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A quasar-type galaxy has been discovered in the primordial universe, when it was only 670 million years old.
The finding provides clues to how supermassive black holes form.
Glimpses of our cosmic past
Quasars (from English 'quasi-stellar') are extreme galaxies that, due to the enormous distances at which they are, appear as small 'quasi-stellar' points of light.
About 200,000 are known and, thanks to their enormous luminosity, they are objects that allow us to study the universe up to very far distances, which is to say,
until very remote moments in our cosmic past
.
We can imagine quasars as very luminous macrogalaxies with supermassive black holes at their centers.
These black holes grow to billions of solar masses, which is why we refer to them as 'ultramassive'.
Naturally, these highly structured galaxies would take a long time to form, but
surprisingly they are found in the earliest universe
.
Understanding how such colossal structures are built in relatively short time intervals is one of the greatest challenges in astrophysics today.
Breaking records
One of the ways to approach this problem is to study the farthest possible quasars, that is, those that we observe in their earliest stage of evolution.
For this reason, in recent years we are witnessing a kind of race in which distance records are broken.
The three most distant known quasars have been discovered in the last three years
.
Astronomer Feige Wang (University of Arizona) and his colleagues have just broken distance records again by detecting a quasar so far away that we see it as it was 670 million years after the big bang.
In astronomical terms this is equivalent to saying that its redshift is z = 7.64.
This object is more than 20 million light-years further than the farthest previously known.
If we compare the current universe with a 50-year-old adult, it turns out that
this quasar shows us what this adult-universe was like when it was a child of only two years old
.
Most surprisingly, this quasar, named J0313-1806, houses an ultramassive black hole at its center that contains 1.6 billion times the mass of the Sun.
The quasar J013-1806 observed with ALMA.ALMA / ApJLett / Wang et al.
Two theories
How can such extremely massive black holes form?
First, a smaller black hole needs to be formed, to act as a 'seed', which will grow over time as it devours the material from its environment.
But there are two schools to explain the growth mechanism.
There are astronomers who think that the seed is formed and feeds with other much smaller black holes, of stellar masses.
The theory predicts that the first generation stars in a galaxy (known as Population III) are much more massive than those of later generations,
reaching several hundred solar masses each
.
The more massive a star is, the faster it burns its energy and the faster it lives and dies.
Thus, relatively quickly, a first generation of stellar-type black holes could form, the merger of which would give rise to a much more massive black hole.
If the stars are part of large clusters, as it often seems to be the case, the process is accelerated.
A second stream of thought ignores the stars to form the seed of the supermassive black hole.
These astronomers think that the black hole would form directly from the gravitational collapse of the great masses of primordial gas in early galaxies.
This process is
not possible in today's galaxies due to the presence of many stars
and the effect they have on the interstellar medium, but it could be possible in early galaxies.
Runaway growth
Thanks to the newly discovered quasar, Wang and colleagues can put the two theories to the test.
Suppose the extreme case in which the seed grows at maximum speed, gaining as much mass as possible all the time (without interruption).
In order to form a black hole of 1.6 billion solar masses, in an interval of only 670 million years, it turns out that the black hole that acts as a seed would have to have a minimum of 10,000 solar masses.
This
is too large a value to be derived from stars, even from a large cluster
.
And the first of the two theories we have presented must be discarded.
It is concluded that, at least in the case of J0313-1806, the initial black hole could only be formed from the collapse of the primordial gas present in the young galaxy.
However, it is unknown how black holes gain mass over time.
The ultramassive of J0313-1806 is feeding with 25 solar masses a year, at the moment in which the observations show us.
But
growth must have been very different in the past
.
In order to elucidate how growth occurs, there is no choice but to continue observing progressively more distant quasars, and continue to break these distance records.
To carry out this study, Wang and collaborators have used different large telescopes in Chile (the Magellan, the Gemini South and ALMA) and in Hawaii (the Gemini North and the Keck).
The results have been published in an article, entitled "A Luminous Quasar at Redshift 7,642", from a recent issue of The Astrophysical Journal Letters.
The manuscript can be consulted at this link.
Rafael Bachiller
is director of the National Astronomical Observatory (National Geographic Institute) and academic of the Royal Academy of Doctors of Spain.
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