by Supermassive black holes are among the most impressive (and terrifying) objects in the universe – with masses about a billion times greater than that of the Sun. And we know they’ve been around for a long time.
In fact, astronomers discovered the extremely bright compact sources located at the centers of galaxies, known as quasars (fast-growing supermassive black holes), when the universe was less than 1 billion years old.
Now our new study, published in Astrophysical diary lettershas 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 could help us understand how they formed – and why many of them appear more massive than expected.
Black holes grow by gobbling up material that surrounds them, in a process known as accretion. This produces enormous amounts of radiation. The pressure of this radiation sets a fundamental limit on how quickly black holes can grow.
Scientists therefore faced a challenge in explaining these early, enormous quasars: Without much cosmic time to feed, they must have either grown faster than physically possible or been born surprisingly large.
Light versus heavy seeds
But how do black holes form in the first place? Various possibilities exist. The first is that so-called primordial black holes have existed since shortly after the Big Bang. Although plausible for low-mass black holes, massive black holes cannot have formed in significant numbers according to the Standard Model of cosmology.
Black holes can certainly 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 rapidly if they are formed in extremely dense star clusters where stars and black holes can merge. It is these ‘stellar mass seeds’ of black holes that are supposed to grow up too quickly.
The alternative is that they could arise from ‘heavy seeds’, with masses about a thousand 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 trap gas clouds while background radiation prevented them from forming stars. Instead, they collapsed into black holes.
The problem 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.
Too many black holes
For years we have had a good idea of how many galaxies existed in the first billion years of cosmic time. But finding black holes in these environments was a huge challenge (only luminous quasars could be proven).
Although black holes grow by engulfing surrounding material, this does not happen at a constant rate; they break their diet into ‘meals’, causing their brightness to vary over time. We monitored some of the earliest galaxies over a period of fifteen years for changes in brightness, and used this to conduct a new count of how many black holes exist.
It turns out that there are several times as many black holes in ordinary early galaxies than we initially thought.
Other recent groundbreaking work with the James Webb Space Telescope (JSTW) is beginning to lead to similar conclusions. In total, we have more black holes than could be created by direct collapse.
There is another, more exotic way to form black holes that can produce seeds that are both massive and plentiful. Stars are formed by gravitational contraction of gas clouds: if significant numbers of dark matter particles can be captured during the contraction phase, the internal structure can be completely changed – and nuclear ignition can be prevented.
Growth could therefore continue many times longer than the typical lifespan of an ordinary star, making them much more massive. But like ordinary stars and objects that collapse instantly, nothing is ultimately able to withstand the overwhelming force of gravity. This means that these ‘dark stars’ should also eventually collapse and form massive black holes.
We now believe that similar processes would have had to occur to form the large numbers of black holes we observe in the early universe.
Future plans
Research into the formation of early black holes has undergone a transformation in the past two years, but in a sense this field is just getting started.
New observatories in space, such as the Euclid mission or the Nancy Grace Roman Space Telescope, will fill in our count of fainter quasars in 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 in early times.
But it’s really the JWST that we need to keep an eye on in the short term. With its sensitivity for imaging and monitoring and its spectroscopic ability to observe very faint black hole activity, we expect that within the next five years the numbers of black holes will really be determined when the first galaxies formed.
We can even observe the formation of black holes in the act, by witnessing the explosions that accompany the collapse of the first pristine stars. Models say this is possible, but it will take a coordinated and dedicated effort from astronomers.
Matthew J. Hayes, Associate Professor of Astrophysics, Stockholm University
This article is republished from The Conversation under a Creative Commons license. Read the original article.
