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The 2Df Galaxy Redshift Survey Matthew Colless eaa.iop.org DOI: 10.1888/0333750888/5485 The 2dF Galaxy Redshift Survey Matthew Colless From Encyclopedia of Astronomy & Astrophysics P. Murdin © IOP Publishing Ltd 2006 ISBN: 0333750888 Institute of Physics Publishing Bristol and Philadelphia Downloaded on Tue Feb 07 18:28:22 GMT 2006 [131.215.103.76] Terms and Conditions The 2dF Galaxy Redshift Survey ENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICS The 2dF Galaxy Redshift Survey The Anglo-Australian Observatory, under Director Russell Cannon and instrumentation scientist Keith Taylor, combined this opportunity with its established The 2dF Galaxy Redshift Survey (2dFGRS) produced a strength in robotic multi-object spectroscopy in order to three-dimensional map of the distribution of 221 000 develop a wide-field, multi-object spectrograph: the 2dF galaxies covering 5% of the sky and reaching out more facility (Taylor and Gray 1990; Gray et al. 1993). With than 3 billion light-years to a redshift z~0.3. The survey massive field-correction optics including an atmospheric used the Anglo-Australian Telescope’s (AAT’s) 2-degree dispersion compensator, a pair of tumbling focal planes Field facility (2dF), which could observe 400 objects to minimize dead time, 400 optic fibers mounted in mag- simultaneously over a 2° diameter field of view. Survey netic buttons, a fully robotic positioning system, and observations began in 1997 and were completed in 2002; end-to-end software for control, configuration, data- the final survey was an order of magnitude larger than taking and reductions, 2dF was, for its time, a visionary any previous redshift survey. instrument. Construction began in 1990. The 2dFGRS yielded the first map of the large-scale From its conception, the main scientific driver for structure in the local (low-redshift) universe to probe a 2dF was a massive redshift survey, sampling a represen- statistically representative volume on scales from tens tative volume of the universe in fine detail in order to of kiloparsecs to hundreds of megaparsecs. Analysis of precisely measure fundamental cosmological parame- the map gave the first direct evidence that the large- ters. The best existing and ongoing surveys in the early scale structure of the universe grew through gravita- 1990s (e.g. the IRAS Point Source Catalogue redshift sur- tional instability, and provided a precise measurement vey and the Las Campanas Redshift Survey) either did of the mean mass density of the universe. The observed not cover sufficiently large volumes to be statistically structure also displayed a weak signature produced by representative of the large-scale structure or covered the interactions between baryons and photons in the large volumes too sparsely to provide precise measure- very early universe, providing an estimate of the ratio ments. An order-of-magnitude increase in the volume of baryonic matter to dark matter. The combination of and sample size of redshift surveys was required to enter the 2dFGRS observations of the structure in the present- the regime of ‘precision cosmology’. This idea was devel- day universe with observations of the structure in the oped into a concrete proposal by the founding members very early universe from the cosmic microwave back- of the survey team during 1993 and 1994. The first survey ground (CMB) gave precise measurements of the proposal was submitted to the telescope time committees Hubble constant, and the cosmological constant and and the AAT Board in 1994, and envisaged a survey of provided an improved upper limit for the mass of the 250000 galaxies over 2000 deg2 to a magnitude limit of neutrino. bJ=19.5. The survey was estimated to require 125 nights The massive sample of galaxies observed by the on the AAT and was projected to be completed by the 2dFGRS has also been used to precisely characterize mul- end of 1996. tiparameter distributions and relationships within the In fact, 2dF was placed on the AAT in November galaxy population: the distribution of galaxies over lumi- 1995, and the first spectra were taken in mid-1996. nosity, surface brightness, spectral type, star-formation Scheduled observations with 2dF at full functionality rate and environment (local density) and the variation in began in September 1997, and the first major redshift sur- clustering for galaxies of different luminosities and spec- vey observing run occurred in October 1997. Progress tral types. The survey also yielded the first measurement with the survey was slow to begin with, but the survey of the galaxy bias parameter, which links the fluctuations passed 50000 redshifts in mid-1999 and 100000 redshifts in the number of galaxies and the amount of dark matter in mid-2000 (Colless 1999). The first 100 000 redshifts and and is a basic constraint on models of galaxy formation spectra were released publicly in June 2001 (Colless et al. and evolution. 2001), and the 200 000-redshift mark was achieved towards the end of 2001. The survey observations were The History of the 2dFGRS completed in April 2002, after 5 years and 272 nights on The seed of the 2dFGRS was planted in the late 1980s, the AAT. when a British astronomy review gave high priority to converting existing telescopes to wide-field science, The 2dF Wide-Field Fiber Spectrograph Facility especially multi-object spectroscopy. For the AAT, the As its name implied, the 2dFGRS was tightly coupled case for such a conversion was greatly strengthened to the capabilities of the 2dF of the AAT. The crucial when Roderick Willstrop and Charles Wynne showed parameters of 2dF for a massive redshift survey were that, with suitable correction optics, the telescope could as follows: (i) the wide (2° diameter) field of view, be adapted to permit a 2° diameter field of view at the which allowed the 1800 deg2 of the survey to be cov- prime focus (Willstrop 1987; Wynne 1989). ered with 900 overlapping fields; (ii) the large number Copyright © Nature Publishing Group 2002 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998 and Institute of Physics Publishing 2002 Dirac House, Temple Back, Bristol, BS21 6BE, UK 1 The 2dF Galaxy Redshift Survey ENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICS galaxies as faint as bJ=19.5 in 1 h observations; (iv) the capacity for rapid reconfiguration, which meant that there was no dead-time between observations of differ- ent fields. To achieve these complex requirements, a complex instrument was required. All the components of 2dF are contained in the 2dF top-end ring structure, which is mounted at the prime focus of the AAT (see figure 1). A photon reflected by the AAT’s 4 m primary mirror towards the prime focus encounters, first, the wide-field corrector optics (including an atmospheric dispersion corrector) that provide a 2° diameter field of view with good imaging properties; then it is collected at the focal plane by the fiber buttons that route it down the optic fibers; finally, it enters one of the twin spectrographs, each of which measures the spectra from 200 fibers at a Figure 1. A schematic of the 2dF facility showing the major time. The total system efficiency is about 5%. components of the instrument. (K. Taylor, Anglo-Australian The fiber buttons are magnetically attached to a metal Observatory.) plate in the focal plane. They are positioned on the plate by a high-precision robot, which moves over the plate on an X–Y stage and has a gripper arm that can be raised or (400) of simultaneous spectra, which allowed the typi- lowered to pick or place a button on the plate. The robot cal number of galaxies in the field of view down to can position a fiber button on the plate with a precision of µ bJ=19.5 to be observed at once; (iii) the 4 m aperture of 0.3 arcsec (20 m) in 6 s. In order to maximize observing the AAT, which allowed redshifts to be measured for efficiency there are two sets of fiber buttons, and the plates Figure 2. A map of the sky showing the locations of the two 2dFGRS survey strips (NGP strip at left, SGP strip at right) and the ran- dom fields. Each 2dF field in the survey is shown as a small blackcircle; the sky survey plates from which the source catalog was con- structed are shown as small red squares. The scale of the strips at the typical redshift of the survey is indicated. (S. J. Mad dox, University of Nottingham.) Copyright © Nature Publishing Group 2002 Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998 and Institute of Physics Publishing 2002 Dirac House, Temple Back, Bristol, BS21 6BE, UK 2 The 2dF Galaxy Redshift Survey ENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICS accepted redshifts (Q>3) were found to be 98% reliable and to have a typical uncertainty of 85 km s–1. The over- all redshift completeness for accepted redshifts was 92%, although this varied with magnitude. Figure 3 shows a thin slice through the three-dimen- sional map of over 221 000 galaxies produced by the 2dFGRS. The observer is at the center of the map, with redshift increasing towards the edge and angle on the sky (RA) marked around the circumference. This 3° thick slice passes through both the NGP strip (at left) and the SGP strip (at right). The decrease in the number of galax- ies towards higher redshifts is an effect of the survey Figure 3. A 3° thick slice through the 2dF Galaxy Redshift selection by magnitude—only intrinsically more lumi- Survey map. The slice cuts through the NGP strip (at left) and nous galaxies are brighter than the survey magnitude the SGP strip (at right). It contains 63 000 galaxies and shows the limit at higher redshifts.
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