When the sparkling new James Webb Space Telescope sent back its first images of distant galaxies, the first reaction was surprise.
There seemed to be more galaxies visible in the early Universe than we expected, they were brighter than many imagined and they had massive black holes that shouldn’t have had time to grow.
The next reaction was confusion, as existing ideas seemed not to match what was being seen.
This led to the cry across the internet that Webb had 'broken' cosmology.
We’ve now reached the stage where new ideas are burgeoning, with seemingly every other astronomer toting a theory about exactly how the black holes in these galaxies assemble.
Once you’ve formed a black hole, it can grow by accreting surrounding material, but there’s a limit to how fast that can happen; a maximum rate obeyed in most circumstances, called the Eddington limit, applies.
This limit comes from the delicate balance between the pull of gravity and the resisting pressure caused by radiation from hot material in the accretion disc.
Solving the early black holes mystery
Seeing black holes weighing in at tens of millions of solar masses early on in the Universe’s history thus poses a puzzle.
They’re too big to have started as a stellar-sized black hole and then grown. But the authors of one study have a solution: forming intermediate-mass black holes directly.
They start by arguing that galaxies in the early Universe are just different from those in the present-day cosmos.
Today, the speed and efficiency of star formation are regulated by what’s called feedback, processes that impede the birth of new stars.
If this team are right, in the early Universe the brakes are off. With feedback ineffective, these early galaxies sparkle with thousands of newly formed star clusters.
How to form a black hole quickly
The resulting clusters are much more compact than those in the Universe today.
That means that interactions between the stars, which may in any case be more massive than normal, can lead to a collapse of the cluster’s core, forming a massive black hole directly without ever needing a supernova.
The team reckon this could happen within just a few million years.
This speed is important, as the clusters will soon be disrupted, scattering to the galactic winds.
If the black holes form first, then this is actually a benefit, as hundreds or even thousands of them will spread through the disc.
They will tend to move towards the systems’ centre, thanks to friction with the disc. Once there, they can merge and then grow rapidly as gas flows into the galactic centre.
It’s an attractive scenario, one that doesn’t need too much new physics, and which might have happened in many different galaxies.
Not that the mystery is solved. Important questions remain about how, exactly, the cores of the clusters could reach the density needed to form a black hole, and how the black holes would find their way to the galactic centre and stay there, even if disrupted by the kind of major galactic mergers that were common back then.
We have now reached the phase where hard, detailed work is needed to test this and all the other ideas for rapid black hole formation.
But this paper is a good start.
Chris Lintott was reading Growth of Massive Black-Holes in FFB Galaxies at Cosmic Dawn by Avishai Dekell et al. Read it online at: arxiv.org/abs/2409.18605.
This article appeared in the December 2024 issue of BBC Sky at Night Magazine
