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

Catherine Ball*, John M. Cannon, Lukas Leisman, Elizabeth A. K. Adams, Martha P. Haynes, Gyula I. G. Jozsa, Kristen B. W. McQuinn, John J. Salzer, Samantha Brunker, Riccardo Giovanelli, Gregory Hallenbeck, William Janesh, Steven Janowiecki, Michael G. Jones, Katherine L. Rhode

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

7 Citations (Scopus)


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 H I) similar to 10 M-circle dot 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-circle dot pc(-2) level. We quantify the relationships between mass surface density of H I gas and star formation on timescales of similar to 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 Ha 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 3 sigma) 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.

Original languageEnglish
Article number65
Number of pages22
JournalThe Astronomical Journal
Issue number2
Publication statusPublished - Feb-2018


  • galaxies: dwarf
  • galaxies: evolution
  • galaxies: individual (Coma P, AGC 229385)
  • galaxies: irregular

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