Astronomers have discovered an extremely massive black hole from a time when the Universe was less than 900 million years old. The result provides insight into the growth of black holes and galaxies in the young Universe. See Letter p.512
It is commonly believed that every massive galaxy in the Universe harbours a supermassive black hole at its centre. These black holes are thought to have formed in the young Universe with initial masses of between 100 and 100,000 times the mass of the Sun1. Over time, some of them have grown to be up to billions of solar masses by pulling (accreting) interstellar material from their surroundings and/or through merging with other black holes. The most massive black holes that have been found in the nearby Universe have masses of more than 10 billion solar masses2,3. For comparison, our own Galaxy harbours a black hole with a mass of between 4 million and 5 million solar masses4. On page 512 of this issue, Wu et al.5 report the discovery of a supermassive black hole with a mass of a remarkable 12 billion solar masses, from a time when the Universe was only 875 million years old — that is, about 6% of its current age of 13.8 billion years.
Wu and colleagues identified this monster in optical and near-infrared imaging data because it was accreting gas at a high rate. This gas is pulled towards the black hole by gravity and can efficiently radiate away part of its potential energy. Accreting supermassive black holes can therefore be very bright, and can be seen across the Universe as luminous sources termed quasars. Because the light coming from a very distant quasar takes billions of years to reach Earth, astronomers can observe such accreting black holes as they were when the Universe was young.
Theoretically, it is not implausible to find a black hole of more than 10 billion solar masses within 1 billion years after the Big Bang. But it is still surprising to uncover such a massive black hole in the early Universe. It must have been accreting gas at close to the maximum rate for most of its existence; the maximum rate is set by the pressure of the radiation emitted by the in-falling material. The prolonged period of almost maximum accretion is puzzling, because the strong radiation emitted by a quasar is generally assumed to be capable of halting accretion, limiting its existence to 10 million to 100 million years. The fact that the supermassive black hole has grown to 12 billion solar masses in less than a billion years implies that the radiation did not inhibit the high accretion.
In general, studies of supermassive black holes at the centres of nearby galaxies have revealed a tight correlation between the mass of the black hole and the total mass in stars of the galaxy hosting it6. Typically, the mass of a black hole is higher when it resides in a more massive galaxy, with the ratio of the black-hole mass to galaxy mass6,7 being about 0.14–0.5%. Therefore, it has been suggested that the growth of both the black hole and the host galaxy are causally connected. If the relation between black-hole mass and host-galaxy mass were to hold true even in the distant Universe, we would expect the galaxy harbouring the 12-billion-solar-mass black hole to contain a whopping 4 trillion to 9 trillion solar masses in stars, which is the same as the most massive galaxies seen in the current Universe. Studying this host galaxy will give us a glimpse of how massive galaxies formed in the early Universe, and of the interplay between the formation of stars in the galaxy and the accretion onto its central black hole.
Intriguingly, the black hole discovered by Wu and collaborators is not only the most massive of its kind known in the early Universe, it is also, owing to the high accretion rate, by far the most luminous object detected at that cosmic epoch. The quasar can therefore be used as a means of learning about the distant cosmos. As the quasar's light travels towards observers on Earth, it passes through the gas of the intergalactic medium. This medium contains hydrogen, helium and various metals (elements heavier than helium that are produced inside stars), which leave an imprint on the spectrum of the quasar by absorbing a small amount of the quasar's light at specific wavelengths. The brighter the quasar, the more comprehensive the investigation of the intervening gas can be. Thus, the extreme brightness of the newly discovered quasar will allow the abundance of metals in the intergalactic medium of the early Universe to be measured in unprecedented detail. Such measurements will provide information about the star-formation processes at work shortly after the Big Bang, which produced these metals.
Finally, quasars as bright as the one reported here could easily be seen at larger distances from Earth than that of this quasar, and hence in an even younger Universe. Although accreting supermassive black holes become increasingly rare at earlier cosmic times8, current and future wide-field near-infrared imaging surveys should be able to uncover such objects. These giants of the Universe will provide the ideal targets from which to learn about the Universe during the first few hundred million years after the Big Bang. Footnote 1
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The Astrophysical Journal (2018)
Frontiers in Astronomy and Space Sciences (2017)
Monthly Notices of the Royal Astronomical Society (2016)