2013ApJ...777..126S

We present high-resolution (∼2.''5) observations of ¹²CO J = 6-5 toward the luminous infrared galaxy VV 114 using the Submillimeter Array. We detect ¹²CO J = 6-5 emission from the eastern nucleus of VV 114 but do not detect the western nucleus or the central region. We combine the new ¹²CO J = 6-5 observations with previously published or archival low-J CO observations, which include ¹³CO J = 1-0 Atacama Large Millimeter/submillimeter Array cycle 0 observations, to analyze the beam-averaged physical conditions of the molecular gas in the eastern nucleus. We use the radiative transfer code RADEX and a Bayesian likelihood code to constrain the temperature (T_kin), density (n_H2), and column density (N₁₂CO) of the molecular gas. We find that the most probable scenario for the eastern nucleus is a cold (T_kin= 38 K), moderately dense (n_H2 = 10^2.89/cm3) molecular gas component. We find that the most probable ¹²CO to ¹³CO abundance ratio ([¹²CO]/[¹³CO]) is 229, which is roughly three times higher than the Milky Way value. This high abundance ratio may explain the observed high ¹²CO/ ¹³CO line ratio (>25). The unusual ¹³CO J = 2-1/J = 1-0 line ratio of 0.6 is produced by a combination of moderate ¹³CO optical depths (τ = 0.4-1.1) and extremely subthermal excitation temperatures. We measure the CO-to-H₂conversion factor, α_CO, to be 0.5_–0.3^+0.6 M_☉(K.km/s.pc2)^–1, which agrees with the widely used factor for ultra luminous infrared galaxies of Downes & Solomon (α_CO= 0.8 M_☉(K.km/s.pc2)^–1).