Scientific Motivation
The nature of large-scale structure at high redshift is unknown
but is now accessible with large area CCDs. In an effort to improve
our understanding of the evolution of large-scale structure, we
have recently surveyed a contiguous 16 square degree area
in the I-band using the prime focus CCD camera on the
Mayall 4m telescope at
Kitt Peak National Observatory.
Our survey is the first of a new generation of deep, wide-area
imaging surveys to study structure at high redshift.
The aim is to use the faint galaxies and
distant galaxy clusters within this survey
to probe large-scale structure evolution over 0 < z < 1.
A map of the ~710,000 galaxies in the survey is shown
above (and in the results section).
The goal is to set accurate limits at z = 1
on the amplitude of structures now seen at z = 0.
Redshift surveys show that
inhomogeneities in the distribution of low redshift (z < 0.1)
galaxies and clusters exist on scales up to 30/h Mpc (and
probably even larger).
Deeper pencil beam surveys hint at equally large-scale perturbations
at moderate redshifts (z ~ 0.5) (
Broadhurst et al. 1990),
but cannot elucidate their nature, given their extremely limited area coverage.
Observational constraints on the evolution of large scale structure
provide strong limits on structure formation theories because
coherent structures on scales >10/h Mpc are still in the linear regime.
It is precisely in the linear regime where important distinctions can
be made between competing theories. Currently, very little is
known about the evolution of large scale structure and this survey will
provide an important breakthrough.
Survey Strategy
To explore large-scale structure at z = 1 , our survey had to subtend
at least 75/h Mpc and hence a minimum angular dimension of 4 degrees
was required. Furthermore, to avoid severe aliasing effects and unfavorable
window functions, we chose to keep both angular dimensions comparable.
A number of previous observational constraints lead us to these
requirements.
First, to extract reliable anisotropy limits on the cluster distribution
demands survey dimensions
comparable with the turnover in the galaxy power spectrum.
Second,
the cluster-cluster correlation length is at least 16/h Mpc at
low redshift. To measure the
high redshift cluster correlation function with accuracies
comparable to those at low redshift
our survey must contain at least 300 clusters
(approximately 25% of the clusters would be at 0.85 < z < 1.15).
Current estimates of the surface density of clusters from small
area surveys (
Gunn et al. 1986, ApJ, 306, 30;
Postman et al. 1996, AJ, 111, 615)
are in the range 20+ clusters/sq deg out
to z = 1, thus an area of 16 square degrees is required.
Lastly, high order correlations can
establish consistency between the clustering properties of galaxies and
of clusters, a link which
must be explained by any theory of galaxy formation. To perform this
test at high redshift requires a survey which is comparable
in dimension to the cutoff in the cluster correlation function: 50/h Mpc.
The KPNO 4m prime focus CCD camera has a 16 arc minute field of view
(0.47 arcseconds per pixel) and, thus, we were able to survey
2 square degrees per clear night (roughly 32 exposures per night).
Each pointing overlaps its adjacent
pointing by 1 arcminute. The overlap assures that we can track any
photometric zeropoint variations to within 2% or better.
We map out 16 square degrees using a
4 degree x 4 degree square
geometry. To cover this area thus requires 256 overlaping exposures.
Our exposures are 900 seconds in duration. This allows us to
reach a 4-sigma completeness limit of I=23.5, 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).
Two examples of distant clusters that have been discovered in this
survey are shown at the bottom
of this page. The field
of view in both images is 4 arcminutes on a side (i.e., only
1/16th of the field of view of a single prime focus CCD image).
Going to this depth was essential -- a shallower survey would only be
sufficient for detecting the very richest z=1 clusters and would limit our
conclusions about evolution of structure. (At I = 23.5, we are able to
detect z ~ 1 Abell richness class 1 systems). Since we also wish to
conduct follow-up spectroscopic surveys based on these images, it was
desireable to have the data reach the spectroscopic limit of 8 - 10m class
telescopes with a reasonable signal-to-noise ratio.
Fringing was minimized by using the prime focus camera scan table.
After some experimentation, it was determined that a 60-pixel scan was the
optimal choice (the fringe amplitude decays more slowly with scan
size when the scan size exceeds 60 pixels but decays quite rapidly
with scan size for scans less than 60 pixels). As can be seen by inspection
of the two example images on this page, the reduced data typically flatten to
1 percent or better!
The Survey Field Position
We have carefully selected the 4 degree x 4 degree survey field
centered at
10h 13m 27.95s +52d 36m 43.5s (J2000)
by virtue of its high galactic latitude (+51 deg),
low HI column density (2.2 x 10^20),
high declination (increased visibility from KPNO), low
IRAS 100 micron cirrus emission,
and the absence of many bright stars or nearby rich clusters.
Click here to see an ascii table of the 256
J2000 Survey Grid Coordinates .
Analysis Plans
Immediate products of the
survey will be objectively derived catalogues of distant galaxies and
clusters. Constraints on structure from the 2D survey alone will be
made by performing galaxy count fluctuation measurements on scales
up to half the survey size and
from an n-point angular correlation function and power spectrum
analysis
on the galaxy and cluster distributions (e.g., the galaxy angular
autocorrelation
function on scales of 2 - 3 degrees to this depth has not yet been
measured -- we will determine if the observed break in the galaxy angular
correlation function persists at faint magnitudes).
We will also investigate photometric cluster
properties as distance indicators to attempt
studies on the large-scale cluster distribution
without the need for a complete redshift survey.
The best constraints on high redshift structures, of course,
will come from a follow-up spectroscopic survey of
a representative sample of the galaxies and clusters.
The large contiguous area will allow
unambiguous identification of the ``spikes" seen
in the redshift histograms from recent deep pencil beam surveys.
Cluster detection will be performed using the matched filter technique
described in
Postman et al. (1996). We will also employ a wavelet cluster
identification technique (Holden and Nichol 1996). The matched filter method
has the advantage that it provides both a physically meaningful richness
estimate and a redshift estimate.
Survey Status
As of March 1996, the imaging phase of this survey has been completed.
As of April 1997, galaxy catalog construction has also been completed.
Cluster detection has been accomplished and the n-point correlation function
analysis has been completed.
Spectroscopic observations of detected clusters are underway using NOAO
facilities. See publications below for details.
Publications Available
Clustering at High Redshift:
Precise Constraints from a Deep, Wide Area Survey,
Postman, M., Lauer, T. R., Szapudi, I., & Oegerle, W.
1998, Astrophys. J., 506, 33.
Click here for Abstract & Article
Observational Constraints on Higher Order Clustering up to z ~ 1
Szapudi, I., Postman, M., Lauer, T., Oegerle, W.
2001, Astrophys. J., 548, 114.
Click here for Abstract & Article
The KPNO/Deeprange Distant Cluster Survey: I. The Catalog and the Space Density of Intermediate
Redshift Clusters
Postman, M., Lauer, T. R., Oegerle, W., Donahue, M. 2002, ApJ, 579, 93.
Click here for Abstract & Article
An I-Band Selected Sample of Radio-Emitting Quasars:
Evidence for a Large Population of Red Quasars.
White, R. L., Helfand, D. J., Becker, R. H., Gregg, M. D., Postman, M.,
Lauer, T. R., Oegerle, W. 2003, AJ, 126, 706.
Click here for Abstract & Article
Data Available!
To access our I-band images: Click Here!
To access the cluster catalog: Click Here!
Related Surveys
Cool Deeprange Figures
Flux weighted Galaxy Map |
Counts vs Magnitude |
2-pt Corr. Function |
Corr. Func. Amplitude vs Mag |
Cluster Candidate Examples
CL1025+5250 (zest = 0.50) |
CL1024+5214 (zest = 0.80) |
© 2002 - 2005 Marc Postman
Last Updated: 28 April 2005
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