GALAXY COUNTS AT 24 mum IN THE SWIRE FIELDS

This paper presents galaxy source counts at 24 microns in the six Spitzer Wide-field InfraRed Extragalactic (SWIRE) fields. The source counts are compare d to counts in other fields, and to model predictions that have been updated since the launch of Spitzer . This analysis confirms a very steep rise in the Euclidean-normalized differential number count s between 2 mJy and 0.3 mJy. Variations in the counts between fields show the effects of sample variance i n the flux range 0.5-10 mJy, up to 100% larger than Poisson errors. Nonetheless, a “shoulder†in th e normalized counts persists at around 3 mJy. The peak of the normalized counts at 0.3 mJy is higher and narrower than most models predict. In the ELAIS N1 field, the 24 micron data are combined with Spitzer -IRAC data and five-band optical imaging, and these bandmerged data are fit with photometric r edshift templates. Above 1 mJy the counts are dominated by galaxies at z < 0 . 3. By 300 microJy, about 25% are between z ∼ 0.3-0.8, and a significant fraction are at z ∼ 1.3-2. At low redshifts the counts are dominated by spirals, and starbursts rise in number density to outnumber the spirals’ contribution to the counts below 1 mJy.