The role of star-formation and AGN in dust heating of z = 0.3-2.8 galaxies - I. Evolution with redshift and luminosity

TL;DR

This study analyzes the infrared spectral energy distributions of 343 luminous infrared galaxies at redshifts 0.3-2.8, decomposing their emission to understand the roles of star formation and AGN activity in dust heating over cosmic time.

Contribution

It introduces a method to decompose IR spectra into star-formation and AGN components, providing new templates and diagnostics for understanding dust heating and AGN influence in distant galaxies.

Findings

AGN contribution to IR luminosity increases with AGN luminosity.

Over 40% of IR-selected galaxies host an AGN, even at faint flux levels.

Warm dust temperature correlates with AGN luminosity.

Abstract

We characterize infrared spectral energy distributions of 343 (Ultra) Luminous Infrared Galaxies from $z=0.3-2.8$. We diagnose the presence of an 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), AGN, 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 $\mu$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 $L_{\rm IR}$. AGN, 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 $\mu$Jy). Our decomposition technique and color diagnostics are relevant given upcoming observations with the James Webb Space Telescope.