Intermediate-mass black holes in globular clusters: observations and simulations

TL;DR

This paper investigates the presence and effects of intermediate-mass black holes in globular clusters through observations and simulations, revealing their influence on cluster dynamics, scaling relations, and observational biases.

Contribution

It provides new observational constraints on IMBH masses in globular clusters, explores their impact on cluster evolution via simulations, and assesses observational biases in velocity dispersion measurements.

Findings

IMBHs follow a flatter M_bh - sigma relation in globular clusters.

IMBH presence prevents core collapse and accelerates ejection of massive objects.

Velocity dispersion measurements are biased due to stellar blends and background light.

Abstract

The study of intermediate-mass black holes (IMBHs) is a young and promising field of research. Formed by runaway collisions of massive stars in young and dense stellar clusters, intermediate-mass black holes could still be present in the centers of globular clusters, today. Our group investigated the presence of intermediate-mass black holes for a sample of 10 Galactic globular clusters. We measured the inner kinematic profiles with integral-field spectroscopy and determined masses or upper limits of central black holes in each cluster. In combination with literature data we further studied the positions of our results on known black-hole scaling relations (such as M_bh - sigma) and found a similar but flatter correlation for IMBHs. Applying cluster evolution codes, the change in the slope could be explained with the stellar mass loss occurring in clusters in a tidal field over its life time. Furthermore, we present results from several numerical simulations on the topic of IMBHs and integral field units (IFUs). We ran N-body simulations of globular clusters containing IMBHs in a tidal field and studied their effects on mass-loss rates and remnant fractions and showed that an IMBH in the center prevents core collapse and ejects massive objects more rapidly. These simulations were further used to simulate IFU data cubes. For the specific case of NGC 6388 we simulated two different IFU techniques and found that velocity dispersion measurements from individual velocities are strongly biased towards lower values due to blends of neighbouring stars and background light. In addition, we use the Astrophysical Multipurpose Software Environment (AMUSE) to combine gravitational physics, stellar evolution and hydrodynamics to simulate the accretion of stellar winds onto a black hole.