Redshift Surveys of the Local Universe

The Two-Degree Field facility (2dF) at the Anglo-Australian Telescope is a 400-fibre optical spectrograph with a 2 $^{\circ}$ -diameter field of view. [Lewis] described how 2dF's fast robotic fibre-positioner and two focal planes makes it ideally suited for massive redshift surveys. The 2dF Galaxy Redshift Survey (2dFGRS) has now measured redshifts for more than 50,000 galaxies, making it the single largest redshift survey to date. [Colless] gave an overview of the survey's status, reporting that, as of August 1999, 227 of the 1093 survey fields have been observed, yielding redshifts for 53,192 objects, of which 50,180 are galaxies (the remainder are mostly stars plus a handful of QSOs). The redshift completeness of the survey is currently 91% and rising, as data quality and reduction methods are refined. The 2dFGRS will eventually provide redshifts and spectral information for 250,000 galaxies with b_J<19.45 (extinction-corrected) over nearly 2000 sq.deg (Colless 1998, 1999). Figure 1 shows the sky coverage of the 2dFGRS and a number of the other surveys mentioned here. Although the survey is at present only 20% complete, it is expected to be finished by the end of 2001.

**Figure 1:** The areas of the sky covered by various surveys mentioned in the text.
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**Figure 2:** Redshift cone diagram showing the distribution of objects from the 2dF galaxy and QSO redshift surveys as of August 1999. The top panel shows the distribution of the galaxies alone, and the bottom panel shows the distributions of both galaxies and QSOs.
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The 2dFGRS has a companion survey in the 2dF QSO Redshift Survey described by [Smith], which aims to obtain redshifts for more than 25,000 QSOs with 18.25<b_J<20.85 in two strips covering 740 sq.deg (see Figure 1). The primary goals of the survey (Boyle et al. 1999) are to measure the primordial fluctuation power spectrum on very large scales, to obtain a geometric determination of the cosmological constant $\Lambda$ , and to investigate the evolution of the QSO luminosity function and QSO clustering. Follow-ups will include studies of absorption-line and damped Lyman- $\alpha$ systems along QSO sight-lines. The multicolour selection criteria for the target sample are expected to give a 93% completeness for the redshift range 0.3<z<2.2. As of August 1999, 13,854 of the 48,078 QSO candidates have been observed, and have yielded 7026 identified QSOs. Figure 2 shows both the 2dF galaxy and QSO surveys on a single redshift cone diagram; the density of sampling of the galaxies at low redshift is complemented by the sparser QSO sample reaching out to beyond z=2.2.

The Sloan Digital Sky Survey (SDSS) will be one of the largest and most comprehensive astronomical datasets ever produced. Overviews of SDSS science and operations were given by [Margon] and [Szalay] (see also Margon 1999). SDSS is both a photometric and a spectroscopic survey of $\pi$ steradians of the northern sky with $b>\vert 30\mbox{$^{\circ}$}\vert$ . The survey uses a purpose-built 2.5m telescope at Apache Point Observatory in New Mexico, supported by a 0.5m telescope providing continuous monitoring of the photometric conditions. The imaging system consists of a high-efficiency drift-scan camera using 30 CCDs to image in five passbands quasi-simultaneously while covering the sky in 3 $^{\circ}$ -wide strips with an effective exposure time of 55s. This will yield images for 10⁸ galaxies and 5 x 10⁷ stars down to $r^\prime$ =23.1 over a quarter of the sky. The unique SDSS photometric system ( $u^\prime$ $g^\prime$ $r^\prime$ $i^\prime$ $z^\prime$ ) was detailed by [Fukugita]. The filter set is specially designed, featuring high-throughput, nearly-disjoint passbands covering the range from 0.3-1.0 $\mu$ m. The imaging survey, which achieved first light in May 1998, has already produced exciting discoveries, including the first z=5 QSO (Fan et al. 1999) and the coolest methane brown dwarf (Strauss et al. 1999). [Margon] listed the scientific strengths of SDSS, in ascending order of importance, as: (i) the large database; (ii) the homogeneity of the data; (iii) the well-characterized sample; (iv) discovery potential; (v) the archival value of the imaging archive.

The volume of data to be generated by SDSS is enormous: 40 Terabytes of raw data, reduced to a `mere' 1 Terabyte when processed. [Szalay] described the complex data processing pipelines and archival database which will generate and make available the SDSS data products, ranging from a heavily compressed all-sky map, through thumbnail images and spectra for every object, to the final photometric and redshift catalogues. Szalay also outlined the sophisticated indexing procedures which will be used to facilitate a rapid response to complex search queries.

[Loveday] described the SDSS spectroscopic survey that will be carried out in parallel with the imaging survey (Loveday & Pier 1998). There are various spectroscopic target samples: the main galaxy sample, of 900,000 galaxies with Petrosian magnitudes brighter than $r^\prime$ $\approx$ 18.1; the Bright Red Galaxy sample, of 100,000 galaxies brighter than $r^\prime$ $\approx$ 19.3, with selection by colour and photometric redshift to give early-type galaxies with 0.25<z<0.45; the QSO sample, of 100,000 QSO candidates down to $r^\prime$ $\approx$ 20 selected by colour or as FIRST or ROSAT sources; and the stellar sample, 100,000 stars of various types. In addition there will be a smaller number of serendipity sources, selected as `interesting' by a variety of criteria. It is important to note that the redshift sample generated by this spectroscopic survey will be supplemented by photometric redshifts with a precision of $\Delta z$ $\approx$ 0.05 for perhaps 10⁸ fainter galaxies, which, in conjunction with `spectral' types based on precision colours, will be invaluable for studying the evolution of the galaxy population out to z $\sim$ 1. A small amount of test data has been taken with the SDSS spectrograph, a 640-fibre manually-configured system with a 3 $^{\circ}$ field of view. Survey observations are expected to begin in early 2000, with the whole SDSS, both photometric and spectroscopic surveys, taking 5 years to complete.

Another view of the nearby universe is provided by the Two-Micron All-Sky Survey (2MASS), whose current status was reviewed by [Huchra]. 2MASS is surveying the whole sky in the near-infrared J, H and K_s bands, and for galaxies expects to achieve limiting magnitudes in these bands of 15.0, 14.3 and 13.5 respectively (Chester et al. 1998). The northern 2MASS survey began in 1997, and the southern survey in 1998. At present 70% of the sky has been covered by 2MASS, and the whole survey is expected to be completed by early 2001.

The 2MASS imaging survey will be followed up by two (coordinated) redshift surveys and two (complementary) peculiar velocity surveys. [Huchra] described the 2MASS Redshift Survey, which has the goal of measuring redshifts for 125,000 galaxies covering 90% of the sky down to K_s=12.2. This would build on existing redshift catalogues and, in the north, fill in the missing redshifts with the 1.3m at FLWO used for the CfA redshift surveys. [Colless] reported plans for a southern counterpart to this survey, using the 6dF fibre spectrograph currently under construction for the AAO Schmidt telescope. The 6dF Galaxy Survey (Mamon 1999) has two phases, the first of which is a redshift survey of 115,000 galaxies down to K_s=13, H=13.5 or J=14.3 over the 17,000 sq.deg of the southern sky with $b>\vert 10\mbox{$^{\circ}$}\vert$ . As well as completing the all-sky 2MASS survey to K_s=12.2 and pushing another 0.8 mag fainter, this first phase of the 6dF survey will provide a volume-limited sample for the second phase, a peculiar velocity survey of at least 15,000 early-type galaxies at distances less than cz=15000kms^-1 using the Fundamental Plane distance estimator. This in turn will complement an all-sky peculiar velocity survey of 10,000 spirals using the infrared Tully-Fisher relation proposed by [Huchra].

[Webster] reviewed the progress and some preliminary results from the HI Parkes All-Sky Survey (HIPASS; Staveley-Smith et al. 1999), which is currently using the Parkes 21cm multi-beam receiver to cover the sky south of $\delta$ =+25 $^{\circ}$ out to a maximum redshift of cz=12700kms^-1. As of August 1999, the southern hemisphere survey is over 90% complete, and the northern extension is over 10% complete. HIPASS provides a complete 21cm survey of the southern sky with 7 arcmin angular resolution down to a 3 $\sigma$ HI mass limit of 1.4 x $10^{10}(D/100\mbox{\,h$^{-1}$\,Mpc})^2M_\odot$ (Kilborn, Webster & Staveley-Smith 1999). When complete, HIPASS is expected to contain $\sim$ 5000 galaxies with a mean redshift of 3000kms^-1. The effective volume of the sample is 6 x 10⁶h^-3Mpc³. HIPASS should offer a very different view of the local universe to optical and near-infrared redshift surveys, and it will be interesting to see the differences in the galaxy population and the large-scale structure, especially for low-mass objects.

Figure 3 compares the number of objects and the volume encompassed by various redshift surveys.

**Figure 3:** Comparison of the size and volume of existing and planned redshift surveys. Surveys are labelled by their standard acronyms (see text).
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