http://arxiv.org/abs/1405.2072
We investigate the consequences of superkicks on the population of supermassive black holes (SMBHs) in the Universe residing in brightest cluster galaxies (BCGs). BCGs are the most massive galaxies in the Universe, sitting at the center of galaxy clusters. There is strong observational evidence that they grew prominently at late times (up to a factor 2-4 in mass from z=1), mainly through mergers with satellite galaxies from the cluster, and they host the most massive SMBHs ever observed, with masses up to ten billion solar masses. Those SMBHs are also expected to grow hierarchically, together with their host galaxies, experiencing a series of mergers with other SMBHs brought in by merging satellites. Because of the asymmetric gravitational wave emission, some net linear momentum is emitted during the last stages of the binary inspiral and the remnant SMBH experiences a kick in the opposite direction. Kicks may be as large as ~5000 Km/s (“superkicks”), pushing the SMBHs out in the cluster outskirts for a time comparable to galaxy-evolution timescales. Therefore, measurements of the SMBH occupation fraction can be used to observationally test the existence of superkicks in nature. Because of their violent merger history, BCGs are the ideal objects to explore this possibility. Series of (super)kicks are expected to take place, increasing the total ejection probability. Moreover, BCGs host the SMBHs with the largest sphere of influence on the surrounding stars, which makes them the easiest targets for SMBH mass measurements. We predict, under a number of plausible assumptions, that superkicks can efficiently eject SMBHs from BCGs, bringing their occupation fraction down to a likely range 0.9<f<0.99 in the local Universe. A single observational confirmation of a missing nuclear SMBH would provide strong evidence for the occurrence of superkicks in the strong-gravity regime of BH mergers.
D. Gerosa and A. Sesana
Mon, 12 May 14
15/40
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