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 mum 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 mum flux densities of 20 muJy (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-. < L8-1000 mu m < 10(12) L-.] in the redshift range 0.5< z< 2.5 constitute greater than 60% of 24 mum sources seen in the flux range 20 muJy< S-nu < 1000 muJy. At least 85%, and possibly all, of the 24 mum sources have a counterpart in the Infrared Array Camera (IRAC) 3.6 and 4.5 mum 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 mum/HK-band flux ratios of the ISOCAM 15 mum sources seen in the Hubble Deep Field - North strongly suggests that faint 24 mum sources are high-redshift analogs of ISOCAM 15 mum sources and that they have the potential to provide an evolutionary connection between the well-studied z similar to 3 Lyman break galaxy population and the dusty starburst galaxies seen at z similar to 1.