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5.1. Survey overview

The largest dataset for which a thorough comparison with the above picture has been made is the 2dF Galaxy Redshift Survey (2dFGRS). This survey was designed around the 2dF multi-fibre spectrograph on the Anglo-Australian Telescope, which is capable of observing up to 400 objects simultaneously over a 2 degree diameter field of view. For details of the instrument and its performance see, and also Lewis et al. (2002). The source catalogue for the survey is a revised and extended version of the APM galaxy catalogue (Maddox et al. 1990a, b, c); this includes over 5 million galaxies down to bJ = 20.5 in both north and south Galactic hemispheres over a region of almost 104 deg2. The bJ magnitude system is related to the Johnson-Cousins system by bJ = B - 0.304(B - V), where the colour term is estimated from comparison with the SDSS Early Data Release (Stoughton et al. 2002).

The 2dFGRS geometry consists of two contiguous declination strips, plus 100 random 2-degree fields. One strip is in the southern Galactic hemisphere and covers approximately 75° × 15° centred close to the SGP at (alpha, delta) = (01h, -30°); the other strip is in the northern Galactic hemisphere and covers 75° × 7.5° centred at (alpha, delta) = (12.5h, +0°). The 100 random fields are spread uniformly over the 7000 deg2 region of the APM catalogue in the southern Galactic hemisphere. The sample is limited to be brighter than an extinction-corrected magnitude of bJ = 19.45 (using the extinction maps of Schlegel et al. 1998). This limit gives a good match between the density on the sky of galaxies and 2dF fibres.

After an extensive period of commissioning of the 2dF instrument, 2dFGRS observing began in earnest in May 1997, and terminated in April 2002. In total, observations were made of 899 fields, yielding redshifts and identifications for 232,529 galaxies, 13976 stars and 172 QSOs, at an overall completeness of 93%. The galaxy redshifts are assigned a quality flag from 1 to 5, where the probability of error is highest at low Q. Most analyses are restricted to Q geq 3 galaxies, of which there are currently 221,496. An interim data release took place in July 2001, consisting of approximately 100,000 galaxies (see Colless et al. 2001 for details). A public release of the full photometric and spectroscopic database is scheduled for July 2003. The completed 2dFGRS yields a striking view of the galaxy distribution over large cosmological volumes. This is illustrated in figure 4, which shows the projection of a subset of the galaxies in the northern and southern strips onto (alpha, z) slices. This picture is the culmination of decades of effort in the investigation of large-scale structure, and we are fortunate to have this detailed view for the first time.

Figure 4

Figure 4. The distribution of galaxies in part of the 2dFGRS: slices 4° thick, centred at declination -2.5° in the NGP and -27.5° in the SGP. This magnificently detailed image of large-scale structure provides the basis for measuring the shape of the primordial fluctuation spectrum and hence constraining the matter content of the universe.

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