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

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 [10 11 L ⊙ < L 8-1000 μm < 10 12 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 < S ν < 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.