Abnormal galaxies: Where’s their dark matter?

Recently, a team led by Researcher Guo Wei of the National Observatory of the Chinese Academy of Sciences discovered a group of abnormal objects — 19 dwarf galaxies that lack dark matter. Previously known dwarf galaxies all contain large amounts of dark matter, but this new collection of dwarf galaxies could change our understanding of “how galaxies are formed.” The study was published in the early hours of November 26 in the journal Nature Astronomy

In the eyes of cosmologists, the materialwes we “see” every day, from all things on Earth to the dots of stars in the night sky, are called heavy matter. Because they are mostly made up of protons (such as familiar protons and neutrons) in mass. A very important feature of heavy matter is that it can interact electromagnetically, in other words, it can glow or reflect light. The “light” is said here, in addition to the light visible to the naked eye, but also includes electromagnetic waves in other bands. Because of this, heavy substances are the most “see” we have.

Although heavy matter is the most visible substance we have seen, recent astronomical observations (such as the rotation altimeter of galaxies, gravitational lenses, cosmic microwave background radiation, etc.) have found that their total is only about one-fifth of all matter in the universe (15.7 percent, according to the latest observations from planck). The biggest difference between the remaining four-fifths of matter and the heavy matter is that they neither glow nor reflect light, so we can’t see them. It is because of this characteristic that these novelty substances are called dark matter.

Abnormal galaxies: Where's their dark matter?

Dark matter played a very important role in the last 13 billion years of the universe. Because of its dominant position in the mass ratio of matter, dark matter determines the distribution of cosmic matter on a large scale – the large-scale structure of the universe. Modern supercomputer simulations reveal that the spatial distribution of dark matter in the universe can reproduce the observed distribution of galaxies, suggesting a inextricable relationship between galaxies and dark matter.

We have known from an early age that we live in a galaxy with a poetic name, the Milky Way. Modern astronomical observations tell us that the Milky Way is just one of the thousands of galaxies in the universe. How are these galaxies formed? Are there any general laws in their formation? One of the most dramatic chapters in modern cosmology and astronomy is the creation of a model of “how galaxies are formed” based on dark matter. Because galaxies are also the main carriers of the observation of the universe, in the past few decades, by studying their nature and formation, greatly contributed to the human understanding of the large-scale structure of the universe and the universe itself. Half of this year’s Nobel Prize in Physics is a tribute to the outstanding contribution of American scientist James Peebles.

According to the current model of galaxy formation, dark matter, which dominates the mass, is first formed by a large, near-spherical cluster of dark matter, which astronomers call a “dark matter halo”; Fall into the dark matter halo. Heavy gas in the dark matter halo through a series of complex physical processes, such as radiation cooling, star formation, star at the end of the evolution of supernova explosions, and so on, and finally formed a thousand postures, colorful glowing galaxies. Our Milky Way galaxy is one of the most beautiful galaxies.

It is precisely because the physics of the heavy subs that formed the glowing galaxy occurs in the dark matter halo, which is also graphically considered to be the cradle and home of the glowing galaxy “born in Stens.” Our Milky Way is now in a dark mass about a trillion times the mass of the sun, and as the sun rotates around the center of the Milky Way, our solar system travels between dark matter. Everyone we know, every story we hear, is in this dark matter that we can’t see in our eyes. Because of this, the “galaxy” in the mouths of astronomers now generally includes visible glowing parts and invisible dark matter halos. The weight of galaxies consists mainly of stars and gases.

Depending on the mass size of the dark matter halo, astronomers have given galaxies different names — larger galaxies that are much larger than the mass of the Milky Way (about a trillion times the mass of the sun) and dwarf galaxies much smaller than the milky milky mass. There are also “big guys” in the universe that make up many galaxies – “galaxy clusters” and “galaxy clusters”.

A cluster of galaxies is usually made up of several to dozens of galaxies. Our Milky Way galaxy and neighbor Andromeda galaxy, for example, and the surrounding dwarf galaxies make up this cluster. Galaxy clusters are the “Big Mac” in the universe — they usually consist of hundreds of galaxies. The dark matter halo of galaxy clusters has a very powerful gravitational pull to capture enough weight matter, so the weight mass of the cluster is about as close as the average weight of the universe (i.e. about one-fifth). In contrast, the dwarf galaxy’s dark matter is less massive, and its gravity is not strong enough to capture some of the higher-temperature heavy gas and the supernova gas that is blown out when a supernova erupts. As a result, dwarf galaxies are usually dominated by dark matter, with the proportion of heavy matter being much less than one-fifth.

Many astronomers use only dark matter instead of weight matter in numerical calculations that simulate the formation of cosmic structures. The numerical simulations they obtained were also very similar to the cosmic structure shown by the heavy submatteric matter (Figure 2).

Abnormal galaxies: Where's their dark matter?

Figure 2. The image on the left shows the distribution of galaxies in the range of about two billion light-years, as observed by Sloan Digital Sky Survey, with each point representing a galaxy. The image on the right shows a supercomputer computer called ELUCID showing the density distribution of dark matter over the same spatial range. From yellow, white, and blue to black, the density of dark matter is shown from high to low. (Photo credit: Wang Huiyuan; Wang et al.) 2016, ApJ, 831, 164)

Vision Sining: Abnormal Galaxies in Galaxy Clusters

Last year, a team led by astronomer Pieter van Dokkum of Yale University observed a strange dwarf galaxy, NGC 1052-DF2, in the NGC 1052 cluster. By measuring the motion of the globular clusters in the galaxy, they calculated the total mass of the galaxy (including the mass of dark matter and the mass of the heavy matter) and found that the total mass was almost equal to the stellar mass estimated by the luminometers of the galaxy (i.e. the weight matter, because the galaxy had few gases). In other words, this galaxy is an anomaly, lacking a lot of dark matter!

Why doesn’t it have dark matter to faint? The appearance of NGC 1052-DF2 has attracted widespread interest from scientists, who have tried to give several different explanations. One natural explanation is that the galaxy lives in a densely dense NGC 1052 galaxy cluster, and if it had moved closer to the center of the galaxy cluster, its material would have been stripped away by the powerful tidal forces of the galaxy cluster. Because dark matter is relatively loosely distributed, it is particularly easy to be stripped. In contrast, the stars at the center of the galaxy are more densely distributed and thus survive. This interpretation of tidal stripping is supported by the results of supercomputer simulations. As a result, in dense, complex environments, small galaxies may be forced to lose their homes and move away from them because of the tidal effects of large galaxies.

It’s worth noting that scientists are still debating whether NGC 1052-DF2 is a galaxy that lacks dark matter because of the controversy surrounding the estimate of the distance it has with us. But in any case, as long as a galaxy lacking dark matter is located in a high-density environment, the current model of galaxy formation can explain its predecessor.

Bigger doubt: Abnormal galaxies are also isolated

If NGC 1052-DF2 is in a dense cluster of galaxies, which can be explained by the tidal effects of large galaxies, then a bold and interesting question is: Are there galaxies in low density regions that lack dark matter? The research team, led by Guo Wei, a scientist at the National Observatory, answered this question, and their discovery of 19 strange galaxies raises greater doubts and challenges.

The Arecibo telescope in Puerto Rico is a well-known radio telescope. It has appeared in Hollywood films of the same name based on Carl Sagan’s sci-fi novel Contact; in the mass media, it is often associated with finding extraterrestrial life; and in 1974, Russell Hulse, a PhD student, and his mentor, Joseph. It was through it that Taylor first discovered the pulsar binary (PSR1913-16), giving circumstantial evidence of the existence of gravitational waves, and thus winning the 1993 Nobel Prize in Physics. In 2005, the Arecibo telescope began a six-year project to observe the ejection signals of neutral hydrogen in more than 30,000 extragalactic galaxies to study the nature of these gas-rich galaxies and how they formed. This sky-watching project is called Arecibo Legacy Fast ALFA Survey, and the acronym ALFALFA happens to be the English name for the plant’s azimuth.

For observational purposes, most of alfalfa’s galaxies are gas-rich and located in low densities. ALFALFA’s radio band data can tell us the mass of gas in these galaxies. But to know the mass of another part of these galaxies, the mass of stars, we also need to know about the visible light bands of these galaxies. Scientists at the National Observatory selected a signal-to-noise ratio from the alfalfa galaxy and another visible light-band galaxy survey project, Sloan Digital Sky Survey (SDSS) from the star table, which has a signal-to-noise ratio that is not facing us (unsodulated). Preliminary screening of 324 dwarf galaxy samples was obtained.

For the 324 galaxy samples, the scientists estimated the mass of their gas from the total radio integral flow of the sub-hydrogen, and the star mass from the visible brightness of the star, the sum of which was the mass of the galaxy’s heavy son. The spectral width of the neutral hydrogen is related to the total mass of the galaxy within the neutral hydrogen radius, so with the help of the spectral width of the neutral hydrogen in ALFALFA, the scientists calculated the total mass of each galaxy. They were surprised to find that of the 324 dwarf galaxies, 19 had a mass of more than half the mass! Unlike normal dwarf galaxies, these 19 dwarf galaxies dominate the weight of the anti-passenger, dominant position of mass. This is a group of abnormal dwarf galaxies that lack dark matter!

Abnormal galaxies: Where's their dark matter?

Figure 3. The relationship between the total mass (vertical axis) and the weight mass (horizontal axis) of 324 galaxies. The gray dotted line marks a relationship in which the total mass is equal to twice the mass of the weight. The blue data points beneath the dotted line mark 19 dwarf galaxies that lack dark matter. (Photo: Qi Guo et al.) )

The good play is still behind it. When scientists looked for large clusters of galaxies like NGC 1052 next to the 19 dwarf galaxies, they found that 14 dwarf galaxies were in a particularly isolated region — located outside the three-fold radius of all nearby galaxy clusters or clusters. The radius of the bit force is usually used to define the radius of a galaxy, and areas outside the radius of the three-fold force are generally thought to be unaffected by the tidal action of the galaxy (or, in image, not within the “sphere of influence” of the galaxy).

The biggest difference between the 19 newly discovered dwarf galaxies lacking dark matter and previous NGC 1052-DF2 is that most of the newly discovered anomalous galaxies are located in low-density, isolated environments that are almost immune to the tidal effects of large galaxies. So the question arises, why are there so little dark matter in these galaxies? Are they born so little dark matter, or have they ever been lost because of some physical processes? This is a new challenge to the current model ingresswith. Scientists don’t have a good answer yet.

Understanding how these isolated dwarf galaxies lack dark matter formed will contribute to a better understanding of the process of galaxy formation. This new discovery also tells us again that one of the most fascinating things about the universe is that it is so vast that it quietly burys all kinds of surprises and eggs in many places we didn’t expect. Every surprise and egg discovery may change human understanding and understanding of the world around it.

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