Gravitational wave sources from inspiralling globular clusters in the Galactic Centre and similar environments

Abstract

We model the inspiral of globular clusters (GCs) towards a galactic nucleus harboring a supermassive black hole (SMBH), a leading scenario for the formation of nuclear star clusters. We consider the case of GCs containing either an intermediate-mass black hole (IMBH) or a population of stellar mass black holes (BHs), and study the formation of gravitational wave (GW) sources. We perform direct summation N -body simulations of the infall of GCs with different orbital eccentricities in the live background of a galaxy with either a shallow or steep density profile. We find that the GC acts as an efficient carrier for the IMBH, facilitating the formation of a bound pair. The hardening and evolution of the binary depends sensitively on the galaxy's density profile. If the host galaxy has a shallow profile the hardening is too slow to allow for coalescence within a Hubble time, unless the initial cluster orbit is highly eccentric. If the galaxy hosts a nuclear star cluster, the hardening leads to coalescence by emission of GWs within 3 − 4 Gyr. In this case, we find a IMBH-SMBH merger rate of Γ IMBH−SMBH = 2.8×10 −3 yr −1 Gpc −3 . If the GC hosts a population of stellar BHs, these are deposited close enough to the SMBH to form extreme-mass-ratio-inspirals with a merger rate of Γ EMRI = 0.25 yr −1 Gpc −3 . Finally, the SMBH tidal field can boost the coalescence of stellar black hole binaries delivered from the infalling GCs. The merger rate for this merging channel is Γ BHB = 0.4 − 4 yr −1 Gpc −3 .

Birth, Life, and Death of Black Hole Binaries around Supermassive Black Holes: Dynamical Evolution of Gravitational Wave Sources

“…A powerful way to test these predictions is via N -body simulations. Unfortunately, simulations that account for different stellar components are extremely expensive from the computational point of view and became affordable only recently, although they still rely on several simplified approximations (see for instance Arca- Sedda & Gualandris 2018;Abbate et al 2018;Baumgardt et al 2018;Panamarev et al 2019).…”

Hypervelocity Stars from a Supermassive Black Hole–Intermediate-mass Black Hole Binary

“…The main alternative was first proposed and discussed by Yu & Tremaine (2003), who suggested the ejection of HVSs as a consequence of star slingshots involving a massive black hole binary composed of the 4 × 10 6 M SMBH already observed in the GC (i.e. SgrA * ) and a putative secondary SMBH or intermediate mass black hole (IMBH; Baumgardt et al 2006;Levin 2006;Fragione et al 2018a,b), possibly delivered by infalling globular clusters due to dynamical friction in their host galaxy (Arca-Sedda & Gualandris 2018;Fragione et al 2018a,b). A scenario subsequently explored in a series of papers by Sesana and collaborators, considering either the ejection of unbound stars (Sesana et al 2006), or the erosion of a pre-existent bound stellar cusp (Sesana et al 2008).…”

The Fifth Candidate for an Intermediate-mass Black Hole in the Galactic Center

“…The IMBH in T1 could have been brought in by a massive cluster that migrated into the Galactic center by dynamical friction. Many members of the parent cluster have possibly been dispersed (Arca- Sedda & Gualandris 2018). Another possible origin of the IMBH is a remnant of "seeds" for supermassive black holes in the early universe, which could be formed in the first star clusters (Sakurai et al 2017).…”