A Far-infrared Characterization of 24 µm Selected Galaxies at 0 < z < 2.5 using Stacking at 70 µm and 160 µm in the COSMOS Field

Abstract

We present a study of the average properties of luminous infrared galaxies detected directly at 24 μm in the COSMOS field using a median stacking analysis at 70 μm and 160 μm. Over 35,000 sources spanning 0 <= z <= 3 and 0.06 mJy <=S ₂₄ <= 3.0 mJy are stacked, divided into bins of both photometric redshift and 24 μm flux. We find no correlation of S ₇₀/S ₂₄ flux density ratio with S ₂₄, but find that galaxies with higher S ₂₄ have a lower S ₁₆₀/S ₂₄ flux density ratio. These observed ratios suggest that 24 μm selected galaxies have warmer spectral energy distributions (SEDs) at higher mid-IR fluxes, and therefore have a possible higher fraction of active galactic nuclei. Comparisons of the average S ₇₀/S ₂₄ and S ₁₆₀/S ₂₄ colors with various empirical templates and theoretical models show that the galaxies detected at 24 μm are consistent with "normal" star-forming galaxies and warm mid-IR galaxies such as Mrk 231, but inconsistent with heavily obscured galaxies such as Arp 220. We perform a χ² analysis to determine best-fit galactic model SEDs and total IR luminosities for each of our bins. We compare our results to previous methods of estimating L _IR and find that previous methods show considerable agreement over the full redshift range, except for the brightest S ₂₄ sources, where they overpredict the bolometric IR luminosity at high redshift, most likely due to their warmer dust SED. We present a table that can be used as a more accurate and robust method for estimating bolometric infrared luminosity from 24 μm flux densities.