The fate of central black holes in merging galaxies

This paper presents results of a series of numerical N-body experiments thatdescribe the merging of galaxy pairs containing a massive black hole in theircore. The aim is to study the orbital evolution of the two black holes throughthe merger event. It is found that merging does NOT lead to the formation of aCLOSE black hole pair} at the center of the remnant when the two progenitorgalaxies have (1) similar masses, (2) different central densities, and (3)relative orbit with non zero angular momentum. Under these conditions, thecentral black hole of the less dense galaxy settles into an wide orbit, i.e.,at a distance from the center comparable to the half mass radius of theremnant, where the time scale for orbital decay by dynamical friction can beseveral Gyrs. The implications of this result are briefly discussed.Comment: 7 pages (4 figures included), uuencoded tared postscript file, accepted to the MNRAS, 1 additional figure available upon request from governato@roma2.infn.i