Mysterious riddle of supermassive black holes

In the vast garden of the universe, the heaviest black holes are born from seeds. Fed by the gas and dust they have consumed or by merging with other dense objects, these seeds have grown and grown to form the center of galaxies, like our own Milky Way. But unlike the plant world, the seeds of giant black holes also had to be black holes. And no one has found these seeds yet – not yet.

One idea is that supermassive black holes – the equivalent of hundreds of thousands to billions of suns en masse – have grown from a population of smaller black holes that has never been view. This elusive group, the "intermediate-mass black holes", would weigh somewhere between 100 and 100,000 suns. Of the hundreds of black holes found so far, there are many relatively small ones, but none for sure in the middle "desert" mass.

Scientists work with NASA's powerful space telescopes, as well as other observatories, to locate distant objects that fit the description of these exotic entities. They have found dozens of possible candidates and are trying to confirm them as black holes. But even if they do, it opens up a whole new mystery: how were black holes of intermediate mass formed?

"What's fascinating, and why people have spent so much time trying to find these intermediate-mass black holes, is because it highlights the processes that took place in the primitive universe: what were relic black hole masses or new black hole formation mechanisms that we have not thought of yet, "said Fiona Harrison, a professor of physics at Caltech in Pasadena, Calif., and senior mission researcher NuSTAR from NASA.

Black hole 101

A black hole is an extremely dense object in the space in which no light can escape. When the material falls into a black hole, there is no exit. And the more a black hole eats, the more it grows in mass and size.

The smallest black holes are called "stellar mass", with between 1 and 100 times the mass of the Sun. They are formed when stars explode in violent processes called supernovae.

Supermassive black holes, on the other hand, are the central anchors of large galaxies – for example, our Sun and all the other stars of the Milky Way gravitate around a black hole called Sagittarius A * that weighs about 4.1 million solar masses. An even heavier black hole, representing 6.5 billion solar masses, is the centerpiece of the Messier 87 (M87) galaxy. The supermassive black hole of M87 appears in the famous Event Horizon telescope image, showing a black hole and its "shadow" for the very first time. This shadow is caused by the horizon of events, the point of no return of the black hole, which flexes and captures the light with its strong gravity.

Supermassive black holes tend to be surrounded by material disks called "accretion disks", made up of extremely hot, highly energetic particles that shine as they approach the event horizon – the region of no return black hole. Those who shine their records because they eat a lot are called "active galactic nuclei".

The density of material needed to create a black hole is staggering. To create a black hole 50 times the mass of the Sun, you must pack the equivalent of 50 suns in a ball of less than 200 km (300 km). But in the case of the centerpiece of M87, it's as if 6.5 billion suns were compressed into a larger ball than the orbit of Pluto. In both cases, the density is so high that the original material must collapse to become a singularity: a tear in the space-time structure.

The key to the mystery of the origin of black holes is the physical limit to which they can grow. Even the giant monsters at the center of galaxies have limitations on their binge eating, as a certain amount of material is repelled by high-energy radiation from accelerated hot particles near the event horizon. By simply eating the surrounding materials, a low-mass black hole could perhaps double its mass only in 30 million years, for example.

"If you start from a mass of 50 solar masses, you simply can not reach a billion solar masses over a billion years," said Igor Chilingarian, astrophysicist at the Astrophysical Observatory. Smithsonian of Cambridge, Massachusetts, and the Moscow State University. But, "as we know, there are supermassive black holes that exist less than a billion years after the formation of the universe."

How to make a black hole that you can not see

At the beginning of the universe's history, the seed of an intermediate-mass black hole could have formed as a result of the collapse of a vast cloud of dense gas or 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 the recipe for a black hole much larger than that of the stars in explosion that are "polluted" by heavy elements in their outer layers and therefore lose more mass because of their stellar winds.

"If we form black holes with 100 solar masses at the beginning of the universe, some of them should merge, but you should then produce a whole range of masses, and some of them should always be present, "said Tod Strohmayer. , astrophysicist at NASA's Goddard Space Flight Center, Greenbelt, Maryland. "So where are they, if they are formed?"

The Observatory of Gravitational Wave Laser Interferometers of the National Science Foundation has suggested that black holes of intermediate mass may still exist, LIGO, a collaboration between Caltech and the Massachusetts Institute of Technology. The LIGO detectors, associated with a European installation in Italy called Virgo, carry out many mergers of black holes through spatio-temporal ripples called gravitational waves.

In 2016, LIGO announced one of the most important scientific discoveries of the last fifty years: the first gravitational wave detection. Specifically, detectors based in Livingston, Louisiana and Hanford, Washington detected the fusion signal from two black holes. The masses of these black holes: 29 and 36 times the mass of the Sun, surprised the scientists. Although they are not yet technically intermediate mass, they are big enough to lift eyebrows.

It is possible that all intermediate-mass black holes have already merged, but this technology has not been developed to locate them.

So where are they?

Look for black holes in the intermediate mass desert is tricky because the black holes themselves do not emit any light. However, scientists can look for specific telltale signs using sophisticated telescopes and other instruments. For example, since the flow of material over a black hole is not constant, the density of the material consumed causes certain variations in the light output in the environment. Such changes are visible more quickly in small black holes than in larger ones.

"On a time scale, you can do the observation campaign that takes months for conventional active galactic nuclei," Chilingarian said.

The most promising intermediate-mass black hole is called HLX-1, with a mass of about 20,000 times that of the Sun. HLX-1 means "hyper-bright X-ray source 1" and its energy efficiency is much higher than Sun-like stars. It was discovered in 2009 by Australian astronomer Sean Farrell, using the XMM-Newton's X-ray space telescope from the European Space Agency. A 2012 study using NASA's Hubble and Swift space telescopes revealed a group of young blue stars in orbit around this object. It was perhaps once the center of a dwarf galaxy swallowed by the largest of the galaxies. ESO 243-49. Many scientists consider HLX-1 to be a proven intermediate-mass black hole, Harrison said.

"The colors of the X-ray light that it emits and its behavior are very similar to those of a black hole," Harrison said. "Many people, including my group, have programs to find things that look like HLX-1, but until now, none is consistent. But the hunt continues.

Less luminous objects that could be intermediate-mass black holes are called ultraviolet X-ray sources or ULX. A scintillating ULX called NGC 5408 X-1 has particularly intrigued scientists looking for black holes of intermediate mass. But NASA's NuSTAR and Chandra X-ray observatories have astonished scientists by revealing that many ULX objects are not black holes. They are rather pulsars, extremely dense stellar remains 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 glitter at timescales corresponding to a black hole of intermediate mass. These changes in brightness are related to the mass of the black hole and are caused by a material in orbit near the internal region of the accretion disk. A study conducted in 2014 examined specific variations in X-ray light and estimated that the M82 X-1 had a mass of about 400 suns. Scientists used NASA's RXTE satellite (Ross X) archive data to study these brightness variations.

More recently, scientists have studied a larger group of possible intermediate-mass black holes. In 2018, Chilingarian and his colleagues described a sample of 10 candidates by re-analyzing the Sloan Digital Sky Survey's optical data and matching the initial perspectives with Chandra X-ray data and XMM-Newton. They are now tracking ground telescopes in Chile and Arizona. Mar Mezcua, from the Spanish Institute of Space Science, conducted a separate study in 2018, also using Chandra data, discovering 40 growing black holes in dwarf galaxies that may be in this intermediate mass range. But Mezcua and his collaborators say that these black holes were formed at the origin of the collapse of giant clouds, rather than by stellar explosions.

And after

Dwarf galaxies are interesting places to keep searching because, in theory, smaller star systems could harbor much smaller black holes than those found at the center of larger galaxies like ours.

Scientists are also looking for globular clusters – spherical concentrations of stars on the periphery of the Milky Way and other galaxies – for the same reason.

"There may be black holes like that, in galaxies like that, but if they do not accumulate a lot of stuff, it may be difficult to see them," Strohmayer said.

Middle-ground black hole hunters are eagerly awaiting the launch of NASA's James Webb Space Telescope, which will be going back to the dawn of the first galaxies. Webb will help astronomers determine what happened first – the galaxy or its central black hole – and how this black hole could be put in place. In combination with X-ray observations, Webb's infrared data will be important to identify some of the oldest black hole candidates.

Another new tool launched in July by the Russian space agency Roscosmos called Spectrum X-Gamma. It is a spacecraft that will scan the sky in X-rays and carries a mirrored instrument developed and built with the NASA Marshall Space Flight Center, Huntsville, Alabama. . The information on gravitational waves from the LIGO-Virgo collaboration will also facilitate the search, as well as the planned mission for the Space Interferometer (LISA) antenna of the European Space Agency.

This fleet of new instruments and technologies, in addition to the current ones, will help astronomers continue to search the cosmic garden for black hole seeds and galaxies like ours.