A mysterious hydrogen gas ring surrounding a faraway galaxy has been discovered by a group of astronomers from the National Centre for Radio Astrophysics (NCRA) in Pune, India. The astronomers observed this with the help of the Giant Metrewave Radio Telescope (GMRT).
With a diameter of nearly 380,000 light-years (around four times that of the Milky Way), the ring is considerably bigger when compared to the galaxy it surrounds.
The galaxy, named AGC 203001, is situated nearly 260 million light-years away from the Earth. Apart from this system, there is only one other known system that has such a huge neutral hydrogen ring. To date, the origin and development of such rings still remain a debatable matter among astrophysicists.
Radio waves emitted by neutral hydrogen atoms, at a wavelength of about 21 cm, has enabled radio astronomers to map the distribution and amount of neutral hydrogen gas not just in the Milky Way galaxy but also in other galaxies in the Universe. Essentially, huge reservoirs of neutral hydrogen gas can be observed in galaxies that are in the process of active formation of new stars.
Although the AGC 203001 galaxy has exhibited no signs of active star formation, it was known to contain enormous amounts of hydrogen. However, the precise distribution of the gas was obscure. The strange nature of this galaxy inspired NCRA astronomers to employ the GMRT to perform high-resolution radio observation of the galaxy to discover where in the galaxy the gas lies.
As part of the GMRT observations, it was found that the distribution of neutral hydrogen is in the form of a huge off-centered ring that extends far beyond the galaxy’s optical extent. A more perplexing fact was the discovery by the astronomers that the current optical images of the ring revealed no sign of any stars in the galaxy.
The NCRA team collaborated with Pierre-Alain Duc and Jean-Charles Cuillandre, two French astronomers, and captured a highly sensitive optical image of the system with the help of the Canada-France-Hawaii-Telescope (CFHT) in Hawaii, United States. Yet, even these images do not reveal any sign of starlight related to the hydrogen ring.
At present, there is no clear solution as to what could result in the formation of such huge, starless hydrogen rings. Traditionally, it was considered that galaxy-galaxy collisions were the reason behind the formation of such off-centered rings surrounding galaxies. But such rings also usually contain stars. This is in contrast to what is observed in the ring. Discovering the origin of this ring is still a challenge to astronomers.
Inspired by the discovery, at present, the researchers are carrying out a large survey to map the neutral hydrogen surrounding various more similar galaxies. In case a few of them also exhibit rings such as this, it would be helpful to gain better insights into the mechanism of formation of such unique rings.
The study was led by Omkar Bait, a doctoral student at NCRA working under the supervision of Yogesh Wadadekar. The research work is a part of Omkar’s doctoral thesis. Another doctoral student at NCRA, Sushma Kurapati, also contributed to the radio observations.
Other expert scientists who took part in the study include Pierre-Alain Duc (Universite de Strasbourg, Strasbourg, France), Jean-Charles Cuillandre (PSL University, Paris, France), Peter Kamphuis (Ruhr University, Bochum, Germany), and Sudhanshu Barway (Indian Institute of Astrophysics, Bengaluru, India).