The ALFALFA Almost Dark Galaxy AGC 229101: A 2 Billion Solar Mass H i Cloud with a Very Low Surface Brightness Optical Counterpart

We present results from deep H i and optical imaging of AGC 229101, an unusual H i source detected at v helio =7116 km s −1 in the Arecibo Legacy Fast ALFA (ALFALFA) blind H i survey. Initially classified as a candidate “dark” source because it lacks a clear optical counterpart in Sloan Digital Sky...

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Veröffentlicht in:The Astronomical journal 2021-12, Vol.162 (6), p.274
Hauptverfasser: Leisman, Lukas, Rhode, Katherine L., Ball, Catherine, Pagel, Hannah J., Cannon, John M., Salzer, John J., Janowiecki, Steven, Janesh, William F., Józsa, Gyula I. G., Giovanelli, Riccardo, Haynes, Martha P., Adams, Elizabeth A. K., Gray, Laurin, Smith, Nicholas J.
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Sprache:eng
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Zusammenfassung:We present results from deep H i and optical imaging of AGC 229101, an unusual H i source detected at v helio =7116 km s −1 in the Arecibo Legacy Fast ALFA (ALFALFA) blind H i survey. Initially classified as a candidate “dark” source because it lacks a clear optical counterpart in Sloan Digital Sky Survey (SDSS) or Digitized Sky Survey 2 (DSS2) imaging, AGC 229101 has 10 9.31±0.05 M ⊙ of H i , but an H i line width of only 43 ± 9 km s −1 . Low-resolution Westerbork Synthesis Radio Telescope (WSRT) imaging and higher-resolution Very Large Array (VLA) B-array imaging show that the source is significantly elongated, stretching over a projected length of ∼80 kpc. The H i imaging resolves the source into two parts of roughly equal mass. WIYN partially populated One Degree Imager (pODI) optical imaging reveals a faint, blue optical counterpart coincident with the northern portion of the H i . The peak surface brightness of the optical source is only μ 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±0.33 M ⊙ , yielding an overall neutral gas-to-stellar mass ratio of M / M * = 98 − 52 + 111 . We demonstrate the extreme nature of this object by comparing its properties with those of other H i -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 H i clouds.
ISSN:0004-6256
1538-3881
DOI:10.3847/1538-3881/ac2a38