Looking for lost members of the black hole family

Black holes are not easy to study. It is so dense that anything that tries to get close to it will be swallowed up, and even light can’t get rid of its clutches, so it doesn’t reflect light. This forced astronomers to take a different approach to its search.

When it is observed that the star is accelerating around the center of an invisible thing, astronomers will speculate that there may be a black hole there. Black holes sometimes produce outward jets during the process of phagocytosis, which is another “leak leaker” of black holes. Recently, astronomers have also observed gravitational waves generated by black hole collisions, so finding black holes through gravitational waves is the third way to find black holes.

Through these observation methods, astronomers’ “shelves” have been placed on large and small black hole “samples”. However, these “samples” lack one that is supposed to be missing – a medium-quality black hole.

Black hole family “three missing one”

The black holes currently found can be roughly classified into two categories according to their quality. One type is the stellar mass black holes, which are formed by supernova explosions when large stars (generally stars with a solar mass more than 20 times) are burned out. Such black holes are usually less than 100 times the mass of the sun, and the largest known currently is 15 times the mass of the sun. The other is a supermassive black hole in the center of the galaxy. The black hole in the center of the Milky Way is 4 million times the mass of the sun. Black holes in other galaxies can reach billions of solar masses. But the medium-quality black hole between the two categories has not been found.

This situation is puzzling. You think, the supermassive black hole must have a place to come. The mainstream view is that they are merged by smaller black holes. However, to grow into a supermassive black hole with 1 billion solar masses, you need to swallow millions of star-rated black holes. This process takes a long time, and even the age of the entire universe is not enough. However, astronomers have discovered that supermassive black holes have existed for the first time after the Big Bang. It seems that there is not enough time for them to grow so fast, unless they have bigger “building blocks”, such as small black holes like constant stars, but medium-quality black holes!

You might say that a medium-quality black hole isn’t coming from a star-studded black hole. Doesn’t that take a long time?

Of course, the star-studded black hole “building blocks” is indeed a way to form medium-quality black holes. But their formation may have another shortcut. A shortcut is generated by the death of the first generation of stars. Because the quality of the first generation of stars is much larger than the current stars, the black holes they formed after the supernova explosion may have become “medium-sized” black holes. Another shortcut is that it is directly collapsed by a large group of gas clouds. With these two shortcuts, the formation of medium-mass black holes saves time, allowing them enough time to merge into supermassive black holes.

However, if this is the case, there must be a lot of medium-quality black holes that have not been merged into massive black holes, and we should be able to observe them in the universe. But the fact is that this black hole is seriously “out of stock” on the “shelf”. Where did they hide?

Finally found a medium quality black hole

In fact, this problem was only brought up by astronomers until recently. Before that, they thought it had been solved.

This is the case. Since the 1980s, radio telescopes around the world have detected a series of unusually bright X-ray bursts. Astronomers initially thought they came from black holes. Because when the substance hits the black hole at a faster and faster speed, the friction with the adjacent material will make it hot, and then emit X-rays. The greater the mass of the black hole, the faster the material surrounding it, the greater the friction, the brighter the X-rays emitted. But now, these X-ray bursts look too bright, more than a million times brighter than the sun, they can’t come from black holes with only the mass of the stars, and the evidence seems to point to medium-quality black holes.

But with the accumulation of data, astronomers first discovered that these X-ray spectra do not match the spectra of medium-mass black holes. Then in 2014, it was discovered that these X-ray bursts are pulsating signals and can be explained by pulsars. So there is no need to think that they are produced from medium-quality black holes.

So, what other observations have been missed by us? In February 2017, an astronomical group at Harvard University in the United States announced the discovery of a medium-quality black hole in a globular cluster called the “Cuckoo 47th Cluster (47 Tuc, located on the southern hemisphere side of the Rhododendron).”

It stands to reason that there should be a black hole in the center of this cluster, but because this cluster is too old, many of the stars have already burned out, and the nearby matter has been swallowed up by it, leaving only the pulsar. Therefore, it is impossible to see the phenomenon that the star moves around an invisible center, nor does it observe the phenomenon that the black hole produces an outward jet during the process of phagocytosis. Therefore, if there is a black hole in the center, two conventional methods for determining the black hole are It doesn’t work here.

Astronomers have taken a different approach by measuring the small changes in the pulsar acceleration in the cluster. They found that the pulsars in the 47th cluster of the Cuckoo, in addition to their gravitational effects, were accelerated by an additional gravitational force that pointed to a center. According to this, they believe that there is a black hole hidden in it. After calculation, the quality is between 1450 and 3800 solar masses, which is just in the medium quality range.

That said, I finally found one, but most people think that just finding one is not enough to solve the problem of why medium-quality black holes are so scarce. In order to fill this “shelf”, someone turned to another type of missing celestial body in the universe – the dwarf galaxies.

Two mysteries are combined into one

While some astronomers search for medium-quality black holes in the sky, others are looking for missing dwarf galaxies. These dwarf galaxies, as the name implies, are not large galaxies, but galaxies that operate around larger galaxies, such as the Milky Way. The problem now is that the dwarf galaxies we observed are too few.

In the standard picture of cosmology, both galaxies and clusters of galaxies are infiltrated by dark matter. Dark matter is an inert, invisible mysterious substance that is the “scaffolding” formed by galaxies. When the astronomers computerized the formation of galaxies in the early universe, they found that there should be many dwarf galaxies that did not merge. However, in the real universe, we see far less. In addition, there is another problem: the stars we see in the center of the dwarf galaxies do not seem to run fast enough. According to cosmological standard theory, in the center of dwarf galaxies, because dark matter is concentrated there, and dark matter is gravitational to ordinary matter (such as stars), it will make the star rotate at a faster speed.

In order to solve these two problems, some people tend to question the nature of dark matter. However, astrophysicist Joseph Hill of the University of Oxford in the United Kingdom believes that there is an easier way to assume that most dwarf galaxies have a medium-quality black hole in their center.

Regardless of everything, Kirk’s proposal is logically justified. Because there are supermassive black holes in the center of large galaxies, small galaxies should also have medium-quality black holes to match.

In the early days of the birth of a dwarf galaxies, medium-quality black holes would consume a lot of gas. But like the star-rated black hole, there is a huge jet in the process of smoking (the brightest type of celestial body in the universe – the quasar is produced in this way). The jets blew most of the material in the galaxy, and the resulting dwarf galaxies were almost very small and not so bright. This may be why we have been difficult to observe dwarf galaxies. In addition, ordinary matter and dark matter have a gravitational effect. When the jet blows away the common matter in the center of the dwarf galaxies, the dark matter also follows the shape and leaves the center. This may explain why the stars in the center of the dwarf galaxies are not running fast enough.

If Hill’s explanation is credible, the dwarf galaxy is the most promising place to look for missing medium-quality black holes. But how to search for faint dwarf galaxies is still a tricky issue for astronomers.

There is another way: look for gravitational waves. So far, the few gravitational wave events detected have come from the collision of neutron stars or star-studded black holes. If we can detect gravitational waves from a medium-mass black hole collision, then it is equivalent to detecting a medium-mass black hole itself.


Star clusters and galaxies

A galaxies are a group of stars that orbit around a center, and a cluster of stars is a relatively concentrated group of stars in the same galaxies. The galaxy must have a center, but the star group does not necessarily have a center. Of course, sometimes we did not find a group of stars at the beginning, so we called them “star clusters”, but later found that there was a center, so we rose from “star clusters” to “galaxies”.