What determines the shape of the local (z < 0.1) infrared galaxy luminosity function?

Abstract

We investigate what shapes the infrared luminosity function of local galaxies by comparing it to the local infrared active galactic nucleus (AGN) luminosity function. The former corresponds to emission from dust heated by stars and AGN, whereas the latter includes emission from AGN-heated dust only. Our results show that infrared emission from AGN starts mixing into the galaxy luminosity function in the luminous infrared galaxy (LIRG) regime and becomes significant in the ultraluminous infrared galaxy (ULIRG) regime, with the luminosity above which local ULIRGs become AGN-dominated being in the log LIRL ∼ 12.2-12.7 range. We propose that as a result of the AGN contribution, the infrared galaxy luminosity function has a flatter high-luminosity slope than UV/optical galaxy luminosity functions. Furthermore, we note that the increased AGN contribution as a function of L IR is reflected in the average dust temperature (T dust) of local galaxies, and may be responsible for the local L IR -T dust relation. However, although our results show that AGN play a central role in defining the properties of local ULIRGs, we find that the dominant power source in the local ULIRG population is star formation.