Spitzer Mid- to Far-Infrared Flux Densities of Distant Galaxies^*

We study the infrared (IR) properties of high-redshift galaxies using deep Spitzer 24, 70, and 160 μm 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_s-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 μm flux densities of 1.5 < z < 2.5 galaxies as a function of 24 μm flux density, X-ray activity, and rest-frame near-IR color. Galaxies with 1.5 < z < 2.5 and S₂₄ = 53-250 μJy have L_IR derived from their average 24-160 μm flux densities within factors of 2-3 of those inferred from the 24 μm flux densities only. However, L_IR derived from the average 24-160 μm flux densities for galaxies with S₂₄ > 250 μJy and 1.5 < z < 2.5 are lower than those inferred using only the 24 μm flux density by factors of 2-10. Galaxies with S₂₄ > 250 μJy have S₇₀/S₂₄ flux ratios comparable to sources with X-ray detections or red rest-frame IR colors, suggesting that warm dust possibly heated by AGNs may contribute to the high 24 μm emission. Based on the average 24-160 μm flux densities, nearly all 24 μm-selected galaxies at 1.5 < z < 2.5 have L_IR < 6 × 10¹² L_☉, which, if attributed to star formation, corresponds to Ψ < 1000 M_☉ yr^-1. This suggests that high-redshift galaxies may have star formation efficiencies and feedback processes similar to those of local analogs. Objects with L_IR > 6 × 10¹² L_☉ are quite rare, with a surface density ~30 ± 10 deg^-2, corresponding to ~2 ± 1 × 10^-6 Mpc^-3 over 1.5 < z < 2.5.