Luminous infrared galaxies with the Submillimeter Array. IV. 12CO J = 6-5 observations of VV 114

Abstract

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 (Tkin), 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 (nH2= 102.89 cm-3) 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-H2 conversion factor, αCO, to be 0.5 M⊙ (K km s-1 pc 2)-1, which agrees with the widely used factor for ultra luminous infrared galaxies of Downes & Solomon (αCO = 0.8 M⊙ (K km s-1 pc2)-1). © 2013. The American Astronomical Society. All rights reserved.