Galactic stellar flow

Once a small galaxy approaches a giant galaxy, it will be twisted and disintegrated by the powerful gravitational tide and pulled into a slender and spectacular star stream.

Finding these interstellar immigrants requires a pair of sharp eyes. In theory, these stars will line up in a long line (that is, a stream) to expose their whereabouts, just like the conga dance team on a crowded dance floor (conga dance originated in Latin America, and dancers will dance together in a long line).

Back to the present, some star streams will lead to globular clusters or a satellite galaxies in the Milky Way, which is probably the birthplace of the stars in the star stream, and some star streams will lead to the only remains of the birthplace. But in fact, because the "aborigines" are relatively evenly distributed in the Milky Way, it is difficult to stand out from the stars and be found.

In order to overcome this problem, many researchers have recently adopted "matched filtering technology", which was developed during World War II to obtain clear images of incoming enemy planes. As long as we know the general forms of indigenous stars and "interstellar immigrants", this technology can filter out the former and let the star stream appear.

The Sagittarius star stream discovered by Ibata and his colleagues in 1994 may be the most impressive known star stream. This star stream is like a giant star necklace, circling the Milky Way. It spans over 65.438+billion light years and contains about 65.438+billion stars. Elliptic galaxy connected to Sagittarius dwarf galaxy ("short" means that the galaxy is very small, and "ellipse" means that the galaxy is ellipsoid in shape). There are 15~20 similar small galaxies around the Milky Way, which revolve around the Milky Way like satellites around planets (hence the name satellite galaxies). These satellite galaxies vary in size, from large magellanic cloud, which is about one-tenth the mass of the Milky Way, to Sagittarius dwarf galaxy, which is one-hundredth the mass of the Milky Way, and even dim galaxies, which are only one-millionth the mass of the Milky Way.

Living near the Milky Way is not a comfortable thing. These small galaxies will gradually deform and eventually disintegrate. Sagittarius dwarf galaxy has been extinct for billions of years, and now it is heading for disintegration. Its stars will be scattered throughout the galaxy, and now such a star stream will gradually disappear, and it will be difficult for future astronomers to distinguish it from the indigenous stars in the galaxy. Several other small galaxies are also being dismembered by the Milky Way, and some of them now have only one star stream. Large magellanic cloud represents another rare phenomenon-the Milky Way gets gas from it, not stars.

The mechanism behind the disintegration of galaxies is the gravitational tidal action that we are very familiar with. The same mechanism also leads to the ebb and flow of the oceans on earth. Tidal force occurs when different parts of the celestial body are subjected to different gravity.

The moon exerts more gravity on the side of the earth facing it than on the side facing away from it. Although the difference between these two forces is small enough to break the earth, it is enough to make the ocean slightly expand. Because the two celestial bodies revolve around each other, the uplift of the ocean will also rotate with it, so that the sea level somewhere on the earth will rise or fall periodically. Similarly, the Milky Way will exert more gravity on one side of the satellite galaxies or cluster than on the other, causing them to distort.

In this way, the stars in the satellite galaxies or cluster will be gradually dragged away by the Milky Way (see the picture below). As time goes on, satellite galaxies will lose more and more stars. These stars, like crumbs falling from bread, form long tails around satellite galaxies.

The star stream will gradually be absorbed by the Milky Way and eventually become invisible. However, in the vast Xinghai, we can find subtle clues about the movement of stars and the clues of these "interstellar immigrants".

Satellite galaxies, such as Sagittarius dwarf galaxy, has contributed to the construction of the Milky Way. These discoveries have completely changed astronomers' original theoretical understanding of galaxy formation: they once thought that all galaxies originated directly from the almost imperceptible fluctuation of material density in the primitive universe (astronomers confirmed that the material density in the early universe was almost the same, with only about three ten thousandths of a difference), and then experienced the early avalanche growth and quickly evolved into this appearance.

Now, based on the observation of stellar flow, researchers generally believe that only dwarf galaxies whose mass does not exceed 65.438 billion times that of the sun have experienced such a rapid formation process; A large galaxy like the Milky Way, with a mass equivalent to hundreds of billions of suns, was gradually formed through the accretion and annexation of dwarf galaxies. This annexation process has continued to this day, but the intensity is not as good as before.

Astronomers caught the process of the Milky Way devouring neighboring galaxies on the spot, and they raised a deeper question: What kind of chemical composition do these ancient galaxy "building materials" have? What is the ratio of "interstellar immigrants" to "aborigines" in large galaxies now? How did the chemical elements brought by these small galaxies change the early evolution history of the Milky Way?

Like fossils, these stellar streams record the history of galaxy construction, and in addition, they can play a huge role in detecting dark matter (see the short article on the next page).

In order to understand these problems, astronomers need to know not only which stars are being plundered by the Milky Way, but also which stars have been captured. The difficulty for researchers is that once foreign stars and gases are mixed into the Milky Way, we can't identify them by their unique spatial distribution. Astronomers must find more subtle clues to trace the origin of these stars, such as their movement patterns and some indelible inherent characteristics in their chemical composition.

We are all used to describing the motion of objects with position and speed. But there are other different characteristics of motion, which can be described by physical quantities such as energy and angular momentum. Just as the spatial position of an object can be described by three coordinates, it is called three-dimensional space, so we can also describe the motion state of the object by position plus momentum (a ***6 physical quantity). This abstract six-dimensional space is called phase space.

The advantage of phase space is that compared with real space, the arrangement pattern of stars in phase space is more reduced (here refers to the ability to recover after great changes). Although the process of annexation and digestion of galaxies usually destroys the spatial structure of stellar streams, it cannot erase their overall structure in phase space [this is an important principle in statistical mechanics-Liouville theorem].

Therefore, by measuring energy, angular momentum and the density of random samples of stars in phase space, researchers can identify clusters that cannot be directly observed. They are the phantoms of satellite galaxies, which were disintegrated by the Milky Way a long time ago. Several research groups, such as Amina Helmi of Kapteyn Astronomical Institute in Groningen, the Netherlands and Chris B. Brook of Washington University in the United States, have used this technology to find some remains of the annexed satellite galaxies.

The remains that have been discovered so far are located near the solar system, because the existing equipment cannot measure the three-dimensional motion of more distant stars accurately enough. Name, source and quality

Length (solar mass)

(Light-years) Dwarf galaxies that have been swallowed up by Arcturus flow in the age of structural discovery are unknown. Old stars lacking heavy elements 197 1 Magellanic current, large Magellanic galaxy and small Magellanic galaxy, 200 million 1 100 million hydrogen 1972 Sagittarius current, Sagittarius short elliptical galaxy 1 100 million. 1 100,000 different stars 1994 pearl star stream (Helmi) has been swallowed up by dwarf galaxies1100,000 to1100,000. Some old stars with complete rings around the Milky Way lack heavy elements 1999 Paloma 5 star stream globular cluster Paloma 5 5000 30000 old star 200 1 Virgo star stream has been swallowed up by dwarf galaxy 30000200 1 unicorn ring dog constellation dwarf galaxy 65438+2000000 middle-aged star 2002 anti-galactic star stream has been.

(45-degree tidal star stream) Globular cluster NGC 546610,000 60,000 Very old star 2006 Orphan star stream Ursa Major dwarf system100,000 20,000 old star stream 2006 Agron star stream.

(Acheson flow) Unknown unknown 2007 wailing star flow in globular clusters

(Cocytus flow) Globular cluster unknown unknown 2007 Forget Sichuan star flow

Unknown globular cluster unknown Stix flow in 2007

(Ghost River) The swallowed dwarf galaxy is unknown. 2007 * Ashley, Kositos, Lette and Styx are all Styx around the underworld in Greek mythology.