HerMES: SPIRE galaxy number counts at 250, 350, and 500 <i>μ</i>m

Emission at far-infrared wavelengths makes up a significant fraction of the total light detected from galaxies over the age of Universe. <i>Herschel<i/> provides an opportunity for studying galaxies at the peak wavelength of their emission. Our aim is to provide a benchmark for models of galaxy population evolution and to test pre-existing models of galaxies. With the <i>Herschel<i/> Multi-tiered Extra-galactic survey, HerMES, we have observed a number of fields of different areas and sensitivity using the SPIRE instrument on <i>Herschel<i/>. We have determined the number counts of galaxies down to ~20 mJy. Our constraints from directly counting galaxies are consistent with, though more precise than, estimates from the BLAST fluctuation analysis. We have found a steep rise in the Euclidean normalised counts <100 mJy. We have directly resolved ~15% of the infrared extra-galactic background at the wavelength near where it peaks.