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arXiv · 2109.12139

The ALFALFA Almost-Dark Galaxy AGC~229101: A Two Billion Solar Mass HI Cloud with a Very Low Surface Brightness Optical Counterpart

Lukas Leisman·Katherine L. Rhode·Catherine Ball·Hannah J. Pagel·John M. Cannon·John J. Salzer·Steven Janowiecki·William F. Janesh·Gyula I. G. Józsa·Riccardo Giovanelli·Martha P. Haynes·Elizabeth A. K. Adams·Laurin Gray·Nicholas J. Smith
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Abstract

We present results from deep HI and optical imaging of AGC 229101, an unusual HI source detected at v$_{\rm helio}$ = 7116 km/s in the ALFALFA survey. Initially classified as a candidate "dark" source because it lacks a clear optical counterpart in SDSS or DSS2 imaging, AGC 229101 has $10^{9.31\pm0.05}$ solar masses of HI, but an HI line width of only 43$\pm$9 km/s. Low resolution WSRT imaging and higher resolution VLA B-array imaging show that the source is significantly elongated, stretching over a projected length of ~80 kpc. The HI imaging resolves the source into two parts of roughly equal mass. WIYN pODI optical imaging reveals a faint, blue optical counterpart coincident with the northern portion of the HI. The peak surface brightness of the optical source is only $\mu_{g}$ = 26.6 mag arcsec$^{-2}$, well below the typical cutoff that defines the isophotal edge of a galaxy, and its estimated stellar mass is only $10^{7.32\pm0.33}$ solar masses, yielding an overall neutral gas-to-stellar mass ratio of M$_{\rm HI}$/M$_*=$~98$_{+111}\atop^{-52}$. We demonstrate the extreme nature of this object by comparing its properties to those of other HI-rich sources in ALFALFA and the literature. We also explore potential scenarios that might explain the existence of AGC~229101, including a tidal encounter with neighboring objects and a merger of two dark HI clouds.

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Lukas Leisman, Katherine L. Rhode, Catherine Ball, Hannah J. Pagel, John M. Cannon, John J. Salzer, Steven Janowiecki, William F. Janesh, Gyula I. G. Józsa, Riccardo Giovanelli, Martha P. Haynes, Elizabeth A. K. Adams, Laurin Gray, Nicholas J. Smith. 2021-09-24. The ALFALFA Almost-Dark Galaxy AGC~229101: A Two Billion Solar Mass HI Cloud with a Very Low Surface Brightness Optical Counterpart. https://doi.org/10.3847/1538-3881%2Fac2a38

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