Mid-Infrared Spectral Diagnostics of Luminous Infrared Galaxies
  We present a statistical analysis of the mid-infrared (MIR) spectra of 248
luminous infrared (IR) galaxies (LIRGs) which comprise the Great Observatories
All-sky LIRG Survey (GOALS) observed with the Infrared Spectrograph (IRS)
on-board the Spitzer Space Telescope. The GOALS sample enables a direct
measurement of the relative contributions of star-formation and active galactic
nuclei (AGN) to the total IR emission from a large sample of local LIRGs. The
AGN contribution to the MIR emission (f-AGN) is estimated by employing several
diagnostics based on the properties of the [NeV], [OIV] and [NeII] fine
structure gas emission lines, the 6.2 microns PAH and the shape of the MIR
continuum. We find that 18% of all LIRGs contain an AGN and that in 10% of all
sources the AGN contributes more than 50% of the total IR luminosity. Summing
up the total IR luminosity contributed by AGN in all our sources suggests that
AGN supply ~12% of the total energy emitted by LIRGs. The average spectrum of
sources with an AGN looks similar to the average spectrum of sources without an
AGN, but it has lower PAH emission and a flatter MIR continuum. AGN dominated
LIRGs have higher IR luminosities, warmer MIR colors and are found in
interacting systems more often than pure starbursts LIRGs. However we find no
linear correlations between these properties and f-AGN. We used the IRAC colors
of LIRGs to confirm that finding AGN on the basis of their MIR colors may miss
~40% of AGN dominated (U)LIRGs
