In an effort to observe the evolution of large-scale structure, Marc Postman (STScI), Tod Lauer (NOAO), Bill Oegerle (JHU), Istvan Szapudi (Durham), and John Hoessel (Wisconsin) have recently completed a deep I-band imaging survey of a contiguous 16 square degree region of sky using the prime focus CCD camera on the KPNO Mayall 4-m Telescope. The survey is the first of a new generation of deep, wide-area imaging surveys to study structure at high redshift. The evolution of large-scale structure (LSS) in the universe depends on the abundance and form of dark matter, the mean baryon density, the turnover scale in the perturbation power spectrum, and the formation processes of galaxies and clusters. A study of this evolution is, thus, fundamental to cosmology.
The goal of Postman and collaborators is to use the faint galaxies and distant galaxy clusters to probe large-scale structure evolution over 0 < z <= 1 by setting accurate limits at z ~ 1 on the amplitude of structures now seen at z ~ 0. Redshift surveys have shown that inhomogeneities in the distribution of low redshift (z < 0.1) galaxies and clusters exist on scales up to 50/h Mpc (and probably even larger). Deeper pencil beam surveys hint at equally large-scale perturbations at moderate and high redshifts (0.5 <= z <= 3) but it is difficult to elucidate their nature, given the extremely limited area coverage of those spectroscopic probes.
To explore large-scale structure at z = 1, an imaging survey has to subtend at least 75/h Mpc; a minimum
angular dimension of 4° is required. Furthermore, to avoid severe aliasing effects, a 4° × 4° survey geometry was
chosen. The field, centered at 10h 13m 27.95s + 52d 36m 43.5s (J2000), was selected by virtue of its high galactic
latitude (+ 51°), low HI column density
(2.2 × 1020), high declination (increased visibility from KPNO), low IRAS
100 µm cirrus emission, and the absence of many bright stars or nearby rich clusters. The 16' field of view of the KPNO
4-m prime focus CCD camera allowed 2 square degrees to be mapped per night (roughly 32 exposures per night).
Each exposure was 900s, which provides a 4 completeness limit of
IAB = 24, sufficient to detect cluster galaxies
2 magnitudes fainter than the typical unevolved first-ranked elliptical at z = 1 (spectral evolution will help, making
the galaxies brighter and easier to detect). Each pointing overlapped its adjacent pointing by 1'. The overlap assures
that photometric zeropoint variations can be measured to an accuracy of 2% or better. The final survey
contains approximately 1 billion pixels of image data and about 710,000 galaxies to a limit of
IAB = 24 (920,000 objects, when stars are counted)! A flux-weighted map of the galaxies is shown in Figure 1.
A cluster catalog has now been constructed using the matched detector scheme outlined in Postman et al.
1996. There are approximately 16 clusters deg-2 with
CL >= 50 (
CL corresponds to the effective number of
L* galaxies in the central 1h-1 Mpc region of the cluster). Estimates of the
n-point moments of the angular galaxy distribution
have also been computed. The results for the two-point function,
(
), are shown in Figure 2. Excellent agreement
with the results of Maddox et al. 1996 is seen for
IAB <= 18.5. Assuming the redshift dependence of the spatial
correlation function,
(r) , can be expressed as
(r,z) = (r/r
o)-
(1 + z)(3+
) then the results in Figure 2 are consistent with
ro = 5.4h1 Mpc,
= 1.75 ± 0.05 , and
0 <=
<= 1. The slope of
(
) (and hence
(r) ) is not dependent on limiting
magnitude down to IAB = 24 on scales less than 30'. These results are novel in that they extend to scales of ~ 1° at faint
magnitudes and cover a large dynamic range in flux from the same, homogeneous dataset.
Future work will focus on spectroscopic follow-up of the cluster candidates to measure the cluster-cluster correlation function over 0.4 < z < 1. Owen et al. have also conducted a deep VLA survey of the central 6 deg2 and, with the above collaborators, will study the evolution of radio galaxies. Updates on the survey can be found at http://landru.stsci.edu:5000/deeprange.html.