ALMA Observations of the Molecular Clouds in NGC 625

Abstract

We present the highest resolution (1") $^{12}$CO(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 $0.4$ kpc$^2$ and does not have well-ordered large-scale motions. We measure a total molecular mass in NGC 625 of $5.3\times 10^6 M_\odot$, assuming a Milky Way CO-to-H$_2$ 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 $M_\odot$pc$^{-2}$. 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 $\sim 3$ 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.