A Herschel Spectroscopic Survey of Warm Molecular Gas in Local Luminous Infrared Galaxies

We describe an on-going Herschel 200–700 μm spectroscopic survey of a flux-limited sample of 125 local luminous infrared galaxies (LIRGs), targeting primarily at CO line emission from warm and dense molecular gas. The program will provide important statistical data on the interplay between warm molecular gas, IR luminosity, star formation rate and efficiency, AGN, and the diverse properties of LIRGs. Of the 30 sample galaxies observed so far (18 by us; 12 taken from Herschel archive), about 15% show a dominant or significant hot CO gas component emitting at J > 10, that is likely heated by AGN. The other 85% is dominated by a warm gas component with CO line emission peaking at J ≤ 8, likely powered by starburst. While the spectral shapes of the warm gas component show little overall dependence on the total IR luminosity L_(IR), the relative contribution from the hot component appears to correlate positively with L_(IR). The tightest one-to-one correlation between CO line luminosity and L_(IR) seems to be for CO(7-6), implying that the bulk of L_(IR) should be correlated with warm molecular gas of such density and temperature that its CO line emission peaks around J ∼ 7