15 nd B SQL | database b ≤ | 10 ◦ strip 2 Publications of the Astronomical Society of Australia

Table 1: The 6dFGS target samples used to define the tiling. There are also samples of 6843 SUMSS sources (Sadler, Sydney; id = 125), 466 Durham/UKST Galaxy Survey sources (Shanks, Durham; id = 78), and 724 Horologium Supercluster sources (Rose et al, North Carolina) not used in the tiling but included for serendipitous observation. A further 4096 orphaned sources were in the original target catalogues but are no longer, due changes in the 2MASS source catalogues. However, their redshift information has been retained under id = 999. Surveys with larger weights have higher priority in the allocation of fields. ‘Coverage’ is as expected from the tiling simulations; in practice fibre breakages and imperfect fibre assignment reduce these numbers, especially for lower priority samples. id Sample (Contact, Institution) Weight Total Coverage 1 2MASS Ks < 12.75(Jarrett,IPAC) 8 113988 94.1% 3 2MASS H < 13.05(Jarrett,IPAC) 6 3282 91.8% 4 2MASS J < 13.75(Jarrett,IPAC) 6 2008 92.7% 7 SuperCOSMOS rF < 15.6(Read,ROE) 6 9199 94.9% 8 SuperCOSMOS bJ < 16.75(Read,ROE) 6 9749 93.8% 90 Shapley (Proust, Paris-Meudon) 6 939 85.7% 113 ROSAT All-Sky Survey (Croom, AAO) 6 2913 91.7% 119 HIPASS (> 4σ) (Drinkwater, Queensland) 6 821 85.5% 126 IRAS FSC 6σ (Saunders, AAO) 6 10707 94.9% 5 DENIS J < 14.00(Mamon,IAP) 5 1505 93.2% 6 DENIS I < 14.85(Mamon,IAP) 5 2017 61.7% 116 2MASS AGN (Nelson, IPAC) 4 2132 91.7% 129 Hamburg-ESO Survey (Witowski,Potsdam) 4 3539 90.6% 130 NRAO-VLASkySurvey(Gregg,UCDavis) 4 4334 87.6%

Total 167133 93.3%

strument, target catalogue construction, and the allocation of fields and fibres to targets. The data reduction and redshifting procedures are also described, along with the main characteristics of the data comprising the First Data Release. The interested reader is referred to this paper for detailed information about the 6dFGS. Campbell, Saunders & Colless (2004) describe the algorithm used to optimise the placement of field centres with respect to the galaxy distribution on the sky. A series of AAO Newsletter arti- cles trace the history of the 6dF instrument (Watson 1998; Watson et al. 1999; Saunders et al. 2001), target selections (Saunders et al. 2003; Mauch 2002), online database (Read et al. 2003) and First Data Re- lease (Saunders et al. 2004). Various conference pro- ceedings give further information about the instrument (Parker, Watson & Miziarski 1998; Watson et al. 2000), survey aims (Wakamatsu et al. 2003; Colless et al. 2003) and First Data Release (Jones et al. 2005). All of these references can be found together on the 6dFGS Publi- Figure 1: Number-redshift distribution for the 3 6dF Galaxy Survey, comprised of First Data Re- cations Page . lease galaxies with redshift quality Q ≥ 3 and The 6dFGS K-band luminosity function (LF; Jones et al. cz > 600 kms−1. The mean redshift for the sur- 2005, Jones et al., in prep), shows excellent agree- vey (¯z = 0.054) is indicated with a vertical solid ment with earlier work using 2MASS with 2dFGRS (Cole et al. 2001), ZCAT (Kochanek et al. 2001) and line. Also shown are the equivalent distributions SDSS (Bell et al. 2003). Considering the precision of for the SDSS (short dashed line,blue) and 2dFGRS these modern LFs and the influence of such factors (dotted line, red). The 6dFGS and SDSS distribu- as sky coverage and nearby large-scale structures, the tions have been normalised to yield the final sam- level of agreement is remarkable. Other near-infrared ple size expected for each, since neither survey is yet to finish. 3Publications: http://www.aao.gov.au/local/www/6df/publications.html www.publish.csiro.au/journals/pasa 3

and optical luminosity functions from 6dFGS for bJrFJH (Jones et al.) also show good agreement. The 6dFGS is being utilised for a variety of other programmes, including: • Study of the AGN and QSO populations from 2MASS (Francis et al., ANU; Cutri et al., Cal- tech). • Radio-selected samples (Mauch and Sadler, Syd- ney; Mamon, IAP; Doyle and Drinkwater, Queens- land). • Galaxy ages and metallicities (Proctor and Forbes, Swinburne; Lah, ANU; Colless, AAO). • Southern sky peculiar velocity survey (Campbell and Colless, AAO; Lucey, Durham), • Galaxy groups, nearby superclusters and voids (Brough, Kilborn and Forbes, Swinburne; Waka- matsu, Gifu; Johnston-Hollitt et al., Tasmania). Figure 3: Galaxy redshift completeness by field, • Large-scale structures and clustering (Lahav and where completeness is the number of 6dF redshifts Rassat, UCL; Fairall et al., Cape Town). over the total 6dF redshifts and failures. The In Section 2 we summarise the scope and features dashed line indicates the cumulative fraction ac- of the Second Incremental Data Release for the 6dFGS. cording to the right-hand axis. Section 3 outlines caveats and cautions that users should bear in mind when using the data. Comments regard- ing the survey timeline from now until completion are given Section 4. δ < 0◦ equatorial band, save for regions around 11 – 13 hr and 03 – 06 hr right ascension. The former is a re- 2 Second Data Release gion already covered extensively by the SGP region of the 2dF Galaxy Redshift Survey (Colless et al. 2001). Substantial progress has also been made on some of 2.1 Scope of DR2 ◦ the δ < −42 polar fields, which have been included in The Second Incremental Data Release (DR2) for the DR2. 6dF Galaxy Survey spans observations during the pe- riod January 2002 to October 2004, including and su- Figure 2(b) shows the distribution of galaxy red- perceding DR1. It contains 89 211 spectra that have shift completeness on the sky. This is the fraction yielded 83 014 unique galaxy redshifts over roughly of galaxies in the original target catalogue with ac- two-thirds of the southern sky. Within these sets, ceptable (Q = 3 or 4) extragalactic redshifts, either the number of database spectra with acceptable qual- new from 6dFGS or existing literature ones. Varia- ity redshift is 76 443, where ‘acceptable’ means quality tions in completeness at this stage in the survey are parameter Q = 3 or 4 (see Jones et al. 2004a). The dominated by variable observing conditions, since it is number of Q = 3, 4 unique redshifts is 71 627. As dis- only in the final stages of the survey that virgin fields cussed below, only redshifts with Q = 3 or 4 should be become sparse enough to allow re-observing fields with used for extragalactic science. low completeness. Figure 2(c) shows the density of Extragalactic redshifts are ranked on a quality scale 6dFGS targets on the sky. The variation of source of Q = 1 to 4 by human operators, according to the density is a major challenge. The strategies devised to definitions given in Sec. 4.4 of Jones et al. (2004a). handle this are described elsewhere (Jones et al. 2004a; Q = 1 represents spectra of no value, Q = 2 for tenta- Campbell, Saunders & Colless 2004). tive redshift values, Q = 3 for probable redshifts and Another measure of survey efficiency is field com- Q = 4 for reliable redshifts. Only Q = 3 or 4 redshifts pleteness, shown in Fig. 3. The field completeness is should be used for astrophysical applications, although the number of Q = 3 or 4 extragalactic redshifts from some QSOs classified earlier in the survey will have a all targets in a field, and as such, only applies to red- Q = 2 because no QSO template existed at the time to shifts from 6dF. This shows little change from DR1: confirm the redshift Recently a new category, Q = 6, about 80 per cent of all fields are 80 per cent com- was introduced and this is discussed in Sect. 3.2. plete or more, and more than half have completeness DR2 takes its data from 936 fields. Figure 2(a) in excess of 85 per cent. shows that their distribution is true to observing strat- egy adopted by 6dFGS from the outset: mid-latitude Figures 4 and 5 show the distribution of DR2 red- fields were tackled first, followed by those nearest the shifts on the sky. They show many local large-scale equator and finished with the polar targets. Field cov- structures, such as the Shapley and Hydra-Centaurus erage along the central declination strip −42◦ <δ< Superclusters; this is the most detailed and compre- ◦ ◦ −23 is largely complete, as is coverage of the −23 < hensive view of the southern local universe to date. 4 Publications of the Astronomical Society of Australia

(a)

(b)

(c)

Figure 2: (a) Location of the observed ﬁelds (solid discs) contributing redshifts to the First Data Release. The tiling pattern of all ﬁelds (open discs) is also shown. (b) Redshift completeness on the sky, combining the 6dF First Data Release redshifts with the literature sources. (c) Density of targets on the sky. www.publish.csiro.au/journals/pasa 5

Figure 4: All-southern-sky projections of the 6dF Galaxy Survey to date. Each panel corresponds to a 2000kms−1-wide hemispherical shell in recession velocity. Polar coverage is still largely incomplete. 6 Publications of the Astronomical Society of Australia

Figure 5: Same as Fig. 4 but rotated through 12 hours in right ascension. www.publish.csiro.au/journals/pasa 7

2.2 Online Database an HTML page returned directly to the user, a down- 4 loadable file, and/or a TAR file of FITS data. Several The 6dF Galaxy Survey Online Database is main- examples of database access are provided on the web tained by the Wide Field Astronomy Unit of the Insti- site5. tute for Astronomy, University of Edinburgh. The site has been operating since an early 6dF data release in 2002 December of 17 000 redshifts. This was followed 2.3 Changes Since DR1 by the First Incremental Data Release of 6dFGS data The structure of the database and the data within in March 2004 (DR1), which increased the online cat- are largely unchanged since DR1. However, improved alogue to 52 048 measured spectra. This paper marks derivations for bJ and rF galaxy photometry have led the Second Data Release; a third and final release will to the introduction of two new magnitude parameters be made available on completion of the survey. in the TARGET table: BMAGSEL and RMAGSEL. These hold The database is based on the format of the 2dF- the old bJ and rF magnitudes used to select the 6dFGS GRS database and uses Microsoft’s SQL Server 2000. samples in these bands and formerly held by the BMAG Each survey object has a corresponding multi-extension and RMAG parameters. The BMAG and RMAG parameters FITS file containing postage-stamp images of the ob- remain, although, the magnitudes they now hold are ject in bJrFJHK and its 6dF-measured spectrum. Struc- from the improved bJ and rF photometry introduced tured Query Language (SQL) is used to interrogate the for the first time in DR2. database and extract its contents. The new magnitudes result from a significant im- Fuller descriptions of the database can be found in provement in the photometric calibration of the pho- Sec. 5.2 of Jones et al. (2004a) and Read et al. (2003). tographic plate material. The original SuperCOSMOS What follows here is a summary of its main features. magnitudes used to select the optical samples suffer Data are grouped into several tables, listed in Ta- from scatter in the field-to-field zeropoints of some TARGET ble 2. At its heart is the table, which contains tenths of a magnitude. In 2003, J. Peacock, N. Ham- the master target list used to define 6dFGS observa- bly and M. Read re-calibrated all of the bJ, rF (and iN) tions. Every table is linked through the parameters SuperCOSMOS magnitudes to a common zeropoint. TARGETID TARGETNAME and which, although unique in This recalibration work was originally done for the TARGET 6 the table, are not necessarily unique in the 2dFGRS and is described on the 2dFGRS web site others (for example, some objects have been observed The resulting zeropoint scatter has now been reduced more than once). The 6dFGS parameterised spectral to a few hundredths of a magnitude (J. Peacock, priv. data comprising the redshift measurements and obser- comm.). Neither the old nor new magnitudes have SPECTRA vational meta-data are held in the table. The been corrected for Galactic extinction. The old mag- other tables constitute related source catalogues (and nitudes must be used when considering the selection their photometric data) that are used to define the of the optical samples, but the new magnitudes are various target subsets summarised in Table 1. otherwise preferred. TARGET In the tables there are 4 177 Q = 2 sources, Pairing the recalibrated photographic magnitudes 5 347 with Q = 3 redshifts and 70 391 with Q =4. In with the 6dFGS targets is not a straightforward pro- SPECTRA the table, however, there are multiple obser- cess, particularly for the brighter sources. A typical vations of the same redshift: 7 855 for Q = 2, 6 031 scenario will be that the target position will be paired for Q = 3 and 70 047 for Q = 4. In addition, there with several objects in a given SuperCOSMOS/UK are 4 865 value-less Q = 1 spectra which are kept only Schmidt Telescope passband. For example, these ob- for archival purposes and 292 recent Galactic sources jects could be the parent object plus several daughter (Q = 6). There are many more confirmed Galactic objects. As the targets are bright and will often have sources with Q = 2 and noted with a comment. structure it is likely that the parent is the optimum ob- TARGETNAME.FITS is the name given to the multi- ject to get the magnitude for. In other cases the parent extension FITS file holding all the data for a given tar- will be a star/galaxy, galaxy/galaxy or star/star blend, get. Postage-stamp FITS images are held in the first and the correct object is one of the daughters. five extensions, from bJ and rF SuperCOSMOS scans We have adopted an empirically-derived scheme and J, H and K 2MASS imaging respectively. As that uses both proximity, magnitude and blend param- 6dF spectra are measured and then ingested into the eter to assign identifications and classify the match. Of database, this information is held in FITS extensions the 179 262 target sources put through the bJ-matching six to eight, holding the V, R and VR-combined spec- exercise, 44 per cent had both proximity and magni- tra respectively. Repeat observations (if any), are ap- tude rankings of 1. Of the remainder, 48 per cent had pended in the form of further FITS extensions. Jones et al. either a proximity or magnitude rank of 1, 7 per cent (2004a) describe this arrangement in more detail. had neither ranked as 1, and 1 per cent had no match The database can be queried in one of two ways. at all. The corresponding numbers for rF-matching One web-based form allows the user to make a number were around 46, 46, 7 and 1 per cent. of basic selections from a number of common opera- The non-matches fall into a few categories. Most tions. Another form permits users conversant in SQL are very bright objects, which in some cases have also to construct their own query directly. Users are also been deblended into multiple targets by 2MASS. Some able to upload a list of objects and cross-match this 5 with the 6dFGS observations. Output is available as http://www-wfau.roe.ac.uk/6dFGS/examples 62dFGRS photometric calibration: 4 Online database: http://www-wfau.roe.ac.uk/6dFGS/ http://www2.aao.gov.au/2dFGRS/Public/Release/PhotCat/photcalib.html 8 Publications of the Astronomical Society of Australia

Table 2: Tables of data in the 6dFGS Database Table name Description Programme ID Numbers TARGET the master target list progid SPECTRA redshifts and observational data − TWOMASS 2MASS input catalogue K, H, and J 1, 3, 4 SUPERCOS SuperCOSMOS bright galaxies bJ and rF 7, 8 FSC sources from the IRAS FAINT Source Catalogue 126 RASS candidate AGN from the ROSAT All-Sky Survey 113 HIPASS sources from the HIPASS HI survey 119 DURUKST extensiontoDurham/UKSTgalaxysurvey 78 SHAPLEY galaxies from the Shapley supercluster 90 DENISI galaxies from DENIS I < 14.85 6 DENISJ galaxies from DENIS J < 13.85 5 AGN2MASS candidate AGN from the 2MASS red AGN survey 116 HES candidate QSOs from the Hamburg/ESO Survey 129 NVSS candidate QSOs from NVSS 130 SUMSS radio source IDs from SUMSS and NVSS 125

are bJ ∼ 14 to 17 galaxies that lie near satellite trails. tures around 4440 A˚ in V, and 6430 and 6470 A˚ in R. Others are bright objects with poor deblending, glob- These are due to ghost reflections caused by the VPH ular clusters, 30 Doradus in the LMC, and other ex- gratings, which have been used continuously from 2002 tended sources; a few are blank bits of sky. The null September onwards. These features are typically ∼ 10 value assigned to BMAG and RMAG in the case of non- pixels wide. matches is 0.00. Figure 6 shows examples where the Occasionally, poorly spliced data will show a spu- SuperCOSMOS deblending has led to incorrect 6dFGS rious feature at the spectral join around 5570 A.˚ target-matching. Flux calibration of spectra remains highly approxi- Users of the 6dFGS database should take account mate and users are discouraged from using the database of these limitations and treat this preliminary match- for spectrophotometric applications. Sec. 4.2 of Jones et al. ing with care. The matching algorithm will be further (2004a) outlines the spectral reduction steps and how refined for the final survey data release. they affect the final spectra. The only other changes to the database are im- Fibre cross-talk can occur when light from bright proved descriptions for some of the table parameters. stars or Galactic emission-line sources finds its way into parked fibres. The bright spectral features con- taminate adjacent spectra, causing spurious features. 3 Caveats and Limitations We strongly advise that any peculiar object spectra discovered by users are checked for this effect. To 3.1 Spectral Data this end, we have made available the fully-reduced 2- d frames of spectra in the Downloads section of the With the 6dFGS still underway at the time of DR2, Online Database. users should be mindful that DR2 is an intermediate data release. Just as DR2 is a significant improvement 3.2 Redshifts on DR1, so the quality, precision and reliability of the survey data will be further refined before the final data For data from 2004 August onwards, a new version release at the completion of the survey. of the redshifting software was used that introduced Sec. 4.3 of Jones et al. (2004a) discusses in depth some changes worth noting. First, only sources with some of the typical problems that can afflict the spec- an initial software-assigned Q-value of 3 or less were tral data. Generally, the splicing of V and R spectral inspected by eye. While greatly increasing the effi- data is acceptable, with the exception of a small num- ciency of redshifting the spectra, the original config- ber of cases. The effects of strong fringing are occa- uration of the software resulted in 38 spurious high sionally evident in the form of an oscillating spectral redshift sources that are in fact more likely misclassi- response. In some cases, the quality of the V and R fied low redshift sources. For DR2, these sources have data are significantly different, usually with the V be- been flagged and re-assigned Q = 2; they will be red- ing worse because of the lower counts in that spectral shifted again manually at a later date. The software is range. While it is possible in principle to derive red- now run in a different configuration that flags sources shifts from V or R halves alone, no half-data have been falling into this category. included in DR2. The second software change affecting post-August Users should also be aware of spurious spectral fea- 2004 data is that confirmed Galactic sources (stars, www.publish.csiro.au/journals/pasa 9

Figure 6: Examples of UK Schmidt telescope ﬁelds (left) that have resulted in poorly deblended SuperCOS- MOS targets (right). Included is one ﬁeld encompassing the well-known Cartwheel Galaxy, ESO350-G040 (upper left panel).

ISM Balmer-line emission, planetary nebulae, and the 4 Summary like) are now assigned Q = 6. Before this, they had been given Q = 2 and their nature noted in a separate comment. Either way, all acceptable extragalactic red- We have described the Second Incremental Data Re- shifts have Q = 3 or 4. The two schemes will be unified lease (DR2) for the 6dF Galaxy Survey and its access for the final data release. through an online database. We have discussed several issues affecting data quality and underscored the pre- A final change to the software was the introduc- liminary and evolving nature of the data set. We urge tion of a QSO template, which has dramatically im- potential users of the data set to familiarise themselves proved the successful identification of the higher red- with these caveats and with the changes implemented shift QSOs uncovered by the survey. since previous data releases. To check the precision of the redshifts we com- Observing for the 6dFGS will nominally finish on pared a sample of 16 127 6dFGS galaxies with previous 31 July 2005, although some clean-up observations may redshifts from Huchra’s ZCAT Catalog (Fig. 7). The occur after that date. A third and final data release − mean offset cz(6dFGS) − cz(ZCAT) was 22.0 kms 1. will be made sometime in 2006. Repeat measurements indicate that most of the scatter is due to the ZCAT sample, for which the source material is highly inhomogeneous. Acknowledgments

3.3 Photometry We are grateful to the staff of the Anglo-Australian Observatory, who were responsible for the design and construction of the 6dF instrument. AAO support for We have already discussed the limitations of the match- the 6dF Galaxy Survey continues in the form of the ing algorithm used to pair the recalibrated bJ and rF observing and data reduction done by staff of the UK SuperCOSMOS magnitudes with 6dFGS targets in in- Schmidt Telescope, without whom the project would stances where source deblending fails. The sources to not have been possible. We also thank E. Westra, < watch are those that are either bright (bJ ∼ 15), or M. Williams, V. Safouris, and S. Prior for their on- have one or more neighbours. Even for isolated tar- going efforts in measuring redshifts from the survey. gets, deblending may still be an issue if the source is a D. H. Jones is supported as a Research Associate by bright spiral galaxy with genuine clumpy structure. In Australian Research Council Discovery–Projects Grant general, users are urged to treat the optical photome- (DP-0208876), administered by the Australian National try with caution. University. 10 Publications of the Astronomical Society of Australia

Figure 7: Redshift residuals cz(6dFGS) − cz(ZCAT) for a sample of 16127 sources with redshifts from 6dFGS and ZCAT. Both axes are in units of kms−1.

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