Shulou(Shulou.com)11/24 Report--

The Milky way disk, nearly 200,000 light-years across, is not flat, but twisted like potato chips. What kind of power can make a galaxy look like this?

Last time, we have made great progress in our understanding of the overall shape of the Milky way, from the first drawing of the shape of the Milky way by Herschel's "counting stars" method to the spiral galaxy drawn through the 21-centimeter line of interstellar neutral hydrogen.

But even so, what we know about the Milky way is still very limited. Although there is a general idea of the shape, there are often unexpected discoveries in details. For example, the disk-shaped spiral shape was later found to be not as flat as expected, and its edges seemed to be distorted by something, showing a warping structure.

In fact, the warping structure of the Milky way was discovered by radio observations in the 1950s. Later, a great deal of observational evidence shows that not only our Milky way, but also the outer disk edges of about 1P3 disk galaxies in the universe will roll up or down more or less.

However, due to dust cover and other reasons, for the distant outer disk of the Milky way, even a more accurate ranging is very difficult. So although there has long been speculation about the warping structure of the Milky way, there has been no direct evidence.

In 2019, the National Astronomical Observatory of the Chinese Academy of Sciences joined with a number of international cooperation teams to determine the warping structure of the Milky way for the first time. The results were published in the February 2019 issue of the journal Nature Astronomy.

Cepheid variables have always been a common tool for astronomers in terms of long-distance ranging in the Milky way.

The Cepheid variable is a special star whose luminosity varies periodically. For this kind of star, the longer the period of light change, the higher the absolute luminosity, that is, the absolute magnitude, and there is a fixed relationship between the two. The absolute luminosity decreases with the increase of distance, so we can calculate the distance of the star based on the apparent magnitude we actually see. Compared with extragalactic supernova ranging, Cepheid variables are very useful and accurate in ranging within the Milky way.

In order to solve the problem of inaccurate ranging caused by dust in the outer disk of the Milky way, the researchers selected 1339 typical Cepheid variables and constructed a more accurate model of the galactic disk.

For these sparkling stars scattered in interstellar space, they are like lighthouses in the fog, giving us a rough outline of the Milky way.

The researchers found that the farther away the stars are from the center of the Milky way, the more they deviate from the galactic disk. For example, the location of the solar system (about 26000 light-years from the center of the galaxy), where the stars are basically within the range of the galactic disk, with a difference of at most a few hundred light-years, but at the edge of the outer disk farther from the center of the galaxy, the stars there can deviate from the galactic plane for thousands of light-years.

And these stars do not deviate in the same direction, some up and some down. In addition, the twist of the disk itself is not fixed, but there is a precession, that is, the direction of the distortion will change with time.

Why do good-end spiral galaxies show such a distorted attitude? What kind of mysterious force can distort an entire galaxy?

Considering that this warping structure is not unique to the Milky way, this suggests that the cause of it is also common, rather than some kind of coincidence.

For example, the researchers believe that this phenomenon may be caused by the huge rotation force of the inner disk of the galaxy. To put it bluntly, the interior of the galaxy is so coiled that the periphery is curled out of shape.

In addition, some people think that this may have something to do with the dark matter halo around the galaxy or the intergalactic magnetic field. In short, there have been different opinions on the causes of this phenomenon.

In 2020, another article on the warping structure of the Milky way was published in Natural Astronomy. Based on the second phase of the ESA Gaia mission, the researchers analyzed the motion of 12 million stars and found that the warping structure of the Milky way is moving much faster than expected. In this way, this rules out the possibility of dark matter halos or interstellar magnetic fields. This rapid change means that the cause of galaxy distortion should be a more powerful force, such as the collision of another galaxy.

The Gaia Telescope has previously found evidence that the Milky way has collided with other galaxies, and this time the researchers believe that it may be one of the Milky way's satellite galaxies, the Sagittarius dwarf galaxy, that may have warped the Milky way.

The Sagittarius dwarf galaxy, also known as the Sagittarius Dwarf Ellipsoidal Galaxy (SagDEG), is a satellite galaxy with a diameter of about 10, 000 light-years orbiting the Milky way in an elliptical orbit. The galaxy may have circled the Milky way "face-to-face" dozens of times over billions of years.

Before the discovery of the Canis Major Dwarf Galaxy, the Sagittarius Dwarf Galaxy was thought to be the closest galaxy to the Milky way, and it is currently only about 50,000 light-years from the center of the Milky way. You know, the radius of the Milky way is only tens of thousands of light-years. As a result, most of the structures of the Sagittarius dwarf galaxy, except for the core, have been nearly destroyed by the tidal forces of the Milky way and have begun to integrate into the Milky way.

The warping structure at the edge of the Milky way may be caused by the collision between the dwarf galaxy of Sagittarius and the Milky way. The dwarf galaxy of Sagittarius is like a stone thrown into the water, while the warping of the Milky way disk is a ripple on the surface of the water.

The researchers speculate that the collision between the dwarf galaxy of Sagittarius and the Milky way may have occurred between 6.2 billion and 4.2 billion years ago, about the same time as the birth of the sun. That is to say, is it possible that this collision between the dwarf galaxy of Sagittarius and the Milky way led to today's solar system? Only then did we have the earth and today's human civilization?

Of course, at present, these are only half-brain conjectures, and there is no relevant evidence. The study of galactic collisions and the evolution of galactic disks billions of years ago can be called "galactic seismology" and belongs to the category of Galactic Archaeology.

Just as people can learn about the internal history of the earth through earthquakes, they can learn more about the sun through solar earthquakes, and starshocks can learn about other stars in the Milky way. Now that we have entered the era of "galactic seismology", we can learn more about the past and present lives of the Milky way by studying the oscillation and evolution of the Milky way disk.

references

[1] https://www.cnsa.gov.cn/n6758824/n6759218/c6805376/content.html

[2] https://www.nature.com/articles/s41550-018-0686-7

[3] https://www.esa.int/Science_Exploration/Space_Science/Gaia/Milky_Way_s_warp_caused_by_galactic_collision_Gaia_suggests

[4] https://www.nature.com/articles/s41550-020-1017-3

[5] https://www.universetoday.com/157868/a-dwarf-galaxy-passed-close-to-the-milky-way-and-left-ripples-in-its-wake

[6] https://academic.oup.com/mnras/article/516/4/4988/6701646

This article comes from the official account of Wechat: Linvo says ID:linvo001, author: Linvo

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