Supermassive black holes are some of the most impressive (and scary) objects in the universe – with masses around one billion times more than that of the Sun. And we know the’ve been around for a long time.
In fact, astronomers have detected the extremely luminous compact sources that are located at the centres of galaxies, known as quasars (rapidly growing supermassive black holes), when the universe was less than 1 billion years old.
Now our new study, published in Astrophysical Journal Letters, has used observations from the Hubble Space Telescope to show that there were many more (much less luminous) black holes in the early universe than previous estimates had suggested. Excitingly, this can help us understand how they formed – and why many of them appear to be more massive than expected.
Black holes grow by swallowing up material that surrounds them, in a process known as accretion. This produces tremendous amounts of radiation. The pressure from this radiation places a fundamental limit on how quickly black holes can grow.
Scientists were therefore faced with a challenge in explaining these early, massive quasars: without much cosmic time in which to feed, they must have either grown quicker than physically possible, or been born surprisingly massive.
But how do black holes form at all? Several possibilities exist. The first is that so-called primordial black holes have been in existence since shortly after the big bang. While plausible for black holes with low masses, massive black holes cannot have formed in significant numbers according to the standard model of cosmology.
Black holes definitely can form (now verified by gravitational wave astronomy) in the final stages of the short lives of some normal massive stars. Such black holes could in principle grow quickly if formed in extremely dense star clusters where stars and black holes may merge. It is these “stellar mass seeds” of black holes that would need to grow up too fast.
The alternative is that they could form from “heavy seeds”, with masses around 1,000 times greater than known massive stars. One such mechanism is a “direct collapse”, in which early structures of the unknown, invisible substance known as dark matter confined gas clouds, while background radiation prevented them from forming stars. Instead, they collapsed into black holes.
The trouble is that only a minority of dark matter halos grow large enough to form such seeds. So this only works as an explanation if the early black holes are rare enough.
For years, we have had a good picture of how many galaxies existed in the first billion years of cosmic time. But finding black holes in these environments was extremely challenging (only luminous quasars could be proven).
Although black holes grow by swallowing surrounding material, this does not happen at a constant rate – they break their feeding into “meals”, which makes their brightness vary over time. We monitored some of the earliest galaxies for changes in brightness over a 15 year period, and used this to make a new census of how many black holes are out there.
It turns out that there are several times as many black holes residing in ordinary early galaxies than we originally thought.
Other recent, pioneering work with the James Webb Space Telescope (JSTW) has begun to reach similar conclusions. In total we have more black holes than can form by direct collapse.
There is another, more exotic, way of forming black holes that could produce seeds that are both massive and abundant. Stars form by gravitational contraction of gas clouds: if significant numbers of dark matter particles can be captured during the contraction phase, then the internal structure could be entirely modified – and nuclear ignition prevented.
Growth could therefore continue for many times longer than the typical lifetime of an ordinary star, allowing them to become much more massive. However, like the ordinary stars and direct collapse objects, nothing is ultimately able to withstand the overpowering force of gravity. This means these “dark stars” should also eventually collapse to form massive black holes.
We now believe that processes similar to this should have taken place to form the large numbers of black holes we observe in the infant universe.
Studies of early black hole formation have undergone a transformation in the last two years, but in a sense this field is only just beginning.
New observatories in space, such as the Euclid mission or the Nancy Grace Roman Space Telescope, will fill in our census of fainter quasars at early times. The NewAthena mission and the Square Kilometer Array, in Australia and South Africa, will unlock our understanding of many of the processes surrounding black holes at early times.
But it is really the JWST that we must watch in the immediate term. With its sensitivity for imaging and monitoring and spectroscopic capabilities to see very faint black hole activity, we expect the next five years to really nail down black hole numbers as the first galaxies were forming.
We may even catch black hole formation in the act, by witnessing the explosions associated with the collapse of the first pristine stars. Models say this is possible, but it will demand a coordinated and dedicated effort by astronomers.
Matthew J. Hayes receives funding from the Knut & Alice Wallenberg Foundation, the Swedish National Space Agency (SNSA), and the Swedish Research Council (Vetenskapsrådet)
