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

September 2004 • 2004ApJS..154...80C

Authors • Chary, R. • Casertano, S. • Dickinson, M. E. • Ferguson, H. C. • Eisenhardt, P. R. M. • Elbaz, D. • Grogin, N. A. • Moustakas, L. A. • Reach, W. T. • Yan, H.

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 [10¹¹L_solar<L_8-1000μm<10¹²L_solar] 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.

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Ranga-Ram Chary

Senior Scientist