The all-sky map of the outer reaches of the Milky Way has been released, and the galaxy’s ＂little tail＂ may be searching for dark matter clues

　　◎ Intern reporter, He Liang

　　On April 21, NASA’s official website announced that astronomers had released a new all-sky map of the Milky Way’s periphery. There was a particularly obvious bright spot on the map, which was a stellar wake caused by a dwarf galaxy that was about to collide with the Milky Way, which also aroused heated discussions among astronomers. Chang Jiang, a doctor of astrophysics at the Zijinshan Observatory of the Chinese Academy of Sciences, told the Science and Technology Daily reporter that this is the first time that humans have directly observed a high-density wake that generates dynamic friction after the theory of dynamic friction was proposed in the 1940s. Its discovery will improve the theoretical framework of galaxy dynamics and play an important role in promoting the theory of galaxy cosmology.

　　Dynamic friction effect has not been directly observed until now

　　Dynamical friction effect is an important physical mechanism in galactic cosmology, but there is no direct observational evidence.

　　Chang Jiang said that in general, the motion of celestial bodies only needs to consider the attractive force, not the friction caused by electromagnetic interactions. For example, the earth revolves around the sun for 5 billion years and is hardly affected by friction.

　　When a massive object is stationary in the background density field, the resultant attractive force in all directions is zero, and the motion properties do not change. However, if it moves relative to the background density field, it will cause its gravitational attraction to surrounding matter and form a high-density wake behind it. This wake will bring a backward static attractive force, which will drag the object down and produce a similar result to friction, so this effect is called "dynamic friction".

　　Imagine a small boat on a big river, and the boat’s rope is tied to the shore. If the tether is released, the boat will be washed away by the river, due to the friction between the river and the boat (stickiness is not considered here). Chang Jiang explained to the Science and Technology Daily reporter that if the motion state of the boat and the river is different, the boat will be carried along with the river because of the friction of the river; if the river has no friction, and the attractive force of the river from all directions is exactly zero, even if the tether is released, the boat will stay in place because of Newton’s first law.

　　However, even in an environment where the river has no friction but attractive force, the flow path will be bent by the attractive force of the boat after the water passes through the boat – the water passing on the left side of the boat will turn right, the water passing on the right side of the boat will turn left, and the stern will produce a high-density wake, so that there is more material behind the boat than in front of the boat, thus creating a net attractive force backwards for the boat, which will cause the boat to move backwards. "Although the river has no friction, due to the accumulation of attractive forces, there will still be a friction-like effect," Chang Jiang emphasizes.

　　Like ships, galaxies move in a background material field, generating dynamic friction that causes them to slow down. The larger the spatial scale, the more pronounced this effect is. In the solar system, it can be ignored, but it is very obvious on the galactic scale.

　　This is the first time that humans have observed this high-density wake, which is the blue, white mass below the Milky Way in the all-sky map. "This shows that the number of stars there is higher than the surrounding ones, and it is likely that the two galaxies on the right side of the Milky Way disk (the Large and Small Magellanic Clouds) produce high-density wake of stars as they move through the Milky Way," Chang Jiang said.

　　New opportunities to study the Large Magellanic Cloud and dark matter

　　The new map also reveals the connection between this high-density wake and a dwarf galaxy called the Large Magellanic Cloud (hereafter referred to as the Barley Cloud) – the Barley Cloud sails through the Milky Way halo like a ship through the water (scientists believe that the Milky Way’s halo is a "hodgepodge" of high-speed stars, star clusters, and diffuse dwarf galaxies thrown out by the Milky Way, including low-density stellar spheres of stars with no obvious structure and various star streams, stellar high-density regions). Its attractive force gathers the stars in the halo and creates a wake behind it.

　　Changjiang told the Science and Technology Daily reporter that the barley cloud is the largest satellite galaxy in the Milky Way, with a total mass of roughly one-tenth of the Milky Way, but it is likely to have just "fallen" in, so it is affected by the tide for too short a time, and the tidal structure is not fully developed, so it is difficult for us to study its "falling" process.

　　This discovery is equivalent to providing a new structure or new perspective to study the falling process of the barley cloud. Chang Jiang said that because the barley cloud is so large, it can serve as a good "probe" to study the structure of the Milky Way and surrounding areas, and even limit cosmology and dark matter theories. Some astronomers believe that the Milky Way is wrapped in a huge dark matter halo, and stars and gas together make up only 3% of the mass of this dark matter halo.

　　"Obviously, according to the kinetic friction theory, this high-density wake is not only composed of high-density stars, but also there should be a more massive dark matter wake at the same location, and the combined force of the two will slow down the barley cloud." Chang Jiang boldly imagined that although dark matter cannot be seen, through the study of the stellar structure and barley cloud dynamics, the mass and range of this wake can be inferred, so as to further infer the ratio of stars and dark matter, the structure of the stellar halo, and the structure, distribution, and mass of the dark matter halo in the Milky Way, and to judge the nature of dark matter, to see if it is cold dark matter or warm dark matter, or some other special particle.

　　Telescopes help humanity understand the Milky Way and the universe

　　"The previous all-sky maps either couldn’t see the outer halo; or the accuracy was not high, and the number of stars was not enough; or the data from multiple telescopes were combined, and the observation accuracy was uncertain." Chang Jiang said that the successful observation of the faint stellar wake by astronomers this time was due to the improvement of observation capabilities and the advantage of space telescopes to achieve all-sky observations.

　　Moreover, since the earth is located on the Milky Way disk, where the density of stars, gas, dust and other celestial bodies is very high, optical observation of the Milky Way disk has always been a big problem. In addition to the structure of the Milky Way disk, observation capabilities also limit the observation of the edge of the Milky Way disk, the Milky Way halo, satellite galaxies in the Milky Way and star streams. "It is difficult to see them in areas that are too far and too dark, and even if you see them, the information obtained is extremely limited," said Chang Jiang. "This requires on the one hand the improvement of telescope technology. On the other hand, it requires multi-band observations, such as seeing high-temperature gas and high-energy processes through X-rays, and observing gas in the Milky Way through radio telescopes. The Purple Mountain Observatory is currently drawing the’galactic picture ‘of the gas distribution in the Milky Way through radio observations