The Strange Case Of The Missing Black Hole
464Astronomers believe that nearly all major galaxies have supermassive black holes at their centres.Not surprisingly, the most massive objects in a galaxy fall to its centre, which is the bottom of the galaxy's gravitation potential.
Most of these black holes are dormant and only a few per cent are active in the sense of drawing material from host galaxy inwards and forming jets and radio structures.
But we have a lot of galaxy types. Does every type of galaxy host such a black hole?
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Probably yes, talking about most typical, like elliptical and spiral ones.
So, firing their telescopes at the centre of a galaxy in the so-called Abell 22561 cluster, astronomers were looking for evidence of a black hole.
But they were not able to find them.
Where is it?
Is it hiding?
Follow me on this video to know more about the strange case of the missing black hole!
It should be there, but we are not able to find it.
And we are not talking about a very small object: we are talking about a supermassive black hole, with the mass of about 10 billion solar masses (just for a term of comparison, the one at the end of our own galaxy is just about 4 million solar masses).
The scene of the crime is located in a galaxy cluster, Abell 2261, at about 3 billion light-years from Earth, in the direction of Hercules constellation.
Among the galaxies that make up the cluster, stands out the one dubbed A2261-BCG, that dominates the group.
It is a giant elliptical galaxy, the biggest type of galaxies known, and it has a very extended nucleus, that reaches 10.000 light-years of amplitude.
For several decades, astronomers have found that a huge black hole hides in the centre of almost all large galaxies: these black holes are called supermassive, to distinguish them from the "normal" ones that are formed by the gravitational collapse of a single star. It is not known exactly how supermassive black holes are formed: they are probably very ancient objects, born in the galaxies of billions of years ago starting from "seeds" of matter, for example from the stars of great mass that existed in the early stages of the life of the Universe. Then, over time, these black holes grew, attracting matter (in the form of stars and gas) that passed near them. And in some cases, two galaxies, each with its own black hole, collided, and the black holes in their cores merged to form a one even larger one.
Supermassive black holes are so famous that Matt Bellamy from the Muse wrote a song about them.
Here's a piece of the lyrics:
“Glaciers melting in the dea* of night
And the superstar's sucked into the supermassive black hole”
Anyway.
In short, it seemed obvious that even A2261-BCG, being a super galaxy with a diameter of one million light-years, that is about 10 times that of our Milky Way, should have a supermassive black hole at its centre. So the hunt to identify him began.
I have a question for you: how do you think astronomers look for black holes? I mean, black holes are...black: we are not able to see them directly.
In fact, the search was complicated by the fact that we were looking for an object that by definition does not emit any type of radiation, and therefore is invisible. But even black holes, if they can't be caught in the act, still leave traces.
For example, stars in the vicinity of a galactic black hole are moving at great speed around something, in fact, of invisible, and by studying their movements one can deduce the presence of the black hole. In 2012 the galaxy A2261-BCG had therefore been observed with the Hubble Space Telescope, but the investigation had led to nothing. Or rather, you could see the central core of the galaxy strangely dilated, but no black hole. Indeed, in the core of A2261-BCG the brightness, instead of increasing as it happens for other galaxies, decreased slightly. And, again, the maximum concentration of stars did not exactly coincide with the centre of the galaxy but was displaced by 2,000 light-years.
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