Astronomers using the Five-hundred-meter Aperture Spherical Telescope (FAST) have discovered a 2-million-light-year-wide cloud of atomic hydrogen around Stephan’s Quintet, a famous compact group of galaxies discovered in 1887.
Contour map of the integrated atomic hydrogen emission in the vicinity of Stephan’s Quintet overlaid on the composite color image. Image credit: NASA / ESA / CSA / STScI.
“Atomic hydrogen is the least bound component of galaxies and is therefore the easiest — and hence first — to be stripped off and spread around during their interactions,” said Dr. Cong Xu, an astronomer with the National Astronomical Observatories at the Chinese Academy of Sciences, and colleagues.
“Thus, the distribution of the very diffuse hydrogen and its velocities can provide new information about the galaxy earliest interactions.”
“To study the diffuse atomic hydrogen associated with Stephan’s Quintet, we carried out deep mapping observations of the 21 cm hydrogen emission over a region centred on Stephan’s Quintet using FAST’s 19-beam receiver.”
Stephan’s Quintet is a visual grouping of five galaxies in the constellation of Pegasus.
The group was discovered by the French astronomer Édouard Stephan in 1877.
Four of the five galaxies — NGC 7317, NGC 7318A, NGC 7318B, and NGC 7319 — form a physical association: the Hickson Compact Group 92 (HCG 92).
The fifth and leftmost galaxy, NGC 7320, is well in the foreground compared with the other four. It resides 40 million light-years from Earth, while the other four galaxies are about 290 million light-years away.
This enormous image from Webb reveals never-before-seen details of Stephan’s Quintet. Image credit: NASA / ESA / CSA / STScI.
“Stephan’s Quintet is unique among compact groups of galaxies,” the astronomers said.
“Observations have previously shown that interactions between multiple members, including a high-speed intruder galaxy currently colliding into the intragroup medium, have probably generated tidal debris and widespread intergalactic shocked gas.”
Using the FAST telescope, they detected a large atomic hydrogen structure with linear scale of around 600,000 parsecs (2 million light-years).
“This is the largest atomic gas structure ever found around a galaxy group,” Dr. Xu said.
“Our observations require a rethinking of properties of gas in outer parts of galaxy groups and demand complex modelling of different phases of the intragroup medium in simulations of group formation.”
The findings were published in October 2022 in the journal Nature.
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C.K. Xu et al. 2022. A 0.6 Mpc H i structure associated with Stephan’s Quintet. Nature 610, 461-466; doi: 10.1038/s41586-022-05206-x
