Tidal Disruption Events and Gravitational Waves from Intermediate-mass Black Holes in Evolving Globular Clusters Across Space and Time

TL;DR

This paper models the evolution of globular clusters with intermediate-mass black holes over cosmic time, predicting tidal disruption event rates and gravitational wave signals that inform observational strategies beyond galactic centers.

Contribution

It introduces a semi-analytic model for globular cluster evolution, estimating TDE and IMBH-SBH merger rates across cosmic time and highlighting off-center TDE observability.

Findings

Integrated TDE rate in GCs can surpass galactic nucleus rates.

Approximately 90% of TDEs occur within 2-15 kpc of galaxy centers.

IMBH-SBH merger rates can reach up to 100 Gpc$^{-3}$ yr$^{-1}$ near GC formation peak.

Abstract

We present a semi-analytic model for self-consistently evolving a population of globular clusters (GCs) in a given host galaxy across cosmic time. We compute the fraction of GCs still hosting intermediate-mass black holes (IMBHs) at a given redshift in early and late type galaxies of different masses and sizes, and the corresponding rate of tidal disruption events (TDEs), both main-sequence (MS) and white dwarf (WD) stars. We find that the integrated TDE rate for the entire GC population can exceed the corresponding rate in a given galactic nucleus and that $\sim 90$% of the TDEs reside in GCs within a maximum radius of $\sim 2-15$ kpc from the host galaxy's center. This suggests that observational efforts designed to identify TDEs should not confine themselves to galactic nuclei alone, but should also consider the outer galactic halo where massive old GCs hosting IMBHs would reside. Indeed, such off-centre TDEs as predicted here may already have been observed. MS TDE rates are more common than WD TDE rates by a factor 30 (100) at $z\leq 0.5$ ($z=2$). We also calculate the rate of IMBH-SBH mergers across cosmic time, finding that the typical IMRI rate at low redshift is of the order of $\sim 0.5-3$ Gpc$^{-3}$ yr$^{-1}$, which becomes as high as $\sim 100$ Gpc$^{-3}$ yr$^{-1}$ near the peak of GC formation. Advanced LIGO combined with VIRGO, KAGRA, ET and LISA will be able to observe the bottom-end and top-end of the IMBH population, respectively.