Gravitational waves from merging intermediate-mass black-holes

Abstract

The discovery of an intermediate-mass black-hole (IMBH) supports a runaway path of a super-massive black-hole (SMBH) formation in a galactic nuclei. A concrete model to explain all the steps to this bottom-up scenario for SMBH is not yet known, but we here propose to use gravitational radiation to probe the merging history of IMBHs. Collisions of black-holes of mass 103 ∼ 106 M· will produce gravitational radiation of 10-1 ∼ 102 Hz in their final merging phase. We simply model a thousand IMBHs with a mass of 103M· in a galactic system, and assume their accumulating history in two extreme cases; hierarchical growth and monopolistic growth. We find that there would be 40 ∼ 100 IMBH merging events a year in the Universe, and that the event numbers of two models apparently differ in the frequency of gravitational radiation. Most of the bursts of gravitational wave by these events are well detectable by currently proposed space gravitational wave antenna, such as LISA or DECIGO. We also show that each burst signal is distinguishable, and conclude that the statistics of the signals would tell us both a galaxy distribution and a formation model of SMBHs. A detailed discussion is presented in Matsubayashi et al.