An article published in the journal “Monthly Notices of the Royal Astronomical Society” reports the discovery of a giant neutral hydrogen ring surrounding a “quenched galaxy” cataloged as AGC 203001. A team of astronomers from the National Center for Radio Astrophysics (NCRA) in Pune, India, used the Giant Metrewave Radio Telescope (GMRT) to observe that ring, much larger than the galaxy itself with a diameter of about 380,000 light years, four tiems the Milky Way’s. The collaboration of French astronomers allowed a further study with the Canada-France-Hawaii-Telescope (CFHT) in Hawaii, USA, which found no signs of stars associated with the ring, an oddity considering that such a structure seems perfect as a nursery for new stars.
About 260 million light years away from Earth, the galaxy AGC 203001 is almost unique, in the sense that so far only one has been found surrounded by such a large neutral hydrogen ring, whose origin and formation are still at the center of discussions. Normally, large reserves of neutral hydrogen in the form of clouds and sometimes in the form of rings are discovered in galaxies where new stars are forming but not in this case. The strange nature of AGC 203001 is exactly what prompted NCRA astronomers to use the GMRT to conduct high definition observations and try to understand where the gas was.
At the time of its construction, the GMRT, which saw the so-called first light in 1995, was the largest interferometric array in the world, essentially the largest set of antennas capable of working together as a single instrument. One of the purposes of the GMRT was to search for the 21-centimeter neutral hydrogen radiation from primordial clouds to study the very first galaxies that existed in the universe. This also makes it useful to study neutral hydrogen in far closer galaxies such as AGC 203001.
The astronomers led by Omkar Bait, an NCRA doctoral student, observed the distribution of neutral hydrogen in the galaxy AGC 203001 revealing that it’s distributed in a large ring that surrounds it and extends far beyond its borders. French astronomers Pierre-Alain Duc and Jean-Charles Cuillandre collaborated in the research, and contributed with observations at optical frequencies conducted with the CFHT. None of the images captured with GMRT and CFHT showed signs of starlight associated with that ring.
The image (Courtesy O. Bait (NCRA-TIFR/GMRT), Duc (ObAS/CFHT). All rights reserved) shows an optical image captured by the CFHT of the galaxy AGC 203001 with the large gas ring in red observed by the GMRT. Two other galaxies in the image have much smaller rings, again shown in red.
Neutral hydrogen rings around galaxies are associated with collisions with other galaxies but in those cases shock waves tend to trigger the formation of new stars. This makes the galaxy AGC 203001 even more anomalous. One possibility is that in this case the density of the gas is insufficient to form stars or that its temperature is too high for that hydrogen to start forming the lumps needed to trigger star formation.
The researchers intend to map neutral hydrogen around many more galaxies of that type to try to find similarities and differences. Understanding better the origin and characteristics of the neutral hydrogen rings around them is the foundation to understand their evolution, especially in relation to star formation.
