In recent years, astrophysicists have found growing evidence that super massive black holes sit at the heart of most galaxies, and certainly those with bulges in the middles.
The one at the centre of our galaxy is about 6 light hours across (not light years, thanks Mark!), about the size of the orbit of Neptune, and is equivalent in mass to several million Suns. We can see one extraordinary star, called S2, orbiting it every 15 years or so.
What’s exciting about this link between black holes and galaxies is that it’s beginning to look as if black holes and galaxies form together. And that the evolution of one regulates the other. That’s a neat and exciting idea but it leads to a puzzle.
Astronomers assume that gravity holds galaxies together. They can get a good idea of this force by measuring the galactic mass. But many galactic discs rotate so quickly that centripetal forces ought to tear them apart.
Something else must be holding these galaxies together. The consensus view is that some kind of invisible stuff, called dark matter, does the trick. In fact, astronomers think that most galaxies sit in a halo of dark matter that is closely linked with the galactic disc.
Of course, if this is true, it’s only natural to assume that dark matter must influence black holes too. So black holes must be able to grow by eating dark matter and that has big implications for how large they can become and for galactic evolution.
But this is where astrophysicists begin scratching their heads. They can see that black holes are often linked to the size of the bulges at the centre of galactic discs but there is no correlation between the size of black holes and the galactic discs themselves.
And that creates the puzzle: how can galactic discs and black holes both be correlated with dark matter but not to each other?
Today, John Kormendy and Ralf Bender at the Max Planck Institute for Extraterrestrial Physics in Germany, provide the answer.
They have studied six galaxies that contain black holes but do not have a bulge, like NGC 2988 shown above. They did this, they say because: “we want to know whether dark matter correlates with black holes in the absence of the component that we know correlates with black holes.”
Their conclusion is that it doesn’t: the size of the dark matter halo has no effect on the size of the black hole at the centre of a galaxy. Their data suggests that bigger black holes simply sit at the centre of bigger galaxies and the dark matter has nothing to do with it.
But in the fine tradition of science, this clears up one mystery only to reveal another. If dark matter has this powerful gravitational influence, why doesn’t it form into black holes?
Nobody knows. But there is another group of astronomers who will be studying this result with interest. These guys think dark matter is an unnecessary invention. Instead, they say, the rotation of galaxies can be explained if the force of gravity is stronger over these galactic distances.
So according to them there is a simpler explanation for the absence of a link between dark matter and black holes: it’s that dark matter doesn’t exist.
It’s fair to say that this idea, Modified Newtonian Dynamics or MOND, is not a mainstream one. But as an alternative hypothesis it could yet give dark matter a run for its money.
Ref: arxiv.org/abs/1101.4650: Supermassive Black Holes Do Not Correlate With Dark Matter Halos Of Galaxies