# Blackhole & Galaxy size

Is there an exact relationship between the size of a galaxy and the density of its black hole?

jimgraber
Gold Member
For big galaxies with bulges, the mass (not density) of the black hole is about 1/700 of the mass of the bulge. No one knows why but some kind of joint evolution is the most popular theory.

Chronos
Gold Member
No firm correlation between galaxy and central black hole mass has been established to date. The central black hole masses of Andromeda and our galaxy are quite modest compared to the monster black holes in some similarly massive galaxies. Black hole density, however, is an easier question. All black holes are equally dense.

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jimgraber
Gold Member
"All black holes are equally dense."
This is not true unless you are thinking of the infinitely dense singularity.
The radius of a black hole is proportional to its mass.
The volume is proportional to its radius cubed.
Therefore its density is inversely proportional to the mass squared.
It is very high for stellar mass black holes, but comparable to ordinary densities for supermassive black holes.

Chronos