Molecular gas in luminous infrared galaxies

Abstract

CO (1 -> 0) emission profiles are presented for 60 infrared luminous, [L-ir_(8-1000 microns) = 5 x 10^10^-3 x 10^12^ L_sun_, H_0_ = 75 km s^-1^ Mpc^-1^], bright (F_60 micron_ > 5.24 Jy), IRAS galaxies at redshifts 1500-25,000 km s^-1^ Most of these galaxies are sufficiently distant that a single 55" beam measures the total CO luminosity and hence the total molecular gas content. The data for these galaxies are analyzed together with that for a sample of 29 closer IRAS bright galaxies with L_ir_ down to 2 x 10^10^ L_sun_ mapped by Tinney et al. (1990). All of the IRAS bright galaxies are extremely rich in molecular gas with M(H_2_) = 1 x 10^9^-6 x 10^10^ M_sun_, corresponding to 0.5-30 times the mass of H_2_ gas in the Milky Way. The dust masses are evaluated from the IRAS data and the mean ratio of H_2_ gas to "warm dust" is 540 +/- 290. Including the mass of heavy elements and the contribution from H I implies a mean gas-to-dust ratio in the range 900-1100. The discrepancy between this value and the standard Galactic gas-to-dust ratio of ~ 150 could be due to the presence of large quantities of cold dust to which IRAS is insensitive, and/or errors in the assumed dust mass absorption coefficient. The infrared luminosity-to-H_2_ mass ratios, L_ir_/M(H_2_), range from 2-220 L_sun_ M^-1^_sun_, up to a factor of 55 times the mean ratio for molecular clouds in the Galaxy. The ratio L_ir_/M(H_2_) increases with increasing L_ir_, but shows no systematic dependence on M(H_2_). The higher values of L_ir_/M(H_2_) obtained in the most luminous galaxies suggests either an increased star formation efficiency and/or an additional source of luminosity. The kinetic energy of colliding galaxies is ruled out as a significant source of the luminosity in view of the independence of the L_ir_/M(H_2_) ratios on the galactic velocity dispersions and the merging time scales which are too long. If the excess luminosity is supplied by star formation, the implied lifetime of the interstellar medium is <= 3 x 10^8^ yr. However, a significant number of the galaxies with the highest L_ir_/M(H_2_) ratios exhibit non-thermal optical emission lines characteristic of an active galactic nucleus (AGN), and an AGN appears to be an important energy source for these galaxies.