ALMA Observations of the Molecular Clouds in NGC 625
Abstract
We present the highest resolution (1") 12CO(1-0) observations of molecular gas in the dwarf starburst galaxy NGC 625, to date, obtained with ALMA. Molecular gas is distributed in discrete clouds within an area of 0.4 kpc2 and does not have well-ordered large-scale motions. We measure a total molecular mass in NGC 625 of 5.3× 106 M, assuming a Milky Way CO-to-H2 conversion factor. We use the CPROPS package to identify molecular clouds and measure their properties. The 19 resolved CO clouds have a median radius of 20 pc, a median line width 2.5 km s-1, and a median surface density of 169 M-2. Larson scaling relations suggest that molecular clouds in NGC 625 are mostly in virial equilibrium. Comparison of our high-resolution CO observations with a star formation rate map, inferred from ancillary optical observations observations, suggests that about 40% of the molecular clouds coincide with the brightest HII regions. These bright HII regions have a range of molecular gas depletion timescales, all within a factor of 3 of the global depletion time in NGC 625 of 106-134 Myr. The highest surface density molecular clouds towards the southwest of the galaxy, in a region we call the Butterfly, do not show strong star formation activity and suggest a depletion time scale longer than 5 Gyr.