Warm and Cold Molecular Gas in Luminous Infrared Galaxies

Abstract

We present the properties of the warm and cold molecular gas in a sample of local Luminous Infrared Galaxies in the Great Observatories All-sky LIRG Survey (GOALS). The rotational transitions of H_2 observed in the Mid-Infrared Spectra from the Spitzer Survey trace gas at temperatures about 100K, while the 12 CO (1-0) maps acquired with the Combined Array for Millimeter Astronomy identify the morphology of the cold (less than 40K). The 17.1 μm S(1) transitions of warm H_2 is detected in 94% of the 247 LIRG nuclei analyzed. The warm H_2 S(0) to S(3) emission is correlated with the 6.2 μm and the [Si II] cooling line. This suggests that the H_2 is mostly excited in PDR. The large scatter in correlation both for merging and non-merging sources suggests that some of the excitation appear to be due to shocks associated with SNe. This is consistent with the high fractions of [FeII]/[OIV] ≥ 1 we measure in about 20% of the sources and the number of sources with detected S(7) emission. The larger scatter in the PAH to H_2 may also be due to both the presence of an AGN hard ionizing continuum and the the large range of interactions observed in LIRGs (about 50% of LIRGs in GOALs are interacting). The 16 12CO(1-0) maps for LIRGs in a merger sequence show that molecular gas surface densities range between about 65 M_☉/pc^2 to 2600 M_☉/pc^2 with a median and mean value of 290 and 571 M^☉/pc^2 respectively. This suggests that LIRGs have a wide range of molecular gas surfaces but in general a factor of a few greater than normal galaxies and not the extreme values seen typically in Ultraluminous Infrared Galaxies (ULIRGs) or PG QSOs. We find that molecular gas depletion times increase with merger stage but do not appear to vary with stellar mass.

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