The mysterious 'gargantuans' that control our galaxies

Last year astronomers were finally able to reveal the first images of the supermassive black hole at the center of our Milky Way galaxy.

However, the black hole itself could not be seen, at least not directly: it is so dense that its gravitational pull prevents even light from escaping.

But the image of Sagittarius A, as our galaxy's black hole is known, revealed a halo of glowing gas around the object;

an object whose mass we know to be a million times that of the Sun. Recent discoveries like this, like many others, have astonished astronomers.

"Over the last few years, everything we thought we knew about black holes has been thrown into question," says Michela Mapelli, an astrophysicist and professor at the University of Padua in Italy.

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Everyone has heard of black holes, but few people realize how much these strange objects continue to vex astronomers.

Last year astronomers were made aware of a black hole destroying and engulfing a star that wandered too close.

Another has been described as the fastest growing black hole ever observed, devouring the equivalent mass of Earth every second and is thus already 3 billion times more massive than our Sun.

This image shows the center of our galaxy.

The black hole is known as Sagittarius A* or Sgr A*.NASA

cosmic minnows

Mapelli studies stellar black holes, which form when a large, fast-burning star implodes.

Compared to supermassive ones, these black holes are cosmic minnows.

Astronomers expected such black holes to be between five and ten times the mass of the Sun, but in fact they have a much wider range of sizes.

In recent years, several of up to 100 solar masses and a small one of only 2.6 have been discovered.

“We have discovered characteristics and mass ranges of black holes that we could not have imagined before the recent observations,” Mapelli says.

He is particularly intrigued by binary black hole systems, in which two black holes orbit around each other.

This phenomenon can occur when two stars orbiting each other end their lives as black holes.

Representation of two black holes before colliding and merging.Mark Myers ( (Mark Myers)

Again, there could be many other ways to form binary black holes, something Mapelli is studying in his DEMOBLACK project, funded by the European Research Council.

"Seven years ago, most people were skeptical about the existence of binary black holes," she says: "Not even theorists were convinced of their existence."

Now, Mapelli said, almost a hundred have been discovered.

They eject gravitational waves, disturbances in space-time that can be picked up with sophisticated detectors at the Laser Interferometry Observatory for Gravitational Waves (LIGO) in the United States and at the Virgo interferometer in Italy.

According to Mapelli, most astrophysicists doubted that two black holes could get close enough to merge, but then gravitational waves began to send signals of collisions between black holes.

In 2019, a peculiar merger event occurred between black holes of sixty and eighty solar masses.

Whether they formed directly from stars is unknown, as the assumption that star-born black holes are between five and ten solar masses has become unfounded.

"Whether the maximum mass of a stellar black hole is only 60 solar masses or if it can reach 90, or even 300, is a huge question mark," explains Mapelli.

"I feel guilty for this great uncertainty, since I have personally contributed to causing this situation."

Graphic visualization of the black hole Gargantua, by astrophysicist Kip Thorne in the Christopher Nolan film 'Interstellar' (2014).Christopher Nolan

galactic monsters

The biggest beasts are found at the center of almost all galaxies.

Almost all of them are active and contain hot gas inside that absorbs gravity.

The mass of some of these black holes is up to 10 billion times that of the Sun.

"Here are some real monsters," says Professor Christopher Reynolds of the University of Cambridge (UK): "Their influence in a galaxy can extend up to a hundred, even two hundred, light-years."

Stars and galaxies feel the gravitational pull of these black holes even at these astronomical distances, but the bursts of energy they generate by consuming matter can be felt even farther away, 100,000 light-years or more away.

As part of the EU-funded DISKtoHALO project, Reynolds is investigating how these supermassive black holes grow, suck in hot gas and generate bursts of energy that are fired outwards.

"We know that these black holes produce jets of energy that send discharges outwards," he details.

One thing that astrophysicists have not yet been able to figure out is why the gas at the core of some galaxies is so hot—between 10 and 100 million degrees Celsius—even though the systems are billions of years old and therefore have had plenty of time to cool down.

The way in which black holes interact with their surroundings and with distant parts of their galaxies is an extremely complex puzzle.

Computer models are of little help because knowledge is required at relatively small scales and, at the same time, at gigantic scales measured in light years.

Nine radio telescopes have managed to capture this gigantic sinkhole in the center of the Centaurus A galaxy.Capella Observatory

“We're talking about something the size of a tennis ball regulating something the size of the Earth,” compares Reynolds.

One way to study these supermassive black holes at the center of galactic clumps is to examine the hot gases in their vicinity.

It is impossible to see these gases through a telescope, but their energy can be observed with the X-rays they emit, given their high temperature.

Why hot gas doesn't cool and form stars remains anyone's guess.

“You need a heat source to emit energy in the middle of the cluster, and the only one powerful enough are black holes,” reflects Reynolds.

It is still a mystery to him and his colleagues how exactly this heat source works.

However, it is clear that supermassive black holes do not lead a quiet life.

Reynolds describes it: “These black holes aren't even spherical;

they rotate on themselves forming a disc full of instability”.

Despite new findings about these strange galactic creatures, the true nature of black holes remains unknown.

Assumptions used in the past have been called into question.

If anything is clear, it is that black holes will continue to intrigue the brightest minds in astronomy.

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