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 |
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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. |
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ISSN: | 0004-6256 1538-3881 |
DOI: | 10.3847/1538-3881/ac2a38 |