Date: November 27, 2011

Title: Black Holes: The Supermassive Ones

Podcaster: Steve Nerlich

Organization: Cheap Astronomy


Description: This is part two of a two part podcast series on black holes.

Bio: Cheap Astronomy offers an educational website where black never goes out of fashion.

Sponsor: This episode of “365 Days of Astronomy” has been sponsored by NO ONE. please consider sponsoring an episode or two so we can continue to bring you daily “info”tainment.


This is the second of a two part series on black holes.

A long, long time ago in a smaller, denser universe, a galaxy may have started to build through the rapid aggregation of gas and dust, forming massive stars, which later collapsed into black holes – which then continued to grow rapidly in size due to the dense surrounding gas and dust that they were able to accrete. Some of them might have gotten bigger by merging together too – but in any case, being a smaller, denser universe this was an age when a black hole could get really big, really fast.

Well, really fast – but within limits. A black hole can’t just suck down everything that comes near it. What happens is that an accretion disk forms around the black hole – in much the same way that water spirals down a plug hole – and this accretion disk represents a bit of a traffic jam. As material spirals in into tighter and tighter circles it is forced to occupy a steadily diminishing volume – so its density increases and it starts heating up.

This is when we start seeing black holes emitting high energy radiation. It’s not the black hole emitting radiation – because, you know, it’s a black hole – all this radiation comes from the accretion disk. For example, Sagittarius A* star was long known as a bright radio source in Sagittarius – which has since been identified as a supermassive black hole – indeed the Blackest brother in the galaxy – and it became visible in x ray once we got space telescopes like Chandra above the atmosphere.

As well as this general glow of radiation from the accretion disk, you also get light year long jets of material pushed out from each pole of a supermassive black hole, due to twisting lines of magnetic force generated within the accretion disk – and a source of even more radiation.

Now there’s a concept called the Eddington limit – which is the point at which the radiation from a star achieves a dynamic balance with the gravitational compression of its mass. For very massive stars, their radiation can begin to exceed their gravity (that is exceed the Eddington limit) so that the star starts blowing off significant proportions of its mass as stellar wind. Stress on the word significant there – nearly all stars have stellar wind, like the Sun does, but only blow off a tiny proportion of their mass in this way. Massive stars, like Wolf-Rayets, can blow off huge amounts of mass just through radiation pressure – depleting much of their hydrogen content, resulting in types 1b or 1c supernovae at the end of their lives.

When an object is blowing off stuff in this manner – its luminosity increases dramatically – which we call Eddington luminosity. Now this Eddington – is Arthur Eddington, who like JA Wheeler was another great advocate of Einstein’s relativity physics. But Eddington was actually a black hole denier, involved in a famous stoush in the 1930s with Subramanyan Chandrasekhar – over the Chandrasekhar limit, which has it that any white dwarf with a mass greater than 1.4 solar masses will collapse and produce a type 1a supernova.
Eddington went to his grave in 1944 in a bit of a huff – still refusing to believe that stars could collapse into nothingness, while the younger Chandrasekhar went on to win a Nobel prize in 1983 – before his death on 1995, after which a space telescope got named after him as well.

And now back to the black holes. The idea of Eddington luminosity can be applied to objects other than stars – one obvious example being accreting black holes. At a certain point, a black hole’s accretion disk is going to reach its own Eddington limit – beyond which it starts emitting Eddington luminosity which will start to blow off gas and dust – which might have otherwise have fed the black hole.

Distant, and highly radiant, quasars may be examples of supermassive black holes which had grown to a galactic scale in the early universe. However, it may be that their growth becomes self-limiting as radiation pressure (that is, Eddington luminosity), from the supermassive black hole’s accretion disk and its polar jets, becomes so intense as to push large amounts of gas and dust out beyond the growing black hole’s gravitational sphere of influence.

The gas and dust that has been pushed out beyond the sphere of influence of a Wolf–Rayet star is called its wind nebula. Now, it’s a bit hard to tell at the distance they’re at, but presumably quasars also have wind nebulae, except that these would be galactic-sized wind nebulae.

The dispersed material in these nebulae could retain angular momentum, first gained from spinning around the accretion disk of the black hole, that would keep all that dispersed material in an orbiting halo – and in these outer regions of material, away from the destructive radiation pressure of the black hole’s accretion disk, star formation could take place.

All this is what you call highly speculative – it’s just my way of talking through what astronomers often call AGN feedback where AGN stands for active galactic nucleus – better known to us a supermassive black hole with an actively radiating accretion disk.

There seems to be an almost linear correlation between the mass of a central supermassive black hole – and the mass of the galactic bulge that surrounds it – so small galaxies have small supermassive black holes – and big ones have, well… big ones. This has led people to suggest that as galaxies grow in size – by new material coming into the galaxy – some of that new material gets absorbed in star formation within the bulge and the residue falls in to the AGN. But if too much material collects around the AGN – exceeding its Eddington limit – that material gets pushed back out into the bulge by the Eddington luminosity – hence the term ‘AGN feedback’.

Maybe in the early dense universe giant black holes were popping up all over the place, sucking down the dense gas and dust around them – but as the intensity of their feeding frenzy increased, they began to starve themselves as their Eddington luminosity built up supermassive wind nebulae made up of material pushed well out of the black hole’s reach. These supermassive wind nebulae would begin to capture any new gas and dust entering the system and pull this material into new star formation, depleting the flow of material towards the supermassive black hole within. And as these supermassive wind nebulae began to concentrate mass into their own structure the nebulae would begin to spin up – perhaps flattening out into a disk structure within which density waves might develop to form spiral arms – and voila galaxy.

So there you go. Black holes – which we normally think of as representing the end of a massive star’s life – could in a different context be the seeds from which galaxies, including our own, first grew. And it’s kind of ironic that the whole business is mediated by processes named after an astronomer who refused to believe that black holes even existed.

Thanks for listening. This is Steve Nerlich from Cheap Astronomy, Cheap Astronomy offers an educational website where black never goes out of fashion. No ads, no profit, just good science. Bye.

End of podcast:

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