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 (AGNs) 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 [Ne v], [O IV], and [Ne II] fine-structure gas emission lines, the 6.2 mu m polycyclic aromatic hydrocarbon (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 AGNs in all our sources suggests that AGNs supply similar to 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 starburst LIRGs. However, we find no linear correlations between these properties and f(AGN). We used the IRAC colors of LIRGs to confirm that finding AGNs on the basis of their MIR colors may miss similar to 40% of AGN-dominated (U)LIRGs.