A search for intermediate-mass black holes in compact stellar systems through optical emissions from tidal disruption events

Abstract

ABSTRACT Intermediate-mass black holes (IMBHs) are expected to exist in globular clusters (GCs) and compact stellar systems (CSSs) in general, but none have been conclusively detected. Tidal disruption events (TDEs), where a star is tidally disrupted by the gravitational field of a black hole, have been observed to occur around the supermassive black holes (SMBHs) found at the centres of galaxies, and should also arise around IMBHs, especially in the dense stellar cores of CSSs. However, to date none have been observed in such environments. Using data from the Zwicky Transient Facility, we search for TDEs associated with CSSs, but none are found. This non-detection allows us to set an upper limit on the TDE rate in CSSs of $n_\text{TDE,Total}\lessapprox 10^{-7} \, \mathrm{ CSS}^{-1}\, \text{yr}^{-1}$, which is 2 dex below the observed TDE rate involving SMBHs interacting with 1 M⊙ main-sequence stars in the nuclei of massive galaxies. We also consider ultracompact dwarfs (UCDs) formed through a tidal stripping process in the surveyed volume. On the assumption that these CSSs contain SMBHs and TDE rates are comparable to current observed optical rates in galactic nuclei ($\approx 3.2 \times 10^{-5}\, \text{gal}^{-1}\, \text{yr}^{-1}$), we determine an upper limit for the number of UCDs formed through a tidal stripping process in the surveyed volume to be NGC,Strip < 1.4 × 104, which we estimate represents $\lt 6~{{\ \rm per\ cent}}$ of the population of GCs >106 M⊙.