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

Abstract

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 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±0.05 M of H i, but an H i line width of only 43±9 km s −1 . 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 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 HI /M * = 98 +111 −52 . We demonstrate the extreme nature of this object by comparing its properties to 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.

The detection of a massive chain of dark HI clouds in the GAMA G23 Field

“…A recent dark cloud discovery (Leisman et al 2021) of a similarly massive neutral hydrogen cloud has identified a dwarf mass stellar counterpart at a surface brightness that is 0.5 to 1.0 mag below our own r-band limit, which emphasises the possibility of an older stellar population at such extreme surface brightnesses, which would be in line with our M H i /M estimate being greater than 1000.…”

“…Using the 3-σ rband luminosity and a dwarf galaxy stellar M/L = 2 (Martin et al 2008;Flynn et al 2006), the limiting stellar mass is 10 6.6 L , and the ratio between the neutral gas and the stellar content, M H i /M , is 10 3.1 . The extremely high mass ratio is well outside of any scaling relation, and is an order of magnitude larger than the estimated value for the recently discovered massive dark cloud, AGC 229101 (Leisman et al 2021). We note that the stellar M/L for dwarf disks and spheroidals may range widely, over an order of magnitude; conservatively using a range between 1-10 (e.g., see Flynn et al 2006), the corresponding range in M H i /M is 10 2.4 -10 3.4 .…”

“…Where did this neutral hydrogen come from, where is it going and why is it not forming stars? Notable examples of massive H i associations are the Leo Ring (Schneider et al 1983;Schneider 1985;Schneider et al 1986;Schneider 1989;Stierwalt et al 2009), with a recently studied analogue AGC203001 (Bait et al 2020), HI 1225+01 (Giovanelli & Haynes 1989), Hanny's Voorwerp (Lintott et al 2009;Józsa et al 2009;Keel et al 2012), HCG 44 (Serra et al 2013;Hess et al 2017) and AGC 229101 (Leisman et al 2021).…”

“…Most notably, using the 40% release data of the blind AL-FALFA survey (Giovanelli et al 2005), Cannon et al (e.g., 2015) estimated that less than 1.5% were not associated with stars, and 25% of those could not immediately be identified as tidal features. This triggered a number of followup studies of the most intriguing cases (Cannon et al 2015;Janowiecki et al 2015;Ball et al 2018;Leisman et al 2021), among others unveiling the characteristics of the extreme high-H i mass LSB object AGC 229101, with M HI /M ≈ 70 and M HI = 10 9.3 M . None of the sources has been found to lack any stellar counterpart, while Kent et al (2007); Kent (2010) confirmed the detection of two formerly unknown dark clouds with M HI ≤ 10 8.5 M in the vicinity of the Virgo cluster.…”

“…In summary, massive (M H i > 10 9 M ) optically dark neutral hydrogen clouds (or, rather, candidates for dark clouds) are so rare that they are the subject of individual studies. Most notable, beneath AGC 229101 (Leisman et al 2021), are the Leo Ring (Schneider et al 1983;Schneider 1985;Schneider et al 1986;Schneider 1989) and the double source HI 1225+01 (Giovanelli & Haynes 1989).…”