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Highlights · 09 Nov 2022

Colossal Atomic-hydrogen Cloud Detected by FAST Hints at Early Interactions between Galaxies in Stephan's Quintet

By SONG Jianlan

The largest gaseous structure in cosmos found so far might be debris left behind by immemorial collisions and crushes between galaxies …

 An international team of astronomers working with FAST, the Five-hundred-meter Aperture Spherical Telescope of China, reported on Oct 19 in Nature their discovery of an atomic-hydrogen cloud 20 times bigger than the Milky Way. This gaseous structure, the largest so far found by human beings, is located in the vicinity of Stephans Quintet, a compact group of galaxies; and might be associated with the violent interactions between galaxies during the Quintet’s early formation and evolution, said the team.  

The newly discovered gaseous structure stretches across a distance as large as two million light years. While sharing a coherent velocity field, this huge structure extends into two parts of different shape: a spherical one centering on Stephans Quintet with higher density, and an elongated, curve one attaching to the south edge of the galaxy group.  

The atomic-hydrogen gaseous structure (the red halo) discovered by the team using FAST, the Five-hundred-meter Aperture Spherical Telescope. (The thicker the red color, the denser is the gas.) At the center is Stephans Quintet, put next to the inset of an infrared image of the same galaxy group released by James Webb Space Telescope. A spherical part of the gaseous structure centers on the quintet; and a curve, diffuse part attaches to the south edge of the Quintet. (Image credit: NAOC, NASA, ESA, CSA & STScI)  

Based on the distribution and velocity of the gas, the team analyzed that it could be the legacy from interactions between galaxies during the early formation of the group, shedding light on some long-existing mysteries about the latter’s history.   

Gases in Galaxy Formation and Evolution

Observations on gaseous structures are important for astronomical research; as such gases are believed to play a key role in galaxy and star formation, and in the origin and evolution of various celestial bodies. For example, the evolution of a galaxy generally features an intake of atomic-hydrogen gas from the surrounding space and a subsequent transformation of the inhaled matter into molecular hydrogen, and finally stars.  

First discovered by French astronomer Édouard Stephan, Stephan’s Quintet is unique and has caught the eye of astronomers for multiple reasons. This keen interest might explain why the group is among the first targets of the James Webb Space Telescope. The interactions between its member galaxies, as well as those between an intruder” and the intragroup medium, are among the many aspects that have fascinated scientists. Such interactions, as shown in previous studies, might have triggered violent gravitational tidal disruptions to leave behind various debris, which has since evolved into the current gaseous structures, stellar filaments and other interesting components, like remains of starbursts. Previous observations by different astronomical instruments have found a “debris field” within Stephan’s Quintet as a possible legacy from this spectacular history. However, the details and chronology of such crushes and collisions are too complicated to understand with the previous observations, which often failed to hear the weak voice from the infant Quintet at its early stages of development. 

Thanks to the FASTs outstanding sensitivity, the team led by Prof. XU Cong from the National Astronomical Observatories, Chinese Academy of Sciences caught the extremely dim radio signals from this thin, diffuse gaseous structure at an unprecedentedly sharp resolution and angular accuracy. They conducted deep mapping observations of the 21 cm atomic-hydrogen emission from the region surrounding the Quintet, focusing on the target 100 times deeper than previous observations. Now the newly detected gaseous structure, at least 1 billion years old in age, might help scientists unveil some details of those ancient interactions.  

Atomic hydrogen is the least bound component of galaxies,explain the authors. This means it is the easiest (and hence the first) to be torn off and spread afar during collisions and crushes. Therefore, the distribution and velocity of this gaseous component can provide new information about the earliest interactions between galaxies.  

Debris from Immemorial Violent Tidal Disruptions

Part of the detected gaseous structure centers around Stephans Quintet like a spherical halo. This part encompasses a region probably associated with the “debris field”, an area within the Quintet with traces from the ancient battlefield of early crushes between member galaxies. This region was previously found by past observations, and might have been involved with the Quintets early formation. 

XU’s team found that the location of this spherical part mostly coincides with an intragroup-medium starburst named SQ-A, which is, according to previous studies, likely a consequent from former galaxy interactions. Before XU’s team, some other instruments, including the Arecibo Telescope and the Green Bank Telescope detected the same source, but with lower sensitivity and hence recognized only the more compact part at the center. The two above-mentioned telescopes found that the source extended to a scale as large as one million light years; now XU’s team identified it as around two million light years in diameter.  

The other part of the newly detected gaseous structure had never been detected by previous observations. It attaches to the south edge of Stephans Quintet, manifests in an elongated, curve shape, like fragments from a broken dish. It appears at the bottom of the FAST mapping and therefore may well reach beyond the map,reported the authors in their Nature paper. This part somehow remains diffuse without being ionized despite the ultraviolet background ubiquitous around the galaxies. This is unusual, as previous research indicated that atomic hydrogen at an extremely low density can merely survive the ionization by the ultraviolet environment over a time no longer than 500 million years, yet this diffuse structure could be as old as 1.5 billion years in age. 

After examining the locations and velocities of nearby galaxies, the team concluded “with high confidence” that the diffuse structure is not associated with a collection of gas-rich galaxies. It would take as many as four galaxies of such – all moving in about the same radial velocity – to produce such a bizarre structure; and the chances to meet the both conditions are extremely slim. Previous simulations indicated that such widely extended disk-like structures could be possible products from inter-galaxy interactions. Given its location and velocity, the team speculated that the “broken disk” is “most likely” to associate with the debris field”, again.  

Given that the location of the diffuse structure is coincident with NGC 7320a, a new source detected by the team themselves, the authors speculated an scenario occurring in the earliest forming stage of the Quintet that might have left behind this debris: about 1.5 billion years ago, NGC 7320a passed through the Quintet, and dragged out some gas from its central region. And that has thus developed into the diffuse feature as now observed. An alternative scenario could be, the team continued, this curve, diffuse structure had resulted from a high-speed head-on collision between an “intruder” and a core member of the quintet. This collision triggered an expanding gas wave, and the residual has evolved into these disk fragments. The team even named the possible intruder – Anon 4 to the southwest of the quintet.   

The team reckons the diffuse gas structure to the south edge of Stephan’s Quintet might be debris from early interactions between member galaxies, possibly a high-speed head-on collision between them, or an intrusion of an external galaxy into the core part of the Quintet. (Image credit: NAOC, NASA, ESA, CSA & STScI) 

Mysterious Atoms

It remains a mystery to unravel how this mass of gaseous hydrogen has survived the potential ionization to remain neutral atoms despite its old age. The team said that they would investigate into this via different methods, with aid from computer simulations.

Our observations require a rethinking of properties of gas in outer parts of galaxy groups and demand complex modeling of different phases of the intragroup medium in simulations of group formation,” concluded the authors.

The exquisite sensitivity of FAST enabled the detection of this tremendous gas structure. The depth of the observation allowed XU et al. to ascertain that the structure is associated with debris created early in the history of Stephan’s Quintet,” comments Julia Blue Bird, astrophysicist from the National Radio Astronomy Observatory, Socorro, USA.  In her commentary entitled The gas about Stephans Quintet reveals a history of collisionpublished in the same issue of Nature, she introduced that cosmological simulations along large temporal and spatial dimensions can provide new insights into galaxy formation. In this direction, observational data, with increasingly improved resolution and accuracy like XU and coauthors’ work with FAST, will help to constrain the physics that serves as an input to these simulations”, and hence contribute to the knowledge of galaxy evolution. 


Bird, J. B. The gas about Stephan’s Quintet reveals a history of collision.  Nature 610, 458–459 (2022). https://doi.org/10.1038/d41586-022-03218-1

Xu, C.K., Cheng, C., Appleton, P.N. et al. A 0.6 Mpc H I structure associated with Stephan’s Quintet. Nature 610, 461466 (2022). https://doi.org/10.1038/s41586-022-05206-x