Galaxy Number Counts in Two Colors From Digitized POSS-II: Evidence for Galaxy Evolution at Low Redshifts?

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Session 71 -- Galaxy and CBR Distribution
Display presentation, Friday, January 14, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[71.02] Galaxy Number Counts in Two Colors From Digitized POSS-II: Evidence for Galaxy Evolution at Low Redshifts?

N.Weir, S.Djorgovski (Caltech), U.Fayyad (JPL)

We have developed a software system call SKICAT for the reduction and analysis of the Palomar-STScI Digital Sky Survey (DPOSS). As a part of the initial scientific verification tests of the system, we measured star and galaxy counts over a limited area near the North Galactic Pole, covering 4 adjacent Survey fields, or $\sim 100$ square degrees. The counts are in two bands, from photographic IIIa-J plates, calibrated to Gunn $g$, and IIIa-F plates, calibrated to Gunn $r$. Extensive CCD calibration has been obtained over this area, with resulting photometric zero-point errors of a few percent r.m.s., both across the individual plates and from plate to plate. The counts reach to an equivalent magnitude of $B \sim 21^m$; approximately 200,000 galaxies are detected down to this flux level. The limit is imposed by the accuracy of our star-galaxy classification, which is correct at about 90\% level or better at this magnitude. The observed power-law slopes of the counts are 0.49 dex/mag in the $g$ band, and 0.46 dex/mag in the $r$ band. Our counts in the $g$ band are consistent with, but slightly lower than, the counts published by the APM group (Maddox et al. 1990) in the $b_J$ band for the Southern sky. Both are significantly in excess over a ``no-evolution'' model in the $B$ band. Our counts in the $r$ band also show an excess over a no-evolution model, reaching a factor of 2 at the limit of our data. The $r$ band is less sensitive to sporadic bursts of star formation and extinction effects than the $g$ or $b_J$ bands. Just like the APM group, we interpret these excess counts at relatively bright magnitudes as possible evidence for field galaxy evolution at low redshifts. However, the data have yet to be tested against newer models which attempt to reconcile photometric and redshift observations in a consistent, no-evolution framework. Therefore, this result must still be considered very preliminary.

We acknowledge a partial support from the NASA contract NAS5-31348, the NSF PYI award AST-9157412, the Caltech President's fund, and JPL. Work at JPL is performed under a contract with the NASA.

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