2004ApJS..154...80C

The Spitzer Space Telescope has undertaken the deepest ever observations of the 24 µm sky in the European Large Area ISO Survey (ELAIS-N1) field, with the Multiband Imaging Photometer (MIPS) instrument, as part of GOODS Science Verification observations. We present completeness-corrected extragalactic source counts down to 24 µm flux densities of 20 µJy (30% completeness), a factor of 10,000 more sensitive than IRAS. The shape of the counts confirms model predictions for a strong evolution of the infrared luminosity function between redshifts of 0 and 1 and suggests a flattening in the evolutionary parameters at higher redshifts. Models that fit the counts indicate that luminous infrared galaxies [10¹¹L_☉<L_8–1000µm<10¹²L_☉] in the redshift range 0.5<z<2.5 constitute greater than 60% of 24 µm sources seen in the flux range 20µJy<S_ν<1000µJy. At least 85%, and possibly all, of the 24 µm sources have a counterpart in the Infrared Array Camera (IRAC) 3.6 and 4.5 µm near-infrared channels, consistent with the expected spectral energy distribution of infrared luminous galaxies at moderate redshift. The similarity between the observed mid-infrared to near-infrared flux ratios of the Spitzer-detected sources and the 15 µm/HK-band flux ratios of the ISOCAM 15 µm sources seen in the Hubble Deep Field-North strongly suggests that faint 24 µm sources are high-redshift analogs of ISOCAM 15 µm sources and that they have the potential to provide an evolutionary connection between the well-studied z∼3 Lyman break galaxy population and the dusty starburst galaxies seen at z∼1.