The Stellar Content of the COSMOS Field as Derived from Morphological and SED-based Star/Galaxy Separation

Abstract

We report on the stellar content of the COSMOS two degree field, as derived from a rigorous star-galaxy separation approach developed for using stellar sources to define the point-spread function variation map used in a study of weak galaxy lensing. The catalog obtained in one filter from the ACS (Advanced Camera for Surveys on the Hubble Space Telescope) is cross-identified with ground-based multiwavelength catalogs obtained using the Suprime-Cam instrument on the Subaru Telescope, which makes possible detailed spectral energy distribution (SED) fitting in order to separate stars from QSOs and compact galaxies. The classification is reliable to magnitude F814W=24, and the sample is complete even fainter. We construct a color-magnitude diagram and color histograms and compare them with predictions of a standard model of population synthesis at (l,b)=(236.816^deg,+42.12^deg). We find features corresponding to the halo subdwarf main-sequence turnoff, the thick disk, and the thin disk. We propose improvements to the standard model that give a better fit: this data set provides constraints on the thick disk and spheroid density laws and on the initial mass function at low mass, although complementary lines of sight would help in lifting the degeneracy between model parameters as well as mitigating any variations in the stellar populations. The depth of this survey makes it possible to explore the spheroid up to distances of ~80 kpc; we find no evidence of a sharp spheroid edge out to this distance, which corresponds to a galactocentric radius of 83 kpc. We identify a blue population of white dwarfs with counts that agree with model predictions. We find a hint for a possible slight stellar overdensity at about 22-34 kpc, but the data are not strong enough at present to claim detection of a stream feature in the halo.

Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS5-26555 also based on data collected at: the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan; the XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA member states and NASA; the European Southern Observatory under Large Program 175.A-0839, Chile; Kitt Peak National Observatory, Cerro Tololo Inter-American Observatory, and the National Optical Astronomy Observatory, which are operated by AURA under cooperative agreement with the National Science Foundation; the National Radio Astronomy Observatory, which is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.; and the Canada-France-Hawaii Telescope (CFHT) with MegaPrime/MegaCam operated as a joint project by the CFHT Corporation, CEA/DAPNIA, the National Research Council of Canada, the Canadian Astronomy Data Centre, the Centre National de la Recherche Scientifique de France, TERAPIX, and the University of Hawaii.