In the vast garden of the universe, the heaviest black holes grew from seeds. These seeds, nourished by the gas and dust they consume, or by the fusion with other dense objects, grew in size and weight, forming the centers of galaxies, such as our own Milky Way. But unlike plants, the seeds of the huge black holes must have been black holes. And no one has ever found these seeds ̵
One idea is that supermassive black holes – the equivalent of hundreds of thousands to billions of suns in mass – have grown out of a population of smaller black holes that have never been seen. This elusive group, the "medium mass black holes," would weigh between 100 and 100,000 suns. Among the hundreds of black holes that have been found so far, there were a lot of relatively small holes, but none with certainty in the middle mass range of the "desert".
Scientists use NASA observatory high-performance space telescopes, among others, to detect distant objects suitable for describing these exotic creatures. They have found dozens of potential candidates and are working to confirm them as black holes. But even if they do, a whole new puzzle opens up: How did black holes form with medium mass?
"What fascinated and why did people spend so much time finding these medium-sized black holes? because it sheds light on processes that took place in the early Universe – what were the masses of black holes or new black hole formation mechanisms that we have not thought of yet, "said Fiona Harrison, a professor of physics at Caltech in Pasadena, California, and principal investigator of NASA's NuSTAR mission.
Black Hole 101
A Black Hole is an extremely dense object in space from which no light can escape. If material falls into a black hole, it has no way out. And the more a black hole eats, the more it grows in mass as well as in size.
The smallest black holes are referred to as "stellar masses" with 1 to 100 times the mass of the Sun. They form when stars explode in violent processes called supernovae.
Supermassive black holes, on the other hand, are the central anchor of large galaxies – for example, our sun and all other stars in the Milky Way circle a black hole called Sagittarius A * that weighs about 4.1 million solar masses. An even heavier black hole – with a whopping 6.5 billion solar masses – is the heart of the galaxy Messier 87 (M87). The supermassive black hole of M87 appears in the famous image of the Event Horizon Telescope and shows for the first time a black hole and its "shadow". This shadow is caused by the event horizon, the point of the black hole where it does not return, the light bends and captures by its strong gravity.
Supermassive black holes usually have disks of material called "accretion disks" made of extremely hot material, high energy particles that shine brightly as they approach the event horizon – the black hole region of no return. Those who light up their slices because they eat a lot are referred to as "active galactic cores."
The density of matter needed to create a black hole is mind-boggling. To make a black hole 50 times the size of the sun, you would have to pack the equivalent of 50 suns into a ball less than 300 kilometers in diameter. In the case of M87, however, 6.5 billion suns are compressed into a sphere wider than the orbit of Pluto . In both cases, the density is so high that the original material has to collapse into a singularity – a rift in the tissue of space-time.
The key to the mystery of the origin of black holes is the physical limit of how fast they can grow. Even the giant monsters in the galaxy's centers have their feeding cravings restricted, as a certain amount of material is forced back by the energetic radiation of hot particles accelerated near the event horizon. If you only eat the surrounding material, a black hole with a small mass may only double its mass in 30 million years.
"Given a mass of 50 solar masses, they simply can not be grown to 1 billion solar masses over 1 billion years," said Igor Chilingarian, astrophysicist at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, and at Moscow State University. But, "as we know, there are supermassive black holes that exist less than 1 billion years after the creation of the universe."
How to Make a Black Hole You Can not See
At the beginning of the universe's history, the germ of a medium-mass black hole could either be due to the collapse of a large, dense gas cloud or through have formed a supernova explosion. The very first stars that exploded in our universe had pure hydrogen and helium in their outer layers, with heavier elements concentrated in the nucleus. This is a recipe for a much more massive black hole than the explosion of modern stars, which are "polluted" with heavy elements in their outer layers and therefore lose more mass due to their stellar winds.
"If we make black holes 100 solar masses early in the universe, some of them should band together, but then basically you should produce a whole bunch of masses, and some of them should still be there," said Tod Strohmayer , Astrophysicist at NASA's Goddard Space Flight Center. Greenbelt, Maryland. "So, where are they when they're formed?"
An indication that medium-sized black holes might still be present came from the Gravity Wave Observatory of the National Science Foundation's Laser Interferometer, LIGO . a collaboration between Caltech and the Massachusetts Institute of Technology. LIGO detectors, combined with a European facility in Italy called the Virgo, spawn many different combinations of black holes through waves in space-time called gravitational waves .
In 2016, LIGO announced one of the most important scientific discoveries of the last half century: the first gravitational wave detection. In particular, the detectors in Livingston, Louisiana, and Hanford, Washington, picked up the signal from two merging black holes. The mass of these black holes: the 29- and 36-times the solar mass surprised the scientists. Although these are technically not an intermediate mass, but large enough to lift the eyebrows.
It is possible that all the black holes in the intermediate mass have already grown together, but the technology has not been precisely tuned to locate them.
So where are they?
The search for black holes in the middle ground desert is difficult because black holes themselves do not emit light. However, scientists can use sophisticated telescopes and other instruments to search for specific telltale signs. For example, because the flux of matter on a black hole is not constant, the clumping of the spent material causes certain fluctuations in the luminous efficacy in the environment. Such changes can be seen more quickly in smaller black holes than in larger ones.
"On a time scale of hours, one can conduct the observation campaign that takes months for classical active galactic nuclei," said Chilingarian.
The most promising The candidate for a medium-mass black hole is called HLX-1 and has a mass approximately 20,000 times that of the Sun. HLX-1 stands for "Hyper-Luminous X-Ray Source 1" and its energy output is much higher than for sun-like stars. It was discovered in 2009 by the Australian astronomer Sean Farrell with the XMM Newton X-ray telescope of the European Space Agency. A 2012 study using NASA's Hubble and Swift space telescopes revealed evidence of a collection of young blue stars orbiting the object. It may once have been the center of a dwarf galaxy swallowed by the larger galaxy ESO 243-49. Many scientists consider HLX-1 to be a proven medium-mass black hole, Harrison said.
"The colors of the X-ray it emits and the way it behaves are very similar to a black hole," Harrison said. "Many people, including my group, have programs to find things that look like HLX-1, but so far none are consistent. But the hunt continues.
Less bright objects, which could be medium-sized black holes, are referred to as ultraluminous X-ray sources or ULX. A flickering ULX called NGC 5408 X-1 fascinated scientists in search of black holes of medium mass. However, NASA's NuSTAR and Chandra X-ray observatories astounded scientists when they found that many ULX objects are not black holes. Instead, they are pulsars, extremely dense star remnants that seem to pulsate like lighthouses.
M82 X-1, the brightest X-ray source in the M82 galaxy, is another very bright object that seems to flicker on time scales that match a black hole of medium mass. These changes in brightness are related to the mass of the black hole and are caused by circulating material near the inner portion of the accretion disk. A 2014 study examined specific differences in X-ray light and estimated that M82 X-1 has a mass of about 400 suns. Scientists used NASA satellite Rossi X-ray Timing Explorer (RXTE) archive data to study these variations in X-ray brightness.
Recently, scientists investigated a larger group of potential medium-mass black holes. In 2018, Chilingarian and colleagues described a sample of 10 candidates by re-analyzing the optical data from the Sloan Digital Sky Survey and comparing the initial outlook with the X-ray data from Chandra and XMM-Newton. They are now tracking ground-based telescopes in Chile and Arizona. Mar Mezcua of the Spanish Institute of Space Sciences carried out a separate study from 2018, in which Chandra data was also used. There were found 40 growing black holes in dwarf galaxies that could be in this particular mass range. However, Mezcua and co-workers argue that these black holes were originally created by the collapse of huge clouds rather than from stellar explosions.
Dwarf galaxies are interesting places to look for, because in theory, smaller star systems could harbor black holes with a much smaller mass than those found in centers of larger galaxies like ours.
Scientists are also looking for globular clusters – spherical concentrations of stars on the edge of the Milky Way and other galaxies – for the same reason.
"There could be black holes in such galaxies, but if they do not accumulate much matter, it may be difficult to see them," said Strohmayer.
The medium-heavy hunters of black holes are eagerly awaiting the launch of NASA's James Webb Space Telescope, which will look back to the dawn of the first galaxies. Webb will help astronomers find out who came first – the galaxy or their central black hole – and how this black hole could have been put together. In combination with X-ray observations, Webb's infrared data will be important in identifying some of the oldest black hole candidates.
Another new tool released by the Russian space agency Roscosmos in July is called Spectrum X-Gamma, which will scan the sky with X-rays and carry an instrument with mirrors mounted with NASA's Marshall Space Flight Center. Huntsville, Alabama, was developed and built. Gravitational wave information resulting from the collaboration between LIGO and Virgo will also be helpful in the search, as will the proposed mission of the European Space Agency for Laser Interferometer Space Antennas (LISA).
This fleet of new instruments and technologies complements the current ones, and will help astronomers continue to search the Cosmic Garden for seeds of black holes and galaxies like ours.