2017ApJ...840....8S

We present new high-resolution ALMA (¹³CO J = 1-0 and J = 2-1) and CARMA (¹²CO and ¹³CO 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 ¹²CO 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 [¹²CO]/[¹³CO] abundance ratio. For Arp 55, an early/intermediate-staged merger, the line measurements are consistent with cold (∼10-20 K), dense (>10^3.5 cm^–3) molecular gas. For NGC 2623, the molecular gas is warmer (∼110 K) and less dense (∼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 ∼kiloparsec scales, of advanced mergers is in general warmer and less dense than early/intermediate stage mergers. We also find that the [¹²CO]/[¹³CO] 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 [¹²CO]/[¹³CO] value is very likely due to stellar nucleosynthesis enrichment of the interstellar medium. On the other hand, Arp 55 has a more Galactic [¹²CO]/[¹³CO] value with the most probable [¹²CO]/[¹³CO] value being 20-30. We measure the CO-to-H₂ conversion factor, α_CO, to be ∼0.1 and ∼0.7 (3 x 10^–4/x_CO) M_☉ (K km s^–1 pc²)^–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.