THE FATE OF CENTRAL BLACK-HOLES IN MERGING GALAXIES

This paper presents results of a series of numerical N-body experiments that describe the merging of galaxy pairs containing massive black holes in their cores. The aim is to study the orbital evolution of the two black holes through the merger event. It is found that merging does not lead to the formation of a close black hole pair at the centre of the remnant when the two progenitor galaxies have (1) similar masses, (2) different central densities, and (3) a relative orbit with non-zero angular momentum. Under these conditions, the central black hole of the less dense galaxy settles into a wide orbit, i.e., at a distance from the centre comparable to the half-mass radius of the remnant, where the time-scale for orbital decay by dynamical friction can be several Gyr. The implications of this result are briefly discussed.