Hydrogen is the lightest and most abundant atomic element in the universe. On Earth, it is found combined with other atoms in molecules or as dihydrogen H2. In the Universe, hydrogen can be found as isolated neutral atoms (HI). The evolution of galaxies is mainly governed by the accretion of this atomic gas from the intergalactic medium, and then by the conversion of this gas into stars. Therefore, observations that detect atomic gas in and around galaxies are crucial for studies of galaxy formation and evolution models.
The most direct method of exploring atomic gas is to observe the 21 cm fine structure emission line of atomic hydrogen in the radio domain. The Five-hundred-metre Aperture Spherical Telescope (FAST), built in Guizhou, China, is currently the largest single-dish radio telescope in the world. The full commissioning of FAST, with its 19-beam receivers, has opened a new window on atomic gas in the Universe, in particular for low-density diffuse gas far away from galaxies.
An international team led by Cong Xu (NAOC), and involving Pierre-Alain Duc (CNRS, Strasbourg Astronomical Observatory), used the FAST radio telescope to carry out a deep mapping of the region surrounding the ‘Stephan’s quintet’ (a well-known compact group of galaxies, which was selected as a target for the first images of the James-Webb Space Telescope). Using the 21 cm emission, they found that the atomic gas extends over 2 million light years (about 20 times the size of the Milky Way). This is the largest structure of atomic gas ever discovered around galaxies. This discovery was made possible by a unique combination of extreme sensitivity, large field of view and competitive angular resolution.
Credits: JWST Space Telescope image: NASA/ESA/CSA, obtained from the Mikulski Space Telescope Archive at STScI, operated by AURA, under NASA contract NAS 5-03127 for JWST. These observations are associated with (ERO) programme #jw02732.
This map of the diffuse gas could be compared with the diffuse stellar component visible in images of the Stephan’s quintet obtained with the MegaCam camera of the Canada-France-Hawaii Telescope (CFHT). The superposition of the HI emission shows that the atomic gas structure extends well beyond the extended stellar halos that envelop the galaxies in the group and that were previously revealed by the CFHT. The origin of this gas remains debated. Was it expelled from galaxies or does it come from the intergalactic medium?
Since its discovery by the French astronomer Edouard Stephan in 1877, Stephan’s quintet has continued to reveal puzzles about the complex interactions between galaxies, but also the interactions between galaxies and the diffuse medium in galaxy groups.
These new observations show that there is a large-scale low-density diffuse gas far from the centre of the group, and that this gas has probably been there for about 1 billion years. The detection of this gas in this atomic form and with such a low density is mysterious, as it is not known how it can survive ionisation by intergalactic ultraviolet radiation over such a long time scale.
Article : A 0.6 Mpc H i structure associated with Stephan’s Quintet, Xu et al. 2022, Nature.
Contact : Pierre-Alain Duc (CNRS) pierre-alain.duc@astro.unistra.fr
Interactive visualization with Aladin Lite
Explore the environment of Stephan’s quintet with Aladin Lite. The deep image obtained at the CFHT with the MegaCam is here superimposed on the PanSTARRS survey. You can use the mouse to zoom or move around the view. You can also overlay the JWST images of the quintet. Click on
to select a different background survey, or change the opacity of the various overlays.