  We investigate what shapes the infrared luminosity function of local galaxies
by comparing it to the local infrared 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(LIR/Lsun)~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 LIR is reflected in the average dust temperature (Tdust) of
local galaxies, and may be responsible for the local LIR-Tdust 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.
