A mysterious ring of hydrogen gas has been discovered around a distant galaxy.
Astronomers at the National Center for Radio Astrophysics (NCRA) in India spotted the anomaly using the Giant Metrewave Radio Telescope (GMRT).
Boasting a diameter about four times that of our Milky Way, the ring is “much bigger” than the galaxy it surrounds—AGC 203001, some 260 million lightyears from us.
“There is only one other such known system with such a large neutral hydrogen ring,” according to lead study author Omkar Bait (who did not reveal its location).
Neutral hydrogen emits radio waves along a 21-centimeter wavelength. This radiation, the NCRA explained, allows astronomers to map the amount and distribution of gas in our Milky Way and beyond.
Large reservoirs of neutral hydrogen are typically found in systems that are actively forming new stars. And while AGC 203001 shows no such signs, it is known to hoard hydrogen.
The unusual nature of this galaxy inspired astronomers at the Pune center to use the GMRT for high-resolution radio observations—in hopes of locating where the gas is distributed.
Their results, published recently in the Monthly Notices of the Royal Astronomical Society, suggest the neutral hydrogen has formed a large off-center ring, extending way beyond the optical extent of the galaxy.
More puzzling is the complete lack of starlight associated with the hydrogen ring—even upon closer investigation using the Canada-France-Hawaii Telescope (CFHT) in Hawaii.
“The origin and formation of such rings is still a matter of debate among astrophysicists,” Yogesh Wadadekar, a professor at the Tata Institute of Fundamental Research in Pune, said in a statement.
There is no clear answer—yet—as to what may cause these large, starless hoops of hydrogen. Scientists once chalked it up to inter-galactic collisions. But those generally come with a remote incandescent body like the Sun.
Encouraged by their discovery, the NCRA team is now conducting a large survey to map the neutral hydrogen around several more galaxies similar to AGC 203001.
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