The Enigmatic (Almost) Dark Galaxy Coma P: The Atomic Interstellar Medium

Abstract

We present new high-resolution H i spectral line imaging of Coma P, the brightest H i source in the system HI 1232+20. This galaxy with extremely low surface brightness was first identified in the ALFALFA survey as an "(Almost) Dark" object: a clearly extragalactic H i source with no obvious optical counterpart in existing optical survey data (although faint ultraviolet emission was detected in archival GALEX imaging). Using a combination of data from the Westerbork Synthesis Radio Telescope and the Karl G. Jansky Very Large Array, we investigate the H i morphology and kinematics at a variety of physical scales. The H i morphology is irregular, reaching only moderate maxima in mass surface density (peak ${\sigma }_{{\rm{H}}{\rm{I}}}\sim 10\,{M}_{\odot }$ pc$^{−2}$). Gas of lower surface brightness extends to large radial distances, with the H i diameter measured at 4.0 ± 0.2 kpc inside the $1\,{M}_{\odot }$ pc$^{−2}$ level. We quantify the relationships between mass surface density of H i gas and star formation on timescales of ~100–200 Myr as traced by GALEX far-ultraviolet emission. While Coma P has regions of dense H i gas reaching the ${N}_{{\rm{H}}{\rm{I}}}={10}^{21}$ cm$^{−2}$ level typically associated with ongoing star formation, it lacks massive star formation as traced by Hα emission. The H i kinematics are extremely complex: a simple model of a rotating disk cannot describe the H i gas in Coma P. Using spatially resolved position–velocity analysis we identify two nearly perpendicular axes of projected rotation that we interpret as either the collision of two H i disks or a significant infall event. Similarly, three-dimensional modeling of the H i dynamics provides a best fit with two H i components. Coma P is just consistent (within 3σ) with the known ${M}_{{\rm{H}}{\rm{I}}}\mbox{--}{D}_{{\rm{H}}{\rm{I}}}$ scaling relation. It is either too large for its H i mass, has too low an H i mass for its H i size, or the two H i components artificially extend its H i size. Coma P lies within the empirical scatter at the faint end of the baryonic Tully–Fisher relation, although the complexity of the H i dynamics complicates the interpretation. Along with its large ratio of H i to stellar mass, the collective H i characteristics of Coma P make it unusual among known galaxies in the nearby universe.