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 interactions. Thus, the very diffuse hydrogen distribution and its velocity field can provide new information about the earliest interactions.
Recent deep mapping observations of 21-cm line emission in the region of the well-known compact group of galaxies “Stephan’s Quintet” using the Five-hundred-meter Aperture Spherical Telescope’s (19-beam receiver) led by XU Cong, a researcher from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), revealed a very large atomic gas structure with a length of about 2 million light years. It is about 20 times the size of the Milky Way.
XU said, “This is the largest atomic gas structure ever found around a galaxy group. The observations reached a sensitivity of 1σ=4.2×1016 cm-2 per channel (Δv=20 km s-1; angular-resolution=4′), making them the most sensitive observations of atomic hydrogen 21-cm line emission at this angular resolution.”
Since the discovery of Stephan’s Quintet in 1877, it continued revealing puzzles related to the complex web of galaxy-galaxy and galaxy-intragroup medium interactions in the group.
The latest findings reveal the presence of large-scale, diffuse, low-density gas far from the group’s center. This gas is likely older than 1 gigayear and has a column identity of less than 1018cm-2. Because it is unclear how the low-density atomic gas can survive being ionized by the intergalactic UV background on such a long time scale, the data cast doubt on the present idea of galaxy-group formation/evolution.