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

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 {σ }_{{H}{{I}}}∼ 10 {M}_⊙ 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}_⊙ 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}_{{H}{{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}_{{H}{{I}}}{--}{D}_{{H}{{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.