Luminous Infrared Galaxies With the Submillimeter Array. IV. 12CO J=6-5 Observations of VV 114

TL;DR

This study uses high-resolution submillimeter observations to analyze the molecular gas properties in the eastern nucleus of VV 114, revealing a cold, moderately dense gas component with a high CO to 13CO abundance ratio and a low CO-to-H2 conversion factor.

Contribution

First detailed high-resolution 12CO J=6-5 observations of VV 114's eastern nucleus combined with multi-line data to constrain molecular gas conditions and abundance ratios.

Findings

Detected 12CO J=6-5 emission only in the eastern nucleus.

Found a cold (38 K), moderately dense (10^2.89 cm^-3) molecular gas component.

Measured a CO-to-H2 conversion factor of 0.5, consistent with ULIRG standards.

Abstract

We present high-resolution (~2.5") observations of 12CO J=6-5 towards 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, that 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 (N(12CO)) 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 cm^-3) molecular gas component. We find the most probable 12CO to 13CO abundance ratio ([12CO]/[13CO]) is 229, 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 (tau = 0.4 - 1.1) and extremely subthermal excitation temperatures. We measure the CO-to-H2 conversion factor, alpha_co to be 0.5 M_sol (K km s^-1 pc^2)^-1, which agrees with the widely used factor for ultra luminous infrared galaxies of Downes & Solomon (1998; alpha_co =0.8 M_sol (K km s^-1 pc^2)^-1).