The Nature of Faint 24 Micron Sources Seen in Spitzer Space Telescope Observations of ELAIS‐N1

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Abstract
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 [1011 L⊙ < L8-1000 μm < 1012 L⊙] in the redshift range 0.5 < z < 2.5 constitute greater than 60% of 24 μm sources seen in the flux range 20 μJy < Sν < 1000 μJy. 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.
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