Far-infrared source counts at 70 and 160 microns in Spitzer deep surveys

We derive galaxy source counts at 70 and 160 mum using the Multiband Imaging Photometer for Spitzer (MIPS) to map the Chandra Deep Field - South (CDF-S) and other fields. At 70 mum, our observations extend upward about 2 orders of magnitude in flux density from a threshold of 15 mJy, and at 160 mum they extend about an order of magnitude upward from 50 mJy. The counts are consistent with previous observations on the bright end. Significant evolution is detected at the faint end of the counts in both bands, by factors of 2 - 3 over no-evolution models. This evolution agrees well with models that indicate that most of the faint galaxies lie at redshifts between 0.7 and 0.9. The new Spitzer data already resolve about 23% of the cosmic far-infrared background at 70 mum and about 7% at 160 mum.