The strange case of the missing black hole.
From our experience, we know that every tree has its own roots.
But what if one day we found a tree that has no roots?
It would be suspicious - or, if you are an AMONG US player, it would be sus.
Anyway, this is pretty much the same thing that happened to astronomers.
In fact, they 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?
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 dead 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.
As we've already said, that was in 2012. In the years since,
the two researchers and their colleagues have been ransacking the galaxy, looking for X-rays
or radio waves from the missing black hole. Because when the matter is about to fall into
a black hole it is accelerated to speeds close to that of light, heated to very high temperatures
and emits a final glow, almost an extreme call for help, in the form of radio waves and X-rays.
Radio analysis of A2261-BCG showed that there were some emissions from the core, but they were
50 million years old: as if at that time the black hole had "turned off", or had escaped.
So where has nature stashed the equivalent of 10 billion suns?
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One possibility is that the black hole is there but has gone silent, having temporarily run out
of anything to eat. But another provocative possibility is that the black hole was
thrown out of the galaxy altogether. In both cases, this would lead to a deeper understanding
of galaxies' formation and evolution. In particular, proving the latter could
provide insight into some of the most violent and dynamic processes in the evolution of galaxies
and the cosmos, about which astronomers theorized but never seen a dance of
titanic forces and swirling worlds that can fling stars and planets across the void.
It is an intriguing mystery. Maybe the upcoming James Webb Space
Telescope (JWST) will have the capability to shed light, so to speak, on the case.
Webb will be the largest, most powerful and complex space telescope ever built
and launched into space. It will fundamentally alter our understanding of the universe.
As we can read on NASA's website, it is an orbiting infrared
observatory that will complement and extend the discoveries of the Hubble Space Telescope,
with longer wavelength coverage and greatly improved sensitivity.
The longer wavelengths enable Webb to look much closer to the beginning of time and
to hunt for the unobserved formation of the first galaxies, as well as to look inside dust
clouds where stars and planetary systems are forming today.
Recent studies in X-rays, however, were conducted by a group led by Kayhan Gültekin,
of the University of Michigan, thanks to the Chandra space telescope.
Research has identified a couple of suspicious agglomerations of matter in the galaxy's core,
which soon turned out to be two mini galaxies engulfed
by the larger one. And nothing that suggests the presence of a supermassive black hole.
The strange characteristics of A2261-BCG suggest a scenario in which it collided with another galaxy
and that the black holes of the two have merged.
This would have been one of the most explosive and energetic events that happened in the universe.
This is what a black hole merging would look, according to NASA:
Many clues converge on this reconstruction: the "dance" between the two black holes before merging
could explain the "enlarged" core of A2261-BCG, the relative scarcity of stars in the centre,
the fact that the brightest area is not exactly in half.
It remains to be seen why the supermassive black hole is not found. And the hypotheses are two:
the first is that he is actually still there, but he is very "quiet" because he devoured
everything he could and there is nothing left that can emit a last cry, like a serial
killer who has run out of own victims. In this case, sooner or later perhaps we will spot it.
In the meantime, some detailed analysis was published of how the merger of
two supermassive black holes could reform the galaxy into what astronomers have found.
In the abstract of “Formation of the largest galactic cores through binary scouring
and gravitational wave recoil” paper from Imran Nasim, University of Surrey, it is stated:
“ In this paper, we run a suite of dry galaxy merger simulations
to explore three different scenarios for central core formation in massive elliptical galaxies:
‘binary scouring’, ‘tidal deposition’ and ‘gravitational wave (GW) induced recoil’ […]
We find that we can only explain the large surface brightness core of A2261-BCG
with a combination of a major merger that produces a small core through binary scouring,
followed by the subsequent GW recoil of its SMBH that acts to grow the core size. We show
that this same model can also explain the bright ‘knots’ observed in the core region of A2261-BCG.”
So, although the strange case of the missing black hole has not yet been solved,
we can say at least we have a lead, or maybe a bunch of possible answers to the question:
where the hell has nature stashed the equivalent of 10 billion suns?
The fact is that nature is always a little bit creative and complicated than we think it is,
so we have to strive in order to find the answers to our questions.
But this is not a bad thing: it pushes us to do better and to get to understand
more about our universe. "Thanks for watching everyone!
Did you enjoy this video? Do you think our black hole is playing hide and seek?
Let us know in the comment below! See you next time on the channel!