Why Do All Galaxies Have a Supermassive Black Hole at Their Centre?

[flotsam]

I watched a tv programme recently in which it said astronomers had found that every galaxy had a supermassive black-hole at its centre. What I found curious was the finding that each specific black hole always made up 0.5% of the mass of the galaxy it was located in. There is clearly an inescapable link between a galaxy and its supermassive central black-hole. What is the current reasearch and theories as to why this is?
Thanks to anyone who posts on this thread.

 

[SpaceTiger]

I watched a tv programme recently in which it said astronomers had found that every galaxy had a supermassive black-hole at its centre. What I found curious was the finding that each specific black hole always made up 0.5% of the mass of the galaxy it was located in.

Neither of those things is really true. It appears that the majority of galaxies have black holes at the center, but not all of them. M33, for example, appears not to have a black hole. As for the total fraction of the galaxy mass, I'm pretty sure that varies a lot as well. For example, in the Milky Way, the total mass of the black hole is a few million solar masses, while the total mass of the galaxy is of order a trillion solar masses. Divide these numbers and you'll get a mass fraction around 0.0001%, not 0.5%.

There are several correlations between the properties of galaxies and their central black holes, but they mostly relate the galaxy's bulge (mass or velocity dispersion) to the black hole.

There is clearly an inescapable link between a galaxy and its supermassive central black-hole. What is the current reasearch and theories as to why this is?

We do indeed believe that there is such a link, but the question has still not been answered to most folks' satisfaction. Really, we're not even sure how the black holes grew to their enormous size in the first place. They could have slowly accreted gas mass from their environment or they could have grown by collisions with smaller black holes. Both of these things could be linked to dynamical processes in the host galaxy and both have theories that claim to create the observed bulge-black hole correlations.