Extragalactic Source Counts at 24 Microns in the <i>Spitzer</i> First Look Survey

We present the Spitzer Multiband Imaging Photometer 24 m 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 m 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 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 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 m 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 m deep counts.