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The status of DPOSS, and some initial scientific applications.
Abstract
The Digital POSS-II (DPOSS) and the resulting Palomar-Norris Sky Catalog
(PNSC) are now nearing completion, with the first anticipated data
releases within a few months. The survey covers the entire sky north of
delta = -3(deg) in 3 bands, photographic JFN calibrated to the Gunn gri
system, reaching to equivalent limiting magnitude of Blim ~
22(m) . As a result of optimal digitisation of the plates, detailed
processing of the scans, and a very extensive CCD calibration program,
the data quality exceedes that of the previous photographically-based
efforts. Object classification is performed using Artificial Neural
Nets, Decision Trees, and other techniques. The end product (PNSC) is
expected to contain > 50 million galaxies and > 2 billion stars.
Numerous scientific projects utilising these data have been started.
DPOSS galaxy counts in 3 bands are described elsewhere at this meeting
by Odewahn et al. They serve as a low-z baseline for deeper galaxy
counts and studies of galaxy evolution. We are also using our galaxy
catalogs to obtain new measurements of galaxy clustering and large-scale
structure. A new catalog of ~ 20,000 galaxy clusters selected in an
objective and statistically well understood manner, is being produced.
Using the catalogs of stellar objects, we search for quasars at z > 4
using gri color selection; to date, over 60 have been found. They are
used for a variety of follow-up studies. We also look for stellar
objects with extreme or peculiar colors or variability, including
optical transients. There is even an exciting possibility of discovering
some previously unknown, rare type of objects, and at least one such
source has been found. DPOSS catalogs are already used for optical IDs
of sources found at other wavelengths, e.g., radio surveys, and
especially the 2MASS. Finally, our galaxy catalogs are used by a number
of other groups for their redshift surveys, e.g., the Palomar-Norris
survey, and perhaps including also the Sloan DSS early observations. We
will present a new photometric catalog of the Coma cluster region,
created for this purpose.
Measurements of galaxy cluster abundances, clustering properties, and mass-to-light ratios in current and future surveys can provide important cosmological constraints. Digital wide-field imaging surveys, the recently demonstrated fidelity of red-sequence cluster detection techniques, and a new generation of realistic mock galaxy surveys provide the means for construction of large, cosmologically interesting cluster samples, whose selection and properties can be understood in unprecedented depth. Here we present the details of the "maxBCG" algorithm, a cluster detection technique tailored to multiband CCD imaging data. MaxBCG primarily relies on an observational cornerstone of massive galaxy clusters: they are marked by an overdensity of bright, uniformly red galaxies. This detection scheme also exploits classical brightest cluster galaxies (BCGs), which are often found at the centers of these same massive clusters. We study the algorithm through its performance on large, realistic, mock galaxy catalogs, which reveal that it is over 90% pure for clusters at 0.1 < z < 0.3 with 10 or more red galaxies, and over 90% complete for halos at 0.1 < z < 0.3 with masses above 2 × 1014 h-1 M☉. MaxBCG is able to approximately recover the underlying halo abundance function and assign cluster richnesses strongly coupled to the underlying halo properties. The same tests indicate that maxBCG rarely fragments halos, occasionally overmerges line-of-sight neighboring (10 h-1 Mpc) halos, and overestimates the intrinsic halo red-sequence galaxy population by no more than 20%. The study concludes with a discussion of considerations for cosmological measurements with such catalogs, including modeling the selection function, the role of photometric errors, the possible cosmological dependence of richness measurements, and fair cluster selection across broad redshift ranges employing multiple bandpasses.
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