Apparently, black holes and dark matter don’t play well together. Broadly speaking, black holes can be considered to be a significant portion of the “missing mass” in the universe, but dark matter is distinguished as “non-baryonic matter”. It seems that this mysterious non-baryonic matter is being used to explain a huge number of unexplained cosmic mysteries, but in the case of supermassive black holes, dark matter plays a very small role insofar as being used as black hole food…
I was pointed in the direction of an interesting paper complied by a French research group entitled “Dark Matter Accretion into Supermassive Black Holes“. In a nutshell they look into dark matter as a possible fuel to make supermassive black holes, well, supermassive. There has been much debate over to what extent dark matter may have to play in the evolution of black holes, so a complex numerical model was used and compared with observations.
This is the cool bit. It turns out that non-baryonic dark matter (a.k.a. dark matter), as opposed to baryonic matter (a.k.a. protons, neutrons, atoms, “normal” stuff), is highly collisionless. This means that dark matter, in its nature, is highly non-interacting. It can be easily scattered by stars and gas clouds, and it is very hard to imagine the stuff create a compact accretion disk around black holes.
Black hole accretion disks generate a lot of radiation, so much so that galactic cores can be observed as quasars. This radiation is produced as “normal” baryonic matter accumulated in the accretion disk falls to its death in the black hole event horizon. The matter has been accelerated to relativistic speeds and about 10% of the mass falling into the ‘hole is converted into energy (i.e. E=mc2). A massive quantity of energy is blasted into space as X-ray emissions.
By taking observations of galactic core quasars (the location of supermassive black holes) and using theoretical models to predict the rate of accretion disk “feeding” into the black hole, a rough estimate on how much dark matter is falling into the black hole can be found. For baryonic matter, the amount of X-rays generated by the quasar is proportional to the amount of matter falling into the black hole. Dark matter on the other hand (theoretically) does not generate X-rays as it falls into a black hole. So, from logical reasoning, if the predicted rate at which mass is falling into the black hole is more than the observed energy emitted, then perhaps some of the mass of the accretion disk is in fact dark matter.
Well, the details are sketchy, but it is looking unlikely that dark matter has any large part to play in the evolution of supermassive black holes. Less than 10% of the mass of the accretion disk will consist of dark matter; the other 90% can be easily accounted for in the form of baryonic matter.
Have a look at my Universe Today article for more on black holes and dark matter…