Who said that the stars are far apart?
If the universe is likened to an endless sea, then galaxies are like islands. The modern standard cold dark matter cosmology believes that the structure of the universe is hierarchically clustered-galaxies and galaxies slowly gather under the action of gravity, and gradually form larger-scale structures. Therefore, in our universe, galaxies usually exist as islands. Our Milky Way is located in a group of galaxies called the Local Group.
Because the universe(https://www.hermes2021.org) is very empty, in general galaxy islands, the distance between galaxies and galaxies is much larger than the scale of the galaxy island itself. However, there is a type of galaxy group that is very special. The distance between islands and islands in these galaxy groups is very small, which is basically the same size as the island itself. Such galaxy groups are called compact galaxy groups.
What can the study of dense galaxy groups tell us? How to judge whether a galaxy is “dense”?
The past and the future of the galaxy can be seen
The first compact galaxy group confirmed in history is the famous Stephen Quintet (see Figure 1). In Stephen’s Quintet, we can clearly see that the distance between the members of these galaxies is so small that they are almost close together, and the image of the galaxy itself is somewhat different from that of an isolated galaxy, which is not so regular.
We know that galaxies are not only called “universal islands” in terms of their large-scale structure, but they are also a complex ecosystem composed of gas, stars, dust, and dark matter. In addition to its own quality (the size of the island https://www.hermesonline.org), the evolution of this ecosystem is also closely related to the surrounding “island” environment. The archipelago such as the dense galaxy group is a very special environment.
In a dense galaxy group, the distance between galaxies and galaxies is very small. The huge mass of the galaxy itself will bring huge tidal effects to adjacent galaxies, which will destroy the structure of the galaxy itself and affect the star formation in it, even including the center. The growth process of black holes. In addition, due to the effect of gravity, galaxies of equal mass in the compact galaxy group will merge quickly (this kind of galaxy merger event of equal mass is called a main merger event in astronomy), and this merger process affects galaxies In terms of its own “ecosystem”, it is almost catastrophic. However, the “catastrophic” event of galaxy merger is an important way for the mass growth of galaxies in the universe, and it is also a hot and difficult point in current galaxy astronomy research. For example, we do not know whether the Milky Way has experienced such catastrophic mergers in its evolutionary history? In the next few billion years, how will the Milky Way and its largest neighbor, the Andromeda Nebula stage such a super merger event?
It is generally believed that the merging process of galaxies in the compact galaxy group will be very rapid. In other words, the dense galaxy group is a special and short-lived state in the evolution of galaxies. Corresponding to this transient transition, dense galaxy groups are not common in observations. Therefore, searching for and establishing a larger and more complete sample of compact galaxy groups and studying its evolution over time is of great significance for understanding the growth of galaxies, especially the merger process.
Finding the true “dense galaxy group” is not easy
The classic method of identifying dense galaxy groups in history is to identify them through images, such as the famous Hickson galaxy group list, which contains 100 dense galaxy groups, published by Canadian astronomer Hickson in 1992. However, this method of image recognition alone has an obvious shortcoming, that is, it lacks distance information. Therefore, it is possible to mistake foreground or background galaxies at different distances as members of the wrong galaxy group. Therefore, in order to verify the compact galaxy group without error, the spectroscopic observations of all member galaxies are required. The redshift of the galaxy can be obtained from the spectral observation of the galaxy, and the distance of the galaxy can be known from the redshift of the galaxy. Except for galaxies that are very close, the spectral redshift is still the most accurate method of galaxy distance measurement.
At present, due to the development of optical fiber spectroscopy technology, the number of galaxy samples with spectral redshift (distance) measurement has greatly increased. For example, the most successful astronomical survey observation by ground-based astronomical telescopes-galaxies with redshift measurement in the Sloan Digital Sky Survey The number of samples reaches millions. However, for dense galaxy groups, the proportion of all members with redshift measurements is not high. This is because general spectral surveys use optical fiber spectroscopy arranged on the focal plane of the telescope to simultaneously shoot multiple targets on the celestial sphere. However, due to the physical collision between the optical fiber and the optical fiber, it is impossible to observe two very close targets on the celestial sphere at the same time. For the dense galaxy group, because its members are very close, it is impossible to arrange multiple optical fibers to observe the spectra of all its members at the same time.
Is there a better way to search for dense galaxies more comprehensively? Of course, the best way is to use other telescopes to make supplementary observations of galaxies that were missed by the collision of optical fibers in the Sloan Digital Sky Survey. In our country’s Guo Shoujing Telescope Spectral Survey, a research team from the Shanghai Astronomical Observatory edited these missing galaxy samples and established them as a supplementary galaxy sample for observation. So far, the Guo Shoujing Telescope has made spectral observations of tens of thousands of supplementary galaxies and obtained accurate redshift measurements. Therefore, combining the observation data of the Sloan Digital Sky Survey and the Guo Shoujing Telescope Survey can carry out unique scientific research on these very close galaxy systems (see Figure 2).
Recently, the Shanghai Astronomical Observatory’s research team has constructed the largest and most complete sample of compact galaxies based on these data, which has become a basic work in related research fields. The research has now been published in the Astrophysics Research Supplement (ApJS).
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Specifically, the study included more than 6,100 dense galaxy groups, which expanded the sample of previous studies by more than an order of magnitude. Among them, the spectra of more than 1,100 dense galaxy groups with member galaxies were exclusively provided by the Guo Shoujing Telescope. It is worth mentioning that this research also innovatively adopted the idea of ”I would rather kill by mistake than let it go”, and constructed more than 8,000 “candidate samples” of dense galaxy groups. This so-called candidate sample means that there are not all spectral redshift measurements in these galaxy group samples, so it cannot be fully confirmed as a true compact galaxy group. It needs to be further verified by spectroscopy in the future. At that time, the observation data of the Guo Shoujing telescope will play a leading role in the related research of compact galaxy systems, thereby producing more original research results in the frontier hot research field of astronomy, which is the merger and interaction of galaxies.
