Extragalactic source counts at 24 microns in the Spitzer First Look Survey

We present the Spitzer Multiband Imaging Photometer 24 mum source counts in the Extragalactic First Look Survey (FLS) main, verification, and European Large Area ISO Survey N1 fields. Spitzer's increased sensitivity and efficiency in large areal coverage over previous infrared telescopes, coupled with the enhanced sensitivity of the 24 mum band to sources at intermediate redshift, dramatically improve the quality and statistics of number counts in the mid-infrared. The FLS observations cover areas of 4.4, 0.26, and 0.015 deg(2), respectively, and reach 3 sigma depths of 0.11, 0.08, and 0.03 mJy. The extragalactic counts derived for each survey agree remarkably well. The counts can be fitted by a super-Euclidean power law of index alpha = - 2.9 from 0.2 to 0.9 mJy, with a flattening of the counts at fluxes fainter than 0.2 mJy. Comparison with infrared galaxy evolution models reveals a peak's displacement in the 24 mum counts. This is probably due to the detection of a new population of galaxies with redshift between 1 and 2, previously unseen in the 15 mum deep counts.