Galaxy with a hydrogen ring

The optical image from the CFHT telescope superposed with the GMRT’s radio image of the distribution of neutral hydrogen in the form of a large ring shown in red. The other two red blobs show the distribution of neutral hydrogen around two other galaxies which are in the vicinity of the ring.

A team of Indian and French astronomers working at the National Centre for Radio Astrophysics (NCRA) in Pune has discovered a mysterious ring of hydrogen gas around a distant galaxy using the Giant Metrewave Radio Telescope (GMRT). The ring is much bigger than the galaxy it surrounds and has a diameter of about 380,000 light years (about four times that of the Milky Way; one light year is about 9.5 trillion kilometres).

The GMRT is located at Narayangaon, about 80 km from Pune, and is run by the NCRA, an institution under the Tata Institute of Fundamental Research (TIFR).

The galaxy (named AGC 203001) is about 260 million light years away from the earth. According to an NCRA press release, the origin and formation of such a large neutral hydrogen ring is still a matter of debate.

Neutral hydrogen emits radio waves at a wavelength of about 21 centimetres. This radiation from neutral hydrogen atoms has allowed radio astronomers to map the amount and distribution of neutral hydrogen gas in the Milky Way and other galaxies. Typically, large reservoirs of neutral hydrogen gas are found in galaxies that actively form new stars. However, despite showing no signs of active star formation, AGC 203001 was found to have large amounts of hydrogen, although its exact distribution was not known. The unusual nature of this galaxy motivated NCRA astronomers to conduct high-resolution radio observation using the GMRT to find out where the gas lies.

The observations revealed that the neutral hydrogen was distributed in the form of a large off-centred ring extending much beyond the optical extent of the galaxy. More puzzlingly, the astronomers found that the existing, or the newly obtained highly sensitive optical images of the ring, showed no signs of it containing stars.

The findings were reported in a recent issue of the Monthly Notices of the Royal Astronomical Society (MNRAS). Figuring out how this ring was formed remains a challenge to astronomers.

Scientists say there is no clear answer to what leads to the formation of such large, starless rings of hydrogen. Conventionally, galaxy-galaxy collisions were thought to lead to the formation of off-centred rings around galaxies. Such rings generally contain stars.

Encouraged by its new discovery, the team is conducting a large survey to map the neutral hydrogen around several other similar galaxies.