In the vast garden of the universe, the heaviest black holes grow from seeds. Fueled by the gas and dust they consume, or by merging with other dense objects, these seeds have grown in size and section to form the centers of galaxies, such as our Milky Way. But unlike the kingdom of plants, the seeds of giant black holes also had to be black holes. And no one has ever found these seeds ̵
One idea is that supermassive black holes – the equivalent of masses of hundreds of thousands to billions of suns – have grown from a population of smaller black holes that have never been seen. This elusive group, the "black holes with intermediate mass", will weigh between 100 and 100,000 suns. Among the hundreds of black holes discovered so far, there are quite a few small ones, but none are certainly in the intermediate wilderness array.
Scientists work with powerful NASA space telescopes, as well as other observatories, to find distant objects that fit the description of these exotic entities. They have found dozens of potential candidates and are working to confirm them as black holes. But even if they do, it reveals a whole new conundrum: How did black holes form an intermediate mass?
"What is fascinating and why have people spent so much time trying to find these black holes with an intermediate mass? because it sheds light on the processes that happened in the early universe – what were the arrays of relict black holes or new black hole formation mechanisms we had never thought of, "says Fiona Harrison, a professor of physics at Caltech in Pasadena , California, and NASA's NuSTAR Principal Investigator.
Black Hole 101
Black Hole is an extremely dense object in space from which no light can escape. he gets into a black hole, he has no way out, and the more he eats one hour hole, the more it grows both in mass and in size.
The smallest black holes are called "star mass" by between 1 and 100 times the mass of the Sun They are formed when stars explode in cruel processes called supernovae.
Supermassive black holes, on the other hand, are the central anchors of large galaxies – for example, our Sun and all other stars in the Milky Way orbit the black a hole called Sagittarius A * that weighs about 4.1 million solar masses. The even heavier black hole – with a huge 6.5 billion solar masses – serves as the center of the Messier 87 (M87) galaxy. The M87 supermassive black hole appears in the famous image from the Event Horizon telescope, showing the black hole and its "shadow" for the first time. This shadow is caused by the event horizon, the point of return of the black hole, the bending and capture of light by its strong gravity.
Supermassive black holes tend to have disks around material called "accumulation disks" made of extremely hot high-energy particles that shine brightly as they approach the event horizon – the black hole region without return. Those who make their discs shine brightly because they eat a lot are called "active galactic nuclei."
The density of matter required to create a black hole is mind-boggling. To make a black hole 50 times larger than the mass of the sun, you will need to collect the equivalent of 50 suns in a ball less than 200 miles (300 kilometers) across. But in the case of the central part of the M87, 6.5 billion suns seem to be compressed into a ball wider than the orbit of Pluto . In both cases the density is so high that the original material must collapse into one peculiarity – a tear in the fabric of space-time.
The key to the mystery of the origin of black holes is the physical limit to how fast they can grow. Even the giant monsters in the centers of the galaxies have feeding restrictions, as a certain amount of material is ejected by high-energy radiation coming from hot particles accelerated near the event horizon. Only by eating surrounding material can a black hole with a low mass be able to double its mass in 30 million years, for example.
"If you start from a mass of 50 solar masses, you just can't increase it to 1 billion solar masses in 1 billion years," said Igor Chillingaryan, an astrophysicist at the Smithsonian Astrophysical Observatory, Cambridge, Massachusetts and Moscow State University. But "as we know, there are supermassive black holes that exist less than 1 billion years after the universe was formed."
How to make a black hole that you can't see
of the history of the universe, the seeds of a black hole with an intermediate mass could be formed either by the collapse of a large, dense gas cloud or by the explosion of a supernova. The first stars to burst into 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 bursting of modern stars, which are "contaminated" with heavy elements in their outer layers and therefore lose more mass through their stellar winds.
"If we form black holes with 100 solar masses early in the universe, some of them have to merge together, but then you generally have to produce a whole range of masses, and then some of them still have to be around, "says Todd Strohmeyer, an astrophysicist at NASA's Goddard Space Flight Center, Greenbelt, Maryland. "So where are they if they were formed?"
One clue that black holes with an intermediate mass may still be there comes from the National Science Foundation's Gravitational-Wave Observatory Laser interferometer LIGO collaboration between Caltech and MIT. LIGO detectors combined with a European facility in Italy called Virgo form many different mergers of black holes through space-time ripples called gravitational waves .
In 2016, LIGO announced one of the best important scientific discoveries of the last half century: the first discovery of a gravitational wave. In particular, detectors based in Livingston, Louisiana and Hanford, Washington, have received a signal to merge two black holes. The masses of these black holes, 29 and 36 times larger than the mass of the Sun, respectively, surprised scientists. Although they are not yet technically intermediate, they are large enough to raise eyebrows.
It is possible that all black holes with intermediate masses have already merged, but also that the technology is not well-tuned to locate them.
And where are they?
Finding black holes in the desert with an intermediate mass is complicated because the black holes themselves do not shed any light. However, scientists can look for specific signal signs using sophisticated telescopes and other tools. For example, because the flow of matter onto a black hole is not constant, the clumsiness of the material consumed causes certain changes in the light emission into the environment. Such changes can be seen more quickly in smaller black holes than larger ones.
"In an hourly schedule, you can build an observational campaign that takes months for classic active galactic nuclei," says Chillingaryan.
The most promising candidate for a black hole with an intermediate mass is called HLX-1, with a mass about 20,000 times larger than that of the Sun. HLX-1 stands for "Hyper-luminous X-ray Source 1", and its energy output is much higher than the Sun-like stars. It was discovered in 2009 by Australian astronomer Sean Farrell using the space agency's XMM-Newton X-ray. A 2012 study using NASA's Hubble and Swift space telescopes found proposals for orbiting a young blue star object. It may have once been the center of a galaxy dwarf absorbed by the larger galaxy ESO 243-49. Many scientists consider the HLX-1 a proven black hole with an intermediate mass, Harrison says.
"The colors of the X-ray light it emits and the way it behaves is very similar to a black hole," Harrison said. "Many people, including my group, have programs to find things that are similar to the HLX-1, but so far none have been consistent. But the hunt continues. ”
The less striking objects that could be medium-sized black holes are called ultraviolet X-ray sources or ULX. The flickering ULX, called the NGC 5408 X-1, was particularly intriguing for scientists looking for black holes with an intermediate mass. But NASA's NuSTAR and Chandra X-ray observatories surprised scientists by finding that many ULX objects are not black holes – instead they are pulsars, extremely dense star-like remnants that look like pulsating headlights.
The M82 X-1, the lightest X source in the M82 galaxy, is another very striking object that seems to flicker along time scales corresponding to a black hole with an intermediate mass. These changes in brightness are related to the mass of the black hole and are caused by orbital material near the inner region of the accretion disk. A 2014 study looked at specific variations in X-ray light and estimated that the M82 X-1 had a mass of about 400 suns. The researchers used archival data from NASA's Rossi X-ray satellite to study these changes in X-ray brightness.
Recently, scientists have examined a larger group of possible black holes with an intermediate mass. In 2018, Chillingaryan and his colleagues described a sample of 10 candidates by re-analyzing the optical data from the Sloan Digital Sky Survey and comparing initial perspectives with the X-ray data from Chandra and XMM-Newton. They are now tracking ground-based telescopes in Chile and Arizona. Mar Mesqua of the Spanish Institute of Space Sciences is conducting a separate study for 2018, also using data from Chandra, finding 40 growing black holes in galaxy dwarfs that could be in this special intermediate mass range. But Mesqua and co-workers claim that these black holes were initially formed by the collapse of giant clouds, not by the occurrence of star explosions.
Dwarf galaxies are interesting places to keep looking, because, in theory, smaller star systems could accommodate black holes with much smaller masses than these, located in the centers of larger galaxies like our own.
Scientists are also looking for globular clusters – spherical concentrations of stars located on the outskirts of the Milky Way and other galaxies – for the same reason.
"There may be such black holes in such galaxies, but if they don't accumulate a lot of matter, it can be difficult to see them," Strohmeier said.
Black hole hunters with an intermediate mass are eagerly awaiting the release of NASA's James Web Web Space Telescope, which will look into the dawn of the first galaxies. The Web will help astronomers understand who comes first – the galaxy or its central black hole – and how that black hole can be harvested. In combination with X-ray observations, Web infrared data will be important in identifying some of the most ancient black hole candidates.
Another new instrument launched in July by the Russian space agency Roscosmos is called Spectrum X-Gamma, a spacecraft that will scan the sky with X-rays and carry a tool with mirrors designed and built by NASA Marshall Center Flight Center, Huntsville. , Alabama. The gravity wave information resulting from the LIGO-Deva collaboration will also help in the search, as will the planned European Space Agency's Laser Interface Space Agency (LISA) mission.
This fleet of new tools and technologies, in addition to the current ones, will help astronomers as they continue to search the cosmos for black hole and galaxy seeds like our own.