Spatial Variations of Galaxy Number Counts in the Sloan Digital Sky Survey. I. Extinction, Large-Scale Structure, and Photometric Homogeneity

Abstract

We study the spatial variation of galaxy number counts using five-band photometric images from the Sloan Digital Sky Survey (SDSS). The spatial variation of this sample of 4.3 million galaxies collected from 2200 deg 2 can be understood as the combination of Galactic extinction and large-scale clustering. With the use of the reddening map of Schlegel, Finkbeiner, & Davis, the standard extinction law is verified for the color bands from u to z within 5% in the region of small extinction values, E ( B - V ) < 0.15. The residual spatial variation of the number counts suggests that the error in the global calibration for SDSS photometry is smaller than 0.02 mag.