Mid-Infrared Spectral Diagnostics of Luminous Infrared Galaxies

We present a statistical analysis of 248 luminous infrared galaxies (LIRGs) which comprise the Great Observatories All-sky LIRG Survey (GOALS) observed with the Infrared Spectrograph (IRS) on-board Spitzer in the rest-frame wavelength range between 5 and 38 µm. The GOALS sample enables a direct measurement of the relative contributions of star-formation and active galactic nuclei (AGN) to the total infrared (IR) emission from a large, statistically complete sample of LIRGs in the local Universe.Several diagnostics effective at isolating the AGN contribution to the Mid-infrared (MIR) emission using [NeV], [OIV] and [NeII] gas emission lines, the 6.2 µm PAH equivalent width (EQW) and the shape of the MIR continuum are compared. The [NeV] line which indicates the presence of an AGN is detected in 22% of all LIRGs. The 6.2 µm PAH EQW, [NeV]/L_(IR), [NeV]/[NeII] and [OIV]/[NeII] ratios, and the ratios of 6.2 µm PAH flux to the integrated continuum flux between 5.3 and 5.8 µm suggest values of around 10% for the fractional AGN contribution to the total IR luminosity of LIRGs. The median of these estimates suggests that for local LIRGs the fractional AGN contribution to the total IR luminosity is ~12%. AGN dominated LIRGs have higher global and nuclear IR luminosities, warmer MIR colors and are interacting more than starburst (SB) dominated LIRGs. However there are no obvious linear correlations between these properties, suggesting that none of these properties alone can determine the activity and evolution of an individual LIRG. A study of the IRAC colors of LIRGs confirms that methods of finding AGN on the basis of their MIR colors are effective at choosing AGN but 50% to 40% of AGN dominated LIRGs are not selected as such with these methods