MIR Diagnostics and Warm Molecular Gas in Local Luminous Infrared Galaxies in the GOALS sample

Andreea Petric (Caltech)


We present spectra taken with the Infrared Spectrograph on Spitzer covering 5-38 microns region of 248 nuclei of luminous infrared galaxies (LIRGs) found as part of the Great Observatories All-sky LIRG Survey (GOALS) to isolate the AGN versus starburst contribution to the MIR emission and determine the faith of the molecular warm and cold gas along a merger sequence of LIRGs. The high ionization lines [NeV] and [OIV] suggest that only 2% of LIRGs are AGN dominated. The EQWs of the 6.2 PAH feature indicate that 16% of sources are AGN dominated. The 6.2 micron PAH EQWs are used to estimate that AGNs in LIRGs contribute no more than 14% to the total IR emission of local LIRGs. We use observations of the pure rotational transitions of molecular hydrogen S0 (28.22 microns), S1 (17.04 microns) , S2 (12.28 microns) and S3 (9.67 microns) used to determine the mass and temperature of the warm gas. We detect the H2 S0, S2 and S3 transitions in about 30-40% or all sources and S1 in 94% of our objects. We find a wide range of ratios of the H2 to the IR luminosities between ~7e-5 and 1.7e-3 with a median of 2e-4. We find a weak correlation between the warm H2 mass and IR luminosity. The derived H2 temperatures seem to change with merger stage in that advanced mergers appear to have higher excitation temperatures. We compare the results obtained for the GOALS sources to similar studies of nearby galaxies and ULIRGs.