Dynamics of intermediate mass black holes in globular clusters

Abstract

Context. We recently introduced a new method for simulating collisional gravitational N-body systems with approximately linear time scaling with N. Our method is based on the multi-particle collision (MPC) scheme, previously applied in fluid dynamics and plasma physics. We were able to simulate globular clusters with a realistic number of stellar particles (at least up to several times 106) on a standard workstation. Aims. We simulated clusters hosting an intermediate mass black hole (IMBH), probing a broad range of BH-cluster and BH–average-star mass ratios, unrestricted by the computational constraints that affect direct N-body codes. Methods. We set up a grid of hybrid particle-in-cell-MPC N-body simulations using our implementation of the MPC method, MPCDSS. We used either single mass models or models with a Salpeter mass function (a single power law with an exponent of −2.35), with the IMBH initially sitting at the centre. The force exerted by and on the IMBH was evaluated with a direct sum scheme with or without softening. For all simulations we measured the evolution of the Lagrangian radii and core density and velocity dispersion over time. In addition, we also measured the evolution of the velocity anisotropy profiles. Results. We find that models with an IMBH undergo core collapse at earlier times, the larger the IMBH mass the shallower they are, with an approximately constant central density at core collapse. The presence of an IMBH tends to lower the central velocity dispersion. These results hold independently of the mass function of the model. For the models with Salpeter MF, we observed that equipartition of kinetic energies is never achieved, even long after core collapse. Orbital anisotropy at large radii appears to be driven by energetic escapers on radial orbits, triggered by strong collisions with the IMBH in the core. We measured the wander radius, that is the distance of the IMBH from the centre of mass of the parent system over time, finding that its distribution has positive kurtosis. Conclusions. Among the results we obtained, which mostly confirm or extend previously known trends that had been established over the range of parameters accessible to direct N-body simulations, we underline that the leptokurtic nature of the IMBH wander radius distribution might lead to IMBHs presenting as off-centre more frequently than expected, with implications on observational IMBH detection.

Interpretable Machine Learning for Finding Intermediate-mass Black Holes

“…Lacking a smoking-gun, uncontroversial direct IMBH detection, indirect detection methods based on IMBH dynamical effects in GCs are useful for compiling a list of candidate hosts for follow up. The observable effects (see Mezcua 2017 for a recent and quite comprehensive review of IMBH detection attempts) include the presence of a central cusp in surface density (Bahcall & Wolf 1976;Newell et al 1976) and/or velocity dispersion (Peterson et al 1989;Noyola et al 2006Noyola et al , 2008Lützgendorf et al 2011Lützgendorf et al , 2012Lützgendorf et al , 2013bLützgendorf et al , 2016Lützgendorf et al , 2017Feldmeier et al 2013;Lanzoni et al 2013;Lanzoni 2016;Lanzoni & Cosmic-Lab Team 2016;Askar et al 2017a), masssegregation reduction (Baumgardt et al 2004;Gill et al 2008;Pasquato et al 2009Pasquato et al , 2016Beccari et al 2010;Umbreit & Rasio 2013;Di Cintio et al 2023), anomalous accelerations from pulsar timing (Peuten et al 2014;Kızıltan et al 2017;Perera et al 2017;Gieles et al 2018), high-velocity stars (Meylan et al 1991;Lützgendorf et al 2011;Fragione & Gualandris 2019), and other clues (Miocchi 2007;Pasquato & Bertin 2008Leigh et al 2014;Pepe & Pellizza 2016;Askar et al 2017a).…”