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

Publication date: 
Main author: 
Ball, Catherine
IAA authors: 
Jones, Michael G.
Ball, Catherine;Cannon, John M.;Leisman, Lukas;Adams, Elizabeth A. K.;Haynes, Martha P.;Józsa, Gyula I. G.;McQuinn, Kristen B. W.;Salzer, John J.;Brunker, Samantha;Giovanelli, Riccardo;Hallenbeck, Gregory;Janesh, William;Janowiecki, Steven;Jones, Michael G.;Rhode, Katherine L.
The Astronomical Journal
Publication type: 
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 }<SUB>{{H</SUB>}{{I}}}∼ 10 {M}<SUB>ȯ </SUB> pc<SUP>−2</SUP>). 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}<SUB>ȯ </SUB> pc<SUP>−2</SUP> 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}<SUB>{{H</SUB>}{{I}}}={10}<SUP>21</SUP> cm<SUP>−2</SUP> 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}<SUB>{{H</SUB>}{{I}}}{--}{D}<SUB>{{H</SUB>}{{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.
ADS Bibcode: 
galaxies: dwarf;galaxies: evolution;galaxies: individual: Coma P;AGC 229385;galaxies: irregular;Astrophysics - Astrophysics of Galaxies