THE ROLE OF STAR FORMATION AND AN AGN IN DUST HEATING OF z = 0.3–2.8 GALAXIES. I. EVOLUTION WITH REDSHIFT AND LUMINOSITY

We characterize infrared spectral energy distributions of 343 (ultra)luminous infrared galaxies from z = 0.3–2.8. We diagnose the presence of an active galactic nucleus (AGN) by decomposing individual Spitzer mid-IR spectroscopy into emission from star formation and an AGN-powered continuum; we classify sources as star-forming galaxies (SFGs), AGNs, or composites. Composites comprise 30% of our sample and are prevalent at faint and bright S 24 , making them an important source of IR AGN emission. We combine spectroscopy with multiwavelength photometry, including Herschel imaging, to create three libraries of publicly available templates (2–1000 μ m). We fit the far-IR emission using a two-temperature modified blackbody to measure cold and warm dust temperatures ( T c and T w ). We find that T c does not depend on mid-IR classification, while T w shows a notable increase as the AGN grows more luminous. We measure a quadratic relationship between mid-IR AGN emission and total AGN contribution to L IR . AGNs, composites, and SFGs separate in S 8 / S 3.6 and S 250 / S 24 , providing a useful diagnostic for estimating relative amounts of these sources. We estimate that >40% of IR-selected samples host an AGN, even at faint selection thresholds ( S 24 > 100 μ Jy). Our decomposition technique and color diagnostics are relevant given upcoming observations with the James Webb Space Telescope .