Gravitational Brownian motion as inhomogeneous diffusion: black hole populations in globular clusters

Abstract

Recent theoretical and numerical developments supported by observational evidence strongly suggest that many globular clusters host a black hole (BH) population in their centers. This stands in contrast to the prior long-standing belief that a BH subcluster would evaporate after undergoing core collapse and decoupling from the cluster. In this work, we propose that the inhomogeneous Brownian motion generated by fluctuations of the stellar gravitational field may act as a mechanism adding a stabilizing pressure to a BH population. We argue that the diffusion equation for Brownian motion in an inhomogeneous medium with spatially varying diffusion coefficient and temperature, which was first discovered by Van Kampen, also applies to self-gravitating systems. Applying the stationary phase space probability distribution to a single BH immersed in a Plummer globular cluster, we infer that it may wander as far as $\sim 0.05,\,0.1,\,0.5{\rm pc}$ for a mass of $m_{\rm b} \sim 10^3,\,10^2,\,10{\rm M}_\odot$, respectively. Furthermore, we find that the fluctuations of a fixed stellar mean gravitational field are sufficient to stabilize a BH population above the Spitzer instability threshold. Nevertheless, we identify an instability whose onset depends on the Spitzer parameter, $S = (M_{\rm b}/M_\star) (m_{\rm b}/m_\star)^{3/2} ,$ and parameter $B = \rho_{\rm b}(0) (4\pi r_c^3/M_b)(m_\star/m_{\rm b})^{3/2} $, where $\rho_{\rm b}(0)$ is the Brownian population central density. For a Plummer sphere, the instability occurs at $(B,S) = (140,0.25)$. For $B > 140,$ we get very cuspy BH subcluster profiles that are unstable with regard to the support of fluctuations alone. For $S > 0.25,$ there is no evidence of any stationary states for the BH population based on the inhomogeneous diffusion equation.