Astrophysics

Making black holes from scratch

The means by which supermassive black holes form and grow have remained largely unclear. Numerical simulations show that the collision of massive galaxies can naturally lead to the creation of these objects.

Black holes are objects in which the pull of gravity is so strong that nothing — not even light — can escape. However, they are far from being 'invisible': they can be detected through the effect they have on their surroundings. Stellar-mass black holes have long been known to exist and to be the evolutionary end point of massive stars. But we now have evidence of another class of black hole, termed supermassive black holes (SMBHs). These have masses of millions, or even billions, of solar masses, and inhabit the centres of galaxies. A black hole of four million solar masses is thought to sit at the centre of the Milky Way. Stars in the Galactic Centre move 'too fast' to be explained solely by the gravitational potential generated by the Galaxy's luminous matter: a massive dark object must be lurking in the depths of the Galaxy, masterminding the movement of its puppet stars. Although the existence of SMBHs can be inferred in nearby and distant galaxies, the way in which they form is still largely mysterious.

On page 1082 of this issue, Mayer et al.1 investigate, by means of high-resolution numerical simulations, the conditions that can drive the formation of an SMBH 'seed' during the merger of massive galaxies in the young Universe. The authors study a process that can leave behind a black hole that is very massive at birth. Once this newly born SMBH seed starts to grow, by accreting matter from the dense cloud of gas in which it is embedded, we can hope to spot it as a quasar — a compact and extremely bright object as small as a star but as luminous as an entire galaxy. SMBHs have been recognized as the 'engines' that power quasars, and the energy emitted by material plunging onto an SMBH as the source of such power. About 10% of every parcel of matter is converted into energy during the infall.

However, SMBHs are not always active as quasars. Quasars were very common in the early Universe — up until the Universe was about 40% of its current age of 14 billion years. But they slowly disappeared, leaving behind quiescent SMBHs. The masses of quiescent SMBHs in nearby galaxies correlate with the properties of their host galaxies2, suggesting that a single mechanism underpins the assembly of both SMBHs and galaxies. However, most of the galaxy-formation models involving quasar and SMBH evolution neglect research into the physical processes that formed SMBHs, and take simplistic approaches. This is a crucial missing ingredient: knowledge about the first SMBH seeds is necessary when investigating how SMBHs grow with their host galaxies over cosmic time.

Mayer et al.1 attack this problem by focusing on the environment in which an SMBH seed might form. They find that the collision and merging of two massive young galaxies produces a massive gas disk, which is born in an unstable configuration, with a spiral pattern that transports mass towards the disk's centre. Within only 100,000 years, more than 100 million solar masses of gas are accumulated in the disk's central region, forming a dense cloud of gas. The core of this cloud eventually collapses into a supermassive star (with masses of tens of thousands of solar masses or more), the core of which ultimately collapses into an SMBH seed. The timescale for the formation of the cloud is much shorter than the 108 years that are needed to convert the disk's gas into stars. Gas is therefore available for SMBH formation rather than being consumed to form stars. Gas collapse driven by galaxy mergers thus overcomes the major difficulty of previous collapse models in isolated galaxies, in which star formation had to be artificially suppressed to prevent it from consuming the gas reservoir.

The highly dynamic, out-of-equilibrium conditions of galaxy mergers, and the details of gas infall, can be studied only with numerical tools such as those used by Mayer and colleagues1. The authors' results tie in nicely with Begelman and colleagues' proposal3,4 that a supermassive star forms when gas-infall rates exceed the large threshold value of about 1 solar mass per year. According to this model3,4, although the star's core collapses into a black hole, material piles up on the star's surface and allows the hole to be efficiently fed; this black-hole-powered star is known as a quasistar3.

Mayer et al. find that such a large gas-infall rate can indeed be produced through galaxy mergers. What's more, the combination of their gas-infall process1 with the quasistar model proposed by Begelman et al. can explain the existence of powerful quasars at a time when the Universe was one-tenth of its current age5. These luminous quasars can be powered only by SMBHs of billions of solar masses. (There are galaxies that weigh, in their entirety, less than these SMBHs.) Explaining the existence of such SMBHs less than one billion years after the Big Bang is challenging, and requires either a very steady growth of the SMBHs or that the seeds are quite massive. Mayer and colleagues' results are appealing in this context because the SMBH seeds that form during galaxy mergers are indeed very massive at birth.

However, the black-hole formation mechanism proposed by Mayer et al. seems to be efficient only for galaxies that were already very massive at early cosmic times; these galaxies are expected to evolve into objects the size of the Milky Way or larger. Although this finding is broadly consistent with observations, which indicate that most SMBHs are hosted by large galaxies, SMBHs have also been found in dwarf galaxies6 (more than a 100 times smaller than the Milky Way). Of course, it is possible that smaller SMBHs in low-mass galaxies may have formed through other mechanisms7, for instance as remnants of the first generation of stars. These stars are predicted to have had masses up to several hundred times that of the Sun, and so probably left behind 'almost-massive' black holes at the end of their life.

The James Webb Space Telescope (the successor to the Hubble Space Telescope), future X-ray missions such as IXO and the gravitational-wave telescope LISA will all have the technical capabilities to detect quasars and SMBHs in the early Universe. These therefore promise to provide further insight into the process that generates SMBHs 'from scratch'.

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Volonteri, M. Making black holes from scratch. Nature 466, 1049–1050 (2010). https://doi.org/10.1038/4661049a

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