ALMA Observations of the Molecular Clouds in NGC 625

Abstract

We present the highest-resolution (1″) ¹²CO observations of molecular gas in the dwarf starburst galaxy NGC 625 to date, obtained with Atacama Large Millimeter/submillimeter Array. The molecular gas, which is distributed in discrete clouds within an area of 0.4 kpc², does not have well-ordered large-scale motions. We measure a molecular mass in NGC 625 of 5.3 × 10⁶ ${M}_{\odot }$ , assuming a Milky Way CO-to- ${{\rm{H}}}_{2}$ 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 linewidth 2.5 $\mathrm{km}\,{{\rm{s}}}^{-1}$ , and a median surface density of 169 ${M}_{\odot }\,{{\rm{pc}}}^{-2}$ . 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, suggests that about 40% of the molecular clouds coincide with the brightest H II regions. These bright H II 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 toward the southwest of the galaxy, in a region we call the Butterfly, do not show strong star formation activity and suggest a depletion timescale longer than 5 Gyr.