Ameliorating systematic uncertainties in the angular clustering of galaxies: a study using the SDSS-III

Abstract

We investigate the effects of potential sources of systematic error on the\nangular and photometric redshift, z_phot, distributions of a sample of redshift\n0.4 < z < 0.7 massive galaxies whose selection matches that of the Baryon\nOscillation Spectroscopic Survey (BOSS) constant mass sample. Utilizing over\n112,778 BOSS spectra as a training sample, we produce a photometric redshift\ncatalog for the galaxies in the SDSS DR8 imaging area that, after masking,\ncovers nearly one quarter of the sky (9,913 square degrees). We investigate\nfluctuations in the number density of objects in this sample as a function of\nGalactic extinction, seeing, stellar density, sky background, airmass,\nphotometric offset, and North/South Galactic hemisphere. We find that the\npresence of stars of comparable magnitudes to our galaxies (which are not\ntraditionally masked) effectively remove area. Failing to correct for such\nstars can produce systematic errors on the measured angular auto-correlation\nfunction, w, that are larger than its statistical uncertainty. We describe how\none can effectively mask for the presence of the stars, without removing any\ngalaxies from the sample, and minimize the systematic error. Additionally, we\napply two separate methods that can be used to correct the systematic errors\nimparted by any parameter that can be turned into a map on the sky. We find\nthat failing to properly account for varying sky background introduces a\nsystematic error on w. We measure w, in four z_phot slices of width 0.05\nbetween 0.45 < z_phot < 0.65 and find that the measurements, after correcting\nfor the systematic effects of stars and sky background, are generally\nconsistent with a generic LambdaCDM model, at scales up to 60 degrees. At\nscales greater than 3 degrees and z_phot > 0.5, the magnitude of the\ncorrections we apply are greater than the statistical uncertainty in w.\n