The Nature of Faint 24 μ m sources Seen in Spitzer Observations of ELAIS - N 1

The Spitzer Space Telescope has undertaken the deepest ever observations of the 24µm sky in the ELAIS-N1 field with the MIPS instrument as part of GOODS Science Verification observations. We present completeness corrected extragalac-tic source counts down to 24µm flux densities of 20µJy (30% completeness), a factor of 10000 more sensitive than IRAS. The shape of the counts confirms model predictions for a strong evolution of the infrared luminosity function between redshift 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 60% of 24µm sources seen in the flux range 20<S ν < 1000 µJy. At least 85%, and possibly all of the 24µm sources have a counterpart in the IRAC 3.6µm and 4.5µm 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 15µm/HK-band flux ratios of the ISOCAM 15µm sources seen in the HDF-N strongly suggests that faint 24µm sources are high-redshift analogs of ISOCAM 15µm 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.