Millimeter Dust Continuum Emission as a Tracer of Molecular Gas in Galaxies

Abstract

Molecular gas in galaxies is the primary fuel of star formation but the exact amount of molecular gas remains unknown since H_2 is not observed directly in cold interstellar regions. CO observations have been so far the best way to trace molecular gas in external galaxies, but in low metallicity environments the gas mass deduced could be largely underestimated due to enhanced photodissociation of CO. In this context, using millimeter dust emission as a dense gas tracer could unveil large H_2 enveloppes in molecular clouds. Mass estimates from millimeter dust emission are compared to virial masses in two giant molecular clouds samples: the local clouds in our Galaxy, and equivalents in the Small Magellanic Cloud. In our Galaxy, virial masses are systematically larger than mass estimates from millimeter emission, confirming previous studies. This is not the case for SMC clouds: molecular gas masses deduced from millimeter observations are systematically higher than the virial masses from CO observations. We show that an additional magnetic field support of the SMC clouds could explain the difference observed.