Luminous Infrared Galaxies with the Submillimeter Array. V. Molecular Gas in Intermediate to Late-stage Mergers

We present new high-resolution ALMA ((CO)-C-13 J = 1-0 and J = 2-1) and CARMA ((CO)-C-12 and (CO)-C-13 J = 1-0) observations of two luminous infrared galaxies (LIRGs), Arp 55 and NGC 2623. The new data are complementary to published and archival submillimeter array observations of (CO)-C-12 J = 2-1 and J = 3-2. We perform a Bayesian likelihood non-local thermodynamic equilibrium analysis to constrain the molecular gas physical conditions such as temperature, column, and volume densities and the [(CO)-C-12]/[(CO)-C-13] abundance ratio. For Arp 55, an early/intermediate-staged merger, the line measurements are consistent with cold (similar to 10-20 K), dense (>10(3.5) cm(-3)) molecular gas. For NGC 2623, the molecular gas is warmer (similar to 110 K) and less dense (similar to 10(2.7) cm(-3)). Because Arp. 55 is an early/intermediate stage merger, while NGC 2623 is a merger remnant, the difference in physical conditions may be an indicator of merger stage. Comparing the temperature and volume density of several LIRGs shows that the molecular gas, averaged over similar to kiloparsec scales, of advanced mergers is in general warmer and less dense than early/intermediate stage mergers. We also find that the [(CO)-C-12]/[(CO)-C-13] abundance ratio of NGC 2623 is unusually high (>250) when compared with the Milky Way; however, it follows a trend seen with other LIRGs in the literature. This high [(CO)-C-12]/[(CO)-C-13] value is very likely due to stellar nucleosynthesis enrichment of the interstellar medium. On the other hand, Arp. 55 has a more Galactic [(CO)-C-12]/[(CO)-C-13] value with the most probable [(CO)-C-12]/[(CO)-C-13] value being 20-30. We measure the CO-to-H-2 conversion factor, alpha(CO), to be similar to 0.1 and similar to 0.7 (3 x 10(-4)/x(CO))M circle dot (K km s(-1) pc(2))(-1) for Arp 55 and NGC 2623, respectively. Because Arp 55 is an early/intermediate-stage merger, this suggests that the transition from a Galactic conversion factor to a LIRG value happens at an even earlier merger stage.