system
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human
Why are the largest black holes we know of so far away from us?

The YouTube channel Kurzgesagt recently uploaded a video on some of the largest known black holes, and I noticed that most of the larger ones they discuss seem to also be the furthest away. OJ 287 and TON 618 are both in the billions of light-years away from us. 

As we look into the distant universe, we are also looking back in time, due to the time it takes the light to travel to us from there. Therefore, we are seeing these ultramassive black holes as they were billions of years ago. But black holes, as far as we understand them, tend to only grow over time. Shouldn't we therefore expect to see smaller black holes when we look back in time, and larger black holes closer to the present, nearer to us?
gpt
Black holes don't grow uniformly at the same rate. The growth rate depends on how much material is nearby, and how much of that stuff gets close enough to the black hole to accrete.

Most massive galaxies seem to have a supermassive black hole in the centre. For this to grow, you need to get gas to the centre of the galaxy. This takes time, because the gas needs to lose angular momentum to fall inwards, and that happens slowly through viscosity and turbulence. But stars *also* form from gas. So as you fuel the central supermassive black hole, you also fuel the formation of stars in the core. As a result, there's a correlation between the mass of a central bulge and the mass of a supermassive black hole - the black hole is about 1/1000th of the mass of the central bulge.

So it comes down to the particulars of how a galaxy gains its mass and how it transports it to the centre. Galaxies vary a lot in their mass and in their history. An interaction or even merger between galaxies can stir up the gas and cause it to fall "downhill" into the centre of the galaxies, fuelling a "starburst" of star formation and dramatic growth of the black hole - the accretion disc around the black hole can become bright enough to become what we call a "quasar". So the black hole growth will depend on the details of not only the mass of its own galaxy, but also on what galaxies are nearby.

This growth isn't steady at all, and has also slowed down over time as more and more gas gets locked up in stars. Only the most massive stars go supernovae, and even then they leave a massive neutron star or black hole remnant, and don't return all their gas to interstellar space. Other stars only eject a little bit of mass, or basically none at all, so we run out of interstellar gas over time. As there's less gas around these days, stars form less often, and black holes grow more slowly. The peak of star formation and black hole growth was several billion years ago. So most things grew dramatically at that point, and haven't grown much since then.

The other thing is that if we're looking for rare objects - the most massive black holes we can find - then you're more likely to find them when you look over a large volume. This is part of what we call "Malmquist bias". In general, massive bright stars and massive black holes are rarer than smaller stars and smaller black holes. The further you look, the larger volume you are searching over, and the more likely you are to see something rare. This means that, even if the universe was static (it isn't), you are more likely to see massive black holes at a larger distance. This can come out as quite a nice correlation, where the maximum mass increases quite tidily with distance. A the same time, smaller objects are harder to see when they're far away, because you need more resolution and sensitivity to see them. So you see fewer small objects and more large objects as you get further away, and it *looks like* there's a correlation between distance and mass, but it's really just a bias from our observations. But this does mean that the most massive black holes we see are most likely to be further away.