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-infrared (MIR) Spitzer Infrared Spectrograph spectra from 5-38 μm of 244 LIRG nuclei. The detailed fits and high-quality spectra allow for characterization of the individual polycyclic aromatic hydrocarbon (PAH) features, warm molecular hydrogen emission, and optical depths for both silicate dust grains and water ices. We find that starbursting LIRGs, which make up the majority of the GOALS sample, are very consistent in their MIR properties (i.e., τ_{9.7 μm}, τ_ice, neon line ratios, and PAH feature ratios). However, as their EQW_{6.2 μm} decreases, usually an indicator of an increasingly dominant active galactic nucleus (AGN), LIRGs cover a larger spread in these MIR parameters. The contribution from PAH emission to the total IR luminosity (L(PAH)/L(IR)) in LIRGs varies from 2%-29% and LIRGs prior to their first encounter show significantly higher L(PAH)/L(IR) ratios on average. We observe a correlation between the strength of the starburst (represented by IR8 = L _IR/L _{8 μm}) and the PAH fraction at 8 μm but no obvious link between IR8 and the 7.7 to 11.3 PAH ratio, suggesting that the fractional photodissociation region (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 obscure sources (s _{9.7 μm} < -1.24). Ice absorption features are observed in ~11% (56%) of GOALS LIRGs (ULIRGs) in sources with a range of silicate depths. 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%) shows excess H₂ emission that is inconsistent with PDR models and may be excited by shocks or AGN-induced outflows.