The author personally interprets: How important is the “cloud” seen by China’s Sky Eye?

In October this year, the National Astronomical Observatory of the Chinese Academy of Sciences used the 500-meter Aperture Spherical Radio Telescope (FAST) to detect the largest atomic gas structure known in the universe. The results were published in the international academic journal Nature at 23:00 Beijing time on October 19, 2022 [1]. Media reports emphasized that it "indicates that there may be more such large-scale low-density atomic gas structures in the universe", but this is far from enough to explain the significance of this research.

To this end, Science Popularization China specially invited the second author of the paper, Mr. Cheng Cheng, to write and provide exclusive and in-depth interpretation .

01

What did astronomers do?

In simple terms, our research team used FAST to see a cloud, a very large cloud. This "cloud" is a neutral hydrogen gas structure about two million light years in size, more than twenty times larger than our Milky Way galaxy.

Clockwise from the upper left corner: NGC7320, NGC7319, NGC7318 (a and b), NGC7317. Image source: esahubble.org

The fact that this cloud is so big is not the main reason why this discovery is so exciting. The astronomical community is more interested in the fact that the discovery of this "cloud" will help us humans further understand the history of galaxy mergers in a very important galaxy group .

This galaxy group is called "Stephan's Quintet". It was discovered by French astronomer Édouard Stephan in 1877 using an 80-centimeter telescope. It is a compact galaxy group consisting of five bright galaxies and some nearby dwarf galaxies. The "quintet" in its name refers to the five bright galaxies that were initially observed. However, subsequent observations have found that NGC7320 among the five bright galaxies is a foreground galaxy, and the galaxy group of Stephan's Quintet is actually mainly composed of four bright galaxies: NGC7319, NGC7318b, NGC7318a, and NGC7317.

Among them, NGC7318b is colliding with the pre-existing intergalactic medium at a speed of about 1000km/s, which is produced by the collision of other galaxies in the galaxy group. In other words, there are complex galaxy interactions in Stephan's Quintet, and a series of observed phenomena caused by them, such as large-scale shock waves, intergalactic medium star formation, star formation on long tidal tails, active galactic nucleus outflows, etc.

Because the interactions in Stephan's Quintet are very active, it has become the most watched galaxy group since its discovery. It is an important sample for studying processes such as galaxy collisions, mergers, intergalactic medium, and tides .

In the study of galaxy clusters, a very important issue is the interaction history of these galaxies, that is, the history of galaxy mergers. However, because the interactions of Stephan's Quintet are relatively complex, the numerical simulation results of different research groups are different, and none of them can explain all the existing observations.

Therefore, the observation of neutral hydrogen distribution in the Stephen Quintet region with the help of FAST is very important. Neutral hydrogen is the lightest atom and the most common baryonic matter. It may be stripped to the periphery of the galaxy cluster in the early stages of galaxy merger. Understanding the distribution of neutral hydrogen in the galaxy cluster, especially the observational data of neutral hydrogen diffused in the outer regions of the galaxy cluster, will be very helpful in constraining the existing merger history models.

02

FAST completed the observation in just 22.4 hours

The area of ​​Stephan's Quintet has been observed by radio telescopes four times using the Arecibo Telescope, the Green Coast Telescope, and the Very Large Array. These observations only detected the distribution of hydrogen atoms near several massive galaxies in an area of ​​about 70 square arc minutes in the center of the galaxy cluster. The observation data were mainly used to study neutral hydrogen radiation in the galaxy cluster, and the data were not sensitive enough to detect more extended neutral hydrogen radiation.

Our research group applied for and obtained observation time from FAST, and conducted neutral hydrogen observations in an area of ​​about 700 square arc minutes in the direction of Stephan's Quintet, which is enough to cover the outer regions of the galaxy cluster and some dwarf galaxies near the cluster.

FAST has a high sensitivity, and this single-aperture radio telescope is very suitable for detecting this diffuse neutral hydrogen gas. FAST's 19-beam receiver covers a large area of ​​the sky, and data from 19 locations can be obtained in one observation. Therefore, FAST can detect neutral hydrogen radiation in this area of ​​the sky with high observation efficiency and sensitivity, making it a very suitable telescope for this topic.

In the fall of 2021, we completed radio observations of 304 locations in this area of ​​the sky in just 22.4 hours, and successfully detected this huge and thin cloud of hydrogen atomic gas.

The distribution of atomic gas in the sky around the famous compact galaxy cluster "Stephen's Quintet" detected by FAST (shown by red halos; thinner halos indicate lower gas column density). The background is a false color image synthesized from the U, G, and R optical band data observed by the Canada-France-Hawaii Telescope. Stephan's Quintet is located in the center of the image. The inset is a color image of the infrared band recently released by the Webb Space Telescope. Stephan's Quintet is one of the first five targets observed by the Webb Space Telescope and shown to the public for the first time, which shows its importance (Image credit: NASA, ESA, CSA, STScI).

03

Behind the “large scale”

These issues are more important

The "cloud" we observed is very thin, and such a thin neutral hydrogen is likely to be ionized by the ultraviolet background radiation of the universe, so this diffuse neutral hydrogen structure is difficult to exist for a long time. Therefore, from a theoretical point of view, how this cloud of gas we observed was produced, what is its connection with Stephan's Quintet, how will these gases evolve in the future, and whether similar atomic gas structures are often produced when galaxy clusters are formed are all issues that are worth careful discussion.

From an observational perspective, how many large-scale neutral hydrogen gas systems there are, whether they are more likely to appear near compact galaxy clusters, and whether they will appear near other celestial systems are also topics that can be studied with the help of FAST's ultra-high sensitivity and observation efficiency. We are also trying to observe some nearby galaxies with similar sensitivity, hoping to detect more similar phenomena.

Take the European collaborators of this article as an example. They have rich experience in numerical simulation of Stephan's Quintet, but their previous numerical simulation did not take into account the ionization and recombination process of atomic gas and gas. Therefore, the European team is now working hard to restore the merger history of these galaxies from the perspective of numerical simulation and combined with the new observational data.

At the same time, our research group's existing neutral hydrogen data also detected neutral hydrogen radiation from several dwarf galaxies near this galaxy cluster. At the end of 2021, we also used the Lijiang 2.4-meter optical telescope to successfully verify the redshift of the spectrum of a galaxy about 800,000 light-years away from the galaxy cluster. It is precisely because of the combination of past and current research that we can discuss the origin of neutral hydrogen structure in this article published in Nature.

This year we applied for some spectral observation time at the Lijiang 2.4m optical telescope. We hope to combine the optical and radio observation data of dwarf galaxies to constrain the star formation history of satellite galaxies near galaxy clusters and understand the role of dwarf galaxies in the evolution of galaxy clusters.

In short, publishing the results in Nature is just the beginning. In the future, "this cloud" can inspire us to do more in-depth research and thinking on topics such as the origin of celestial bodies in the universe . The high sensitivity of FAST also provides us with a broader window to explore the more diffuse neutral hydrogen radiation in the universe. Let us look forward to it together.

References:

[1] Xu, CK, Cheng, C., Appleton, PN et al. A 0.6 Mpc HI structure associated with Stephan's Quintet. Nature 610, 461–466 (2022). https://doi.org/10.1038/s41586-022-05206-x

Author: Cheng Cheng, Associate Researcher at the South American Astronomical Research Center of the Chinese Academy of Sciences

Review｜Han Wenbiao, Researcher at Shanghai Astronomical Observatory, Chinese Academy of Sciences

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