MERGING OF 2 GALAXIES WITH CENTRAL BLACK-HOLES

OBSERVATIONS of elliptical galaxies show a positive correlation 1,2 between total luminosity and core radius, defined as the radius at which surface luminosity becomes half of the central value. This has been taken as evidence against the idea that ellipticals are the result of galaxy mergers, because in simulated mergers 3-5 the core radius remained almost constant. In those simulations, the galaxies contained no central black holes, but there is a body of evidence to suggest that such black holes are common in ellipticals 6. Here, we present simulations of the merging of identical galaxies with and without central black holes, and find that when black holes are included, the merged galaxy acquires an isothermal core comparable in mass to the sum of the two initial black holes. Furthermore, the ratio of the core radius to the half-mass radius is approximately the same as the ratio of the black hole mass to the total galaxy mass, a result also consistent with observational evidence. These results, which can be understood by means of simple analytical arguments, suggest that most elliptical galaxies contain central black holes with masses comparable to the mass of their cores.