LABOCA observations of giant molecular clouds in the south west region of the Small Magellanic Cloud

TL;DR

This study uses LABOCA observations to compare molecular cloud masses derived from dust emission and virial methods in the Small Magellanic Cloud, revealing a significant mass discrepancy that challenges current understanding of molecular gas tracers at low metallicity.

Contribution

First detailed comparison of dust-based and virial mass estimates for GMCs in the SMC using LABOCA data, highlighting a persistent mass discrepancy at low metallicity.

Findings

Virial masses are on average 4 times smaller than dust-based masses.

The mass discrepancy remains unexplained and differs from Galactic observations.

Dust emission dominates the sub-mm emission in the observed GMCs.

Abstract

The amount of molecular gas is a key for understanding the future star formation in a galaxy. Because H2 is difficult to observe directly in dense and cold clouds, tracers like CO are used. However, at low metallicities especially, CO only traces the shielded interiors of the clouds. mm dust emission can be used as a tracer to unveil the total dense gas masses. The comparison of masses deduced from the continuum SIMBA 1.2 mm emission and virial masses in a sample of giant molecular clouds (GMCs), in the SW region of the Small Magellanic Cloud (SMC), showed a discrepancy that is in need of an explanation. This study aims at better assessing possible uncertainties on the dust emission observed in the sample of GMCs from the SMC and focuses on the densest parts of the GMCs where CO is detected. New observations were obtained with the LABOCA camera on the APEX telescope. All GMCs previously observed in CO are detected and their emission at 870microns is compared to ancillary data. The different contributions to the sub-mmm emission are estimated, as well as dust properties, in order to deduce molecular cloud masses precisely. The (sub-)mm emission observed in the GMCs in the SW region of the SMC is dominated by dust emission and masses are deduced for the part of each cloud where CO is detected and compared to the virial masses. The mass discrepancy between both methods is confirmed at 870microns with the LABOCA observations: the virial masses are on average 4 times smaller than the masses of dense gas deduced from dust emission, contrary to what is observed for equivalent clouds in our Galaxy. The origin of this mass discrepancy in the SMC remains unkown. The direct interpretation of this effect is that the CO linewidth used to compute virial masses do not measure the full velocity distribution of the gas. Geometrical effects and uncertainties on the dust properties are also discussed.