Astronomers from the Pune-based National Centre of Radio Astrophysics (NCRA-TIFR) and the University of California, Santa Cruz (UCSC) have used the Giant Metrewave Radio Telescope (GMRT) to map the distribution of atomic hydrogen gas from the galaxy hosting a fast radio burst (FRB) for the first time, revealing important clues on the origin of the burst.
FRBs are transient, bright pulses of emission from distant galaxies at radio wavelengths (seen mostly at wavelengths of tens of centimeters) whose bursts last at most for a few milliseconds. Although FRBs were first detected 15 years ago, and more than 1,000 have been found so far, radio astronomers are still searching for clues as to what kind of astronomical objects can produce so much energy in so little time.
Observations of the gas and stars in the vicinity of FRBs, within their host galaxies, are critical to understand how the bursts were formed.
The GMRT results, published earlier this month in The Astrophysical Journal Letters (ApJL), indicate that the FRB host galaxy has undergone a recent ‘merger’, and that the FRB ‘progenitor’ is most likely a massive star formed due to this merger event.
This is the first case of direct evidence for a recent merger in an FRB host, a major step towards understanding the origins of the FRBs, said the astronomers at NCRA.
“Our target, FRB20180916B, produces repeated very short bursts, and these have been found to arise in the outskirts of a spiral galaxy just half a billion light years away. FRB20180916B is thus one of the closest known FRBs, an ideal candidate to study the local burst environment”, said Balpreet Kaur, a Ph.D. student at NCRA-TIFR and the lead author of the study.
According to Prof. Nissim Kanekar of the NCRA-TIFR and a co-author of the study, the great utility of the GMRT is owing to the use of different combinations of its 30 antennae to both map the atomic hydrogen within the FRB host galaxy in detail, and also search for hydrogen in nearby companion galaxies.
“The GMRT 21cm image shows that the FRB host has four companions, one of which is just 70,000 light years [one light year equals approximtely 9.7 trillion kilometres] from the FRB location,” he explained.
Surprising find
Ms. Kaur said that the first surprise during their observation was the amount of atomic hydrogen in the FRB galaxy, which was around 10 times more than that found in similar nearby galaxies.
“Since atomic gas is the primary fuel for star formation, one would expect a galaxy with a high gas mass to have high star-formation activity. However, though it has a lot of gas, the FRB galaxy is not actively forming stars. This suggests that it has acquired the gas quite recently, probably during ‘merger’ with a smaller companion,” she observed.
While the FRB viewed by the Hubble Space Telescope (HST) optical image shows it as a beautiful spiral disk galaxy, the GMRT images show that the gas distribution in the FRB galaxy is “highly disturbed”, noted Ms. Kaur.
“Contrary to the Hubble Space Telescope (HST) image, the galaxy doesn’t look like a spiral disk in the GMRT images at all. It seems pretty clear that another galaxy or gas cloud collided with the FRB galaxy a few tens of millions of years ago. This would have disturbed the gas distribution, producing the gas tails as seen in the GMRT image. The collision is likely to have also compressed the gas in the galaxy outskirts, giving rise to a burst of star formation near the FRB position,” she said, adding that the GMRT images suggest that the FRB progenitor is likely to be a massive star formed due to the merger event.
Ms. Kaur said that this is the first case of direct evidence for a recent merger in an FRB host and showcased the ability of the GMRT to identify such mergers via disturbances in the gas distribution. The authors plan to carry out similar GMRT observations of more FRB host galaxies in order to test whether merger events might be a common trigger for an FRB formation.
