Spitzer Mid‐ to Far‐Infrared Flux Densities of Distant Galaxies

Abstract

We study the infrared (IR) properties of high-redshift galaxies using deep Spitzer 24, 70, and 160 micron data. Our primary interest is to improve the constraints on the total IR luminosities, L(IR), of these galaxies. We combine the Spitzer data in the southern Extended Chandra Deep Field with a K-band-selected galaxy sample and photometric redshifts from the Multiwavelength Survey by Yale-Chile. We used a stacking analysis to measure the average 70 and 160 micron flux densities of 1.5 < z < 2.5 galaxies as a function of 24 micron flux density, X-ray activity, and rest-frame near-IR color. Galaxies with 1.5 < z < 2.5 and S(24)=53-250 micro-Jy have L(IR) derived from their average 24-160 micron flux densities within factors of 2-3 of those derived from the 24 micron flux densities only. However, L(IR) derived from the average 24-160 micron flux densities for galaxies with S(24) > 250 micro-Jy and 1.5 < z < 2.5 are lower than those derived using only the 24 micron flux density by factors of 2-10. Galaxies with S(24) > 250 micro-Jy have S(70)/S(24) flux ratios comparable to sources with X-ray detections or red rest-frame IR colors, suggesting that warm dust possibly heated by AGN may contribute to the high 24 micron emission. Based on the average 24-160 micron flux densities, nearly all 24 micron-selected galaxies at 1.5 < z < 2.5 have L(IR) < 6 x 10^12 solar luminosities, which if attributed to star formation corresponds to < 1000 solar masses per year. This suggests that high redshift galaxies may have similar star formation efficiencies and feedback processes as local analogs. Objects with L(IR) > 6 x 10^12 solar luminosities are quite rare, with a surface density ~ 30 +/- 10 per sq. deg, corresponding to ~ 2 +/- 1 x 10^-6 Mpc^-3 over 1.5 < z < 2.5.