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

TL;DR

This study provides detailed high-resolution HI imaging of Coma P, revealing its irregular morphology, complex kinematics, and potential signs of a past collision or infall, contributing to understanding of low surface brightness galaxy dynamics.

Contribution

It offers the first detailed kinematic analysis of Coma P using combined radio observations, highlighting its unusual HI properties and complex dynamics among similar galaxies.

Findings

Coma P has irregular HI morphology with moderate surface density peaks.

HI kinematics suggest a collision or infall event.

Coma P's properties are consistent with known scaling relations within scatter.

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 $\sigma_{\rm HI}\sim 10$ $M_{\odot}$ 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$_{\rm HI}$ -- D$_{\rm HI}$ 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]