Mid-Infrared Properties of Luminous Infrared Galaxies II: Probing the
  Dust and Gas Physics of the GOALS Sample
  The Great Observatories All-Sky LIRG Survey (GOALS) is a comprehensive,
multiwavelength study of luminous infrared galaxies (LIRGs) in the local
universe. Here we present the results of a multi-component, spectral
decomposition analysis of the low resolution mid-IR Spitzer IRS spectra from
5-38um of 244 LIRG nuclei. The detailed fits and high quality spectra allow for
characterization of the individual PAH features, warm molecular hydrogen
emission, and optical depths for silicate dust grains and water ices. We find
that starbursting LIRGs, which make up the majority of GOALS, are very
consistent in their MIR properties (i.e. tau_9.7um, tau_ice, neon line and PAH
feature ratios). However, as their PAH EQW decreases, usually an indicator of
an increasingly dominant AGN, LIRGs cover a larger spread in these MIR
parameters. The contribution from PAHs to the total L(IR) in LIRGs varies from
2-29% and LIRGs prior to their first encounter show higher L(PAH)/L(IR) ratios
on average. We observe a correlation between the strength of the starburst
(IR8) and the PAH fraction at 8um but not with the 7.7 to 11.3 PAH ratio,
suggesting the fractional PDR emission, and not the overall grain properties,
is associated with the rise in IR8 for galaxies off the starburst main
sequence. We detect crystalline silicate features in 6% of the sample but only
in the most obscured sources (s_9.7um < -1.24). Ice absorption features are
observed in 11% (56%) of GOALS LIRGs (ULIRGs). Most GOALS LIRGs have
L(H2)/L(PAH) ratios elevated above those observed for normal star-forming
galaxies and exhibit a trend for increasing L(H2)/L(PAH) ratio with increasing
L(H2). While star formation appears to be the dominant process responsible for
exciting the H2 in most of the GOALS galaxies, a subset of LIRGs (10%) show
excess H2 emission that is inconsistent with PDR models and may be excited by
shocks or AGN-induced outflows.
