How Black Holes Shape Galaxies

Posted by ap507 at Jan 22, 2016 03:30 PM |
Professor Andrew King discusses the science behind black holes
How Black Holes Shape Galaxies

Professor Andrew King

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Black holes are always news, as I know from personal experience. A recent theoretical scientific paper of mine suggesting that they could not easily grow beyond a mass of about fifty billion suns got a (to me) surprising amount of publicity in news media around the world.

The popular fascination with black holes rests largely on the exotic effects associated with them – even light can’t escape them, you would be stretched out very long and thin if you fell into one, time runs very differently near them, and many more.

But to astronomers like me, black holes have a simpler but more pervasive significance. They are the ultimate power source on cosmic scales, and have massive effects on the galaxies around them. The stellar architecture of almost every galaxy has been sculpted by the power output of a giant black hole in its centre. Astronomers can measure the cumulative effects of this black hole power,  exerted over eons, but also occasionally get glimpses of it in action (`Black hole caught `burping’ galactic gas supply’).

Black holes throw vast amounts of energy out into space as matter falls into them because of gravity. The infall always involves gas spiralling inwards in a flat disc, speeding up almost to the velocity of light just before the final plunge. The heat generated in the disc makes it radiate the energy of this speedup. This is vast: about ten percent of the maximum energy (`E = mc^2’) which Einstein’s theory of relativity says can be extracted from matter.

Ten percent may not sound a lot, but it is more than ten times what a star can do - the nuclear reactions powering the sun and all the stars liberate at most about 0.7 percent of this maximum energy. The only process which would liberate more - 100 percent - is to get matter and antimatter to annihilate each other. But this is a non-starter as a cosmic energy source, as there is so little antimatter in the universe. So gas infall into a black hole is the most energetic process in the large-scale universe. So it must power the most luminous objects in the universe - quasars - which can shine more than a hundred trillion times brighter than the sun. This black hole power shapes galaxies. 

Black hole with jet, X-ray source (artist's concept).
The centre of almost every galaxy hosts a supermassive black hole, many times heavier than any star. Our own Milky Way has a black hole of about 4 million sun masses in its centre. In cosmic terms this is modest - there are many that are much more massive, even approaching the suggested upper limit of fifty billion suns. The black holes gained this mass through gas infall, and so pumped out huge amounts of energy as they grew. In fact this energy release is about a thousand times bigger than what would in principle be enough to blow away the entire central bulge of the host galaxy, dispersing hundreds of billions of sun masses of gas and stars into space.

Given this, the wonder is perhaps not that black holes can have a big effect on their surrounding galaxies, but that these galaxies survive at all. Growing a galaxy is rather like building a house with an unexploded bomb in the basement. In fact what actually happens is that the motions of the hundreds of  billions of stars in the central bulge of a galaxy - a region tens of thousands of light years in size, are specified to remarkable precision simply by the mass of the black hole in the centre. This is not because these stars’ orbits are controlled by the black hole’s gravity - this is too weak to influence stellar motions beyond a few light years from the hole. So where does this strangely precise relation between black hole mass and stellar motion come from?

The hole’s vast power output supplies a simple and elegant answer. Until the black hole’s mass grows to a certain size, almost all of its power escapes freely into space as light, and it influences only a relatively small region near itself. But once the black hole mass reaches a critical value, related to the large-scale gravity of the galaxy  (and so to the motions of the stars in it) a dramatic change happens.

Almost the full power emitted by the black hole suddenly smashes into the galaxy as a high-speed wind of gas, rather than escaping as light. This wind leaves the stars relatively untouched, but blows away all the gas which would have fuelled further growth of the hole itself. So the hole mass remains stuck at the critical value, as it has starved itself of further food.

The hole has now reached the limits of growth in its host galaxy, and this is how we see many supermassive black holes. The only way one of them can grow any further is if its host galaxy gains more gas itself, by merging with   another galaxy. This is just what is happening to the `burping’ black hole. It lives in a relatively small galaxy, which is observed to be falling into a much bigger nearby galaxy.

This refuels the black hole host galaxy with gas, and allows the hole to start growing again. The repeated `burps’ of outflowing gas show us the wind from the black hole colliding with the new gas in the galaxy and driving it out. Black hole violence shapes galaxies.

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