Limits on Runaway Growth of Intermediate Mass Black Holes from Advanced LIGO

TL;DR

This paper explores how advanced LIGO can constrain the formation of intermediate-mass black holes in globular clusters through runaway mergers, despite the gravitational wave frequencies being mostly out of reach.

Contribution

It demonstrates that aLIGO can effectively probe the IMBH formation mechanism via merger signatures in the lower frequency range, setting limits on the occupation fraction of such black holes.

Findings

aLIGO can probe IMBH formation with a sensitivity to occupation fractions below 3%.

Current observations do not limit the IMBH occupation fraction in globular clusters.

A six-year aLIGO run at design sensitivity can significantly constrain IMBH formation scenarios.

Abstract

There is growing evidence that intermediate-mass black holes (IMBHs), defined here as having a mass in the range M=500-10^5 Msun, are present in the dense centers of certain globular clusters (GCs). Gravitational waves (GWs) from their mergers with other IMBHs or with stellar BHs in the cluster are mostly emitted in frequencies <10 Hz, which unfortunately is out of reach for current ground-based observatories such as advanced LIGO (aLIGO). Nevertheless, we show that aLIGO measurements can be used to efficiently probe one of the possible formation mechanisms of IMBHs in GCs, namely a runaway merger process of stellar seed BHs. In this case, aLIGO will be sensitive to the lower-mass rungs of the merger ladder, ranging from the seed BH mass to masses >~50-300 Msun, where the background from standard mergers is expected to be very low. Assuming this generic IMBH formation scenario, we calculate the mass functions that correspond to the limiting cases of possible merger trees. Based on estimates for the number density of GCs and taking into account the instrumental sensitivity, we show that current observations do not effectively limit the occupation fraction f_occ of IMBHs formed by runaway mergers of stellar BHs in GCs. However, we find that a six-year run of aLIGO at design sensitivity will be able to probe down to f_occ<3% at a 99.9% confidence level, either finding evidence for this formation mechanism, or necessitating others if the fraction of GCs that harbor IMBHs is higher.