FLASHES OF OUR COSMIC PAST
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. Quasars (from English ‘quasi-stellar’) are extreme galaxies that, due to the enormous distances at which they are, appear as small points of light ‘quasi-stellar’. Some 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.
One of the ways to approach this problem is the study of the most distant 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 farther 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 only two years old. Most surprisingly, this quasar, named J0313-1806, houses an ultra massive black hole at its center that contains 1.6 billion times the mass of the Sun.
How can such extremely massive black holes form? First, a smaller black hole needs to be formed, acting as a ‘seed’, which will grow over time as it devours the material in its environment. But there are two schools to explain the growth mechanism. Some astronomers think that the seed forms and feeds with other much smaller black holes, with 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, the 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 often appears to be the case, the process is accelerated. The 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 primitive 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.
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). 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 that 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 observations show us. But growth must have been very different in the past. 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 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.