For the third time in history, we have directly discovered the undeniable signature of black holes: gravitational waves from their merger. In combination with what we already know about stellar orbits near the galactic center, X-ray and radio observations of other galaxies, measurements of the speed of gas movement, it is impossible to deny the existence of black holes. But will we have enough information, from these and other sources, to tell us how many black holes there really are in the Universe and how they are distributed?
Indeed, how many black holes are there in the Universe compared to visible stars?
The first thing you would like to do is move on to direct observation. And this is a great start.
7 million seconds exposure map by Chandra Deep Field-South. There are hundreds of supermassive black holes in this region
Our best X-ray telescope to date is the Chandra X-ray Observatory. From its position in Earth's orbit, it can identify even single photons from distant X-ray sources. By creating deep images of significant portions of the sky, it can identify literally hundreds of X-ray sources, each of which corresponds to a distant galaxy beyond our own. Based on the energy spectrum of the received photons, we see supermassive black holes at the center of every galaxy.
But as incredible as this discovery is, there are many more black holes in the world than one per galaxy. Of course, in every galaxy, on average, there are at least millions or billions of solar masses, but we do not see everything.
The masses of known binary black hole systems, including three verified mergers and one merger candidate from LIGO
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LIGO recently announced its third direct detection of a powerful gravitational signal from a merger of binary black holes, confirming the prevalence of such systems throughout the universe. We don't have enough statistics yet to get a numerical estimate because the error threshold is too high. But if we take as a basis the current threshold of LIGO and the fact that it finds a signal every two months (on average), we can safely say that in every galaxy the size of the Milky Way that we can probe, there are at least a dozen such systems.
Advanced LIGO range and its ability to detect merging black holes
Moreover, our X-ray data show that there are many binary black holes with lower mass; perhaps considerably more than the massive ones LIGO can find. And this is not even taking into account the data indicating the existence of black holes, which are not included in rigid binary systems, and there must be a majority of them. If our galaxy has dozens of black holes of medium and high mass (10-100 solar masses), there must be hundreds (3-15 solar masses) of binary black holes and thousands of isolated (non-binary) black holes of stellar mass.
The emphasis here is on "at least".
Because black holes are so damn hard to find. So far, we can only see the most active, the most massive and the most prominent. Black holes that spiral and coalesce are great, but such configurations should be cosmologically rare. The ones that Chandra has seen are the most massive, active and all, but most black holes are not monsters in the millions, billions of solar masses, and most of the large black holes are currently inactive. We observe only a small fraction of black holes, and this is worth understanding, despite the magnificence of the observed.
What we perceive as a burst of gamma radiation can occur from the merging of neutron stars, which eject matter into the universe and create the heaviest known elements, but also create a black hole in the end.
And yet we have a way to get a qualitative estimate of the number and distribution of black holes: we know how they form. We know how to make them from young and massive stars that go supernovae, from neutron stars that merge, and in the process of direct collapse. And although the optical signatures of the creation of a black hole are extremely ambiguous, we have seen enough stars, their deaths, catastrophic events and star formation throughout the history of the universe to be able to find exactly the numbers we are looking for.
The remnants of a supernova born from a massive star leave behind a collapsing object: either a black hole or a neutron star, from which a black hole can later form under certain conditions
These three ways of creating black holes all have their roots, if you follow them all the way, to massive regions of star formation. To obtain:
- Supernova, you need a star that will be 8-10 times the mass of the Sun. Stars larger than 20-40 solar masses will give you a black hole; smaller stars - a neutron star.
- A neutron star merging into a black hole needs either two neutron stars dancing in spirals or colliding, or a neutron star that sucks the mass out of the companion star up to a certain limit (about 2.5-3 solar masses) to become a black hole.
- Direct collapse of a black hole, you need enough material in one place to form a star 25 times more massive than the Sun, and certain conditions to accurately get a black hole (not a supernova).
Hubble photographs show a massive star 25 times more massive than the Sun, which simply disappeared without supernova or other explanation. Direct collapse will be the only possible explanation
In our vicinity, we can measure, of all the stars that are forming, how many of them have the correct mass to potentially become a black hole. We find that only 0.1-0.2% of all nearby stars have enough mass to go supernova, with the vast majority forming neutron stars. About half of the systems that form binary (binary) systems, however, include stars of comparable masses. In other words, most of the 400 billion stars that have formed in our galaxy will never become black holes.
A modern spectral classification system for Morgan-Keenan systems with the temperature range of each star class in Kelvin. The vast majority (75%) of stars today are M-class stars, of which only 1 in 800 are massive enough to go supernova
But that's okay, because some of them will. More importantly, many have already become, albeit in the distant past. When stars form, you get a mass distribution: you get a few massive stars, slightly larger than average ones, and a lot of low-mass ones. So many that low-mass M-class stars (red dwarfs) with a mass of only 8-40% of the solar mass account for three quarters of the stars in our vicinity. New clusters of stars will not have many massive stars that can go supernova. But in the past, the star-forming regions were much larger and richer in mass than the Milky Way is today.
The largest stellar nursery in the local group, 30 Doradus in the Tarantula Nebula, contains the most massive stars known to man. Hundreds of them (in the next few million years) will become black holes
Above you see 30 Doradus, the largest star-forming region in the local group, with a mass of 400,000 suns. There are thousands of hot, very blue stars in this region, of which hundreds will go supernova. 10-30% of them will turn into black holes, and the rest will become neutron stars. Assuming that:
- there were many such regions in our galaxy in the past;
- the largest star-forming regions are concentrated along the spiral arms and towards the galactic center;
- where we see pulsars (the remains of neutron stars) and sources of gamma rays today, there will be black holes, - we can make a map and show on it where the black holes will be.
NASA's Fermi satellite has mapped the high energies of the universe in high resolution. Black holes in a galaxy on a map are likely to follow small scatter ejections and be resolved by millions of separate sources.
This is Fermi's map of gamma ray sources in the sky. It is similar to the star map of our galaxy, except that it strongly highlights the galactic disk. Older sources are depleted in gamma rays, so they are relatively new point sources.
Compared to this map, the black hole map will be:
- more concentrated in the galactic center;
- slightly more blurred in width;
- include galactic bulge;
- consist of 100 million objects, plus or minus the error.
If you create a hybrid of the Fermi map (above) and the COBE galaxy map (below), you can get a quantitative picture of the location of black holes in the galaxy.
Galaxy visible in infrared from COBE. Although this map shows stars, black holes will follow a similar distribution, albeit more compressed in the galactic plane and more centralized toward the bulge.
Black holes are real, common, and the vast majority of them are extremely difficult to detect today. The universe has been around for a very long time, and although we see a huge number of stars, most of the most massive stars - 95% or more - have long since died. What have they become? About a quarter of them have become black holes, millions are still hiding.
A black hole billions of times more massive than the Sun feeds an X-ray jet at the center of M87, but there must be billions of other black holes in this galaxy. Their density will be concentrated in the galactic center
Elliptical galaxies swirl black holes into an elliptical swarm that swarms around the galactic center, much like the stars we see. Many black holes eventually migrate to the gravity well in the center of the galaxy - which is why supermassive black holes become supermassive. But we don't see the whole picture yet. And we will not see until we learn how to qualitatively visualize black holes.
In the absence of direct visualization, science only gives us this and tells us something remarkable: for every thousand stars we see today, there is roughly one black hole. Not a bad statistics for completely invisible objects, you must agree.
ILYA KHEL