The Herschel-ATLAS: Extragalactic Number Counts from 250 to 500 Microns
  Aims. The Herschel-ATLAS survey (H-ATLAS) will be the largest area survey to
be undertaken by the Herschel Space Observatory. It will cover 550 sq. deg. of
extragalactic sky at wavelengths of 100, 160, 250, 350 and 500 microns when
completed, reaching flux limits (5 sigma) from 32 to 145mJy. We here present
galaxy number counts obtained for SPIRE observations of the first ~14 sq. deg.
observed at 250, 350 and 500 microns. Methods. Number counts are a fundamental
tool in constraining models of galaxy evolution. We use source catalogs
extracted from the H-ATLAS maps as the basis for such an analysis. Correction
factors for completeness and flux boosting are derived by applying our
extraction method to model catalogs and then applied to the raw observational
counts. Results. We find a steep rise in the number counts at flux levels of
100-200mJy in all three SPIRE bands, consistent with results from BLAST. The
counts are compared to a range of galaxy evolution models. None of the current
models is an ideal fit to the data but all ascribe the steep rise to a
population of luminous, rapidly evolving dusty galaxies at moderate to high
redshift.
