Physical Conditions in the LMC's Quiescent Molecular Ridge: Fitting Non-LTE Models to CO Emission

TL;DR

This study uses new high-resolution ALMA and APEX observations combined with non-LTE modeling to accurately determine the physical conditions of molecular gas in the quiescent Molecular Ridge of the LMC, revealing links to star formation activity.

Contribution

The paper introduces a novel multi-line non-LTE fitting method that improves the accuracy of physical parameter estimates in molecular clouds compared to traditional LTE and X_CO methods.

Findings

Fitted H2 densities correlate with YSO presence and mass.

The new method reduces effects of optical depth and line-of-sight projection.

Traditional star formation diagnostics show weak correlation with YSO properties.

Abstract

The Molecular Ridge in the LMC extends several kiloparsecs south from 30 Doradus, and it contains ~30% of the molecular gas in the entire galaxy. However, the southern end of the Molecular Ridge is quiescent - it contains almost no massive star formation, which is a dramatic decrease from the very active massive star-forming regions 30 Doradus, N159, and N160. We present new ALMA and APEX observations of the Molecular Ridge at a resolution as high as ~16'' (~3.9 pc) with molecular lines 12CO(1-0), 13CO(1-0), 12CO(2-1), 13CO(2-1), and CS(2-1). We analyze these emission lines with our new multi-line non-LTE fitting tool to produce maps of T_kin, n_H2, and N_CO across the region based on models from RADEX. Using simulated data for a range of parameter space for each of these variables, we evaluate how well our fitting method can recover these physical parameters for the given set of molecular lines. We then compare the results of this fitting with LTE and X_CO methods of obtaining mass estimates and how line ratios correspond with physical conditions. We find that this fitting tool allows us to more directly probe the physical conditions of the gas and estimate values of T_kin, n_H2, and N_CO that are less subject to the effects of optical depth and line-of-sight projection than previous methods. The fitted n_H2 values show a strong correlation with the presence of YSOs, and with the total and average mass of the associated YSOs. Typical star formation diagnostics, such as mean density, dense gas fraction, and virial parameter do not show a strong correlation with YSO properties.