Cold
gas in the form of neutral hydrogen atoms provides the reservoir for
star formation in galaxies from the distant to the nearby Universe.
Understanding how it accretes onto galaxies is of crucial importance
because fresh supplies of gas fuel the ongoing star-forming. In the
most popular version, accretion onto the galaxy occurs along cosmic
filaments, and at least in more massive galaxies is heated by shocks in
the process; in smaller galaxies the infalling material stays relatively
cool. Since galaxies in the early universe are smaller, it is thought
that this cold process of growth is more typical for them as well.
Astronomers studying accretion need to look at nearby galaxies both
because they are brighter and because they have distinguishable spatial
features such as tails, bridges, ringlike structures, warped discs, or
lopsidedness that could result from accumulating gas. The GALEX Arecibo
SDSS Survey (GASS) is a multi-wavelength, deep survey designed
specifically to search for galaxies rich in atomic hydrogen. CfA
astronomer Sean Moran and five colleagues searched GASS to select one
object, GASS 3505, that has nearly ten billion solar-masses of atomic
hydrogen and a round, relatively unstructured appearance in the optical.
The team followed up with deep radio maps of the hydrogen emission
using the Jansky Very Large Array.
The astronomers found that the cold gas is distributed in a ring
around the galaxy about one hundred and sixty thousand light-years in
diameter, within which extremely inefficient star formation is happening
(about ten times less than the Milky Way’s value). The ring, it turns
out, is connected to a complex stream of material that is a signature
for infall and accretion; the stream is a reminder of how important
faint morphological features are in understanding a galaxy’s evolution.
The scientists, among other analysis, perform computer simulations of a
merger that helps to explain the activity of GASS 3505, with slight
discrepancies identifying the possible presence of some additional
activity still to be confirmed and identified. Future surveys with a new
generation of radio telescopes will be able to study these gas rich
systems at cosmological distances.
Reference(s):
“GASS
3505: The Prototype of HI-Excess, Passive Galaxies,” Geréb, K.,
Catinella, B. Cortese, L., Bekki, K., Moran, S. M., and Schiminovich,
D., MNRAS 462, 382, 2016.
