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

Abstract

We present new high-resolution HI spectral line imaging of Coma P, the brightest HI source in the system HI 1232$+$20. This extremely low surface brightness galaxy was first identified in the ALFALFA survey as an "(Almost) Dark" object: a clearly extragalactic HI 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 HI morphology and kinematics at a variety of physical scales. The HI morphology is irregular, reaching only moderate maxima in mass surface density (peak ${\sigma }_{\mathrm{HI}}\sim 10$ $M_{\odot }$ pc${}^{-2}$). Gas of lower surface brightness extends to large radial distances, with the HI diameter measured at 4.0$\pm$0.2 kpc inside the 1 $M_{\odot }$ pc${}^{-2}$ level. We quantify the relationships between HI gas mass surface density and various types of star formation by considering GALEX far ultraviolet observations and H$\alpha$ nondetections. We describe Coma P's complex HI kinematics using spatially resolved position-velocity analysis and three-dimensional modeling. Both methods of analysis suggest that Coma P's kinematics show signatures of either the collision of two HI disks or a significant infall event. Coma P is just consistent (within 3$\sigma$) with the known M${}_{\mathrm{HI}}$ -- D${}_{\mathrm{HI}}$ scaling relation. It is either too large for its HI mass, has too low an HI mass for its HI size, or the two HI components artificially extend its HI size. Coma P lies within the empirical scatter at the faint end of the baryonic Tully--Fisher relation, although the complexity of the HI dynamics complicates the interpretation. The collective HI characteristics of Coma P make it unusual among known galaxies in the nearby universe. [Abridged]

26 pages, 17 figures. Published in the Astronomical Journal 155, 65

References in corpus (15)

• High-resolution mass models of dwarf galaxies from LITTLE THINGS
• Kinematic modelling of disk galaxies I. A new method to fit tilted rings to data cubes
• A Catalog of Ultra-compact High Velocity Clouds from the ALFALFA Survey: Local Group Galaxy Candidates?
• The ALFALFA "Almost Darks" Campaign: Pilot VLA HI Observations of Five High Mass-to-Light Ratio Systems
• (Almost) Dark HI Sources in the ALFALFA Survey: The Intriguing Case of HI1232+20
• The Star Forming Main Sequence of Dwarf Low Surface Brightness Galaxies
• ALFALFA Discovery of the Nearby Gas-rich Dwarf Galaxy Leo P. V. Neutral Gas Dynamics and Kinematics
• SHIELD: Neutral Gas Kinematics and Dynamics
• SHIELD: Comparing Gas and Star Formation in Low Mass Galaxies
• ALFALFA and WSRT Imaging of Extended HI Features in the Leo Cloud of Galaxies
• Characterizing the Star Formation of the Low-Mass SHIELD Galaxies from Hubble Space Telescope Imaging
• AGC198606: A gas-bearing dark matter minihalo?
• Identifying galaxy candidates in WSRT HI imaging of ultra-compact high velocity clouds
• Detection of an Optical Counterpart to the ALFALFA Ultra-compact High Velocity Cloud AGC 249525
• Rotational Dynamics and Star Formation in the Nearby Dwarf Galaxy NGC 5238

Cited by in corpus (1)

• The Enigmatic (Almost) Dark Galaxy Coma P: Distance Measurement and Stellar Populations from HST Imaging