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

Abstract

In this project, we have taken two separate data sets, and applied them to a search for tidal disruption events (TDEs), to provide evidence for the existence of intermediate-mass black holes (IMBH). The search was performed primarily on the optical archives of the Zwicky Transient facility (ZTF), using light curves and alerts produced by the facility. Follow-up and contextual imaging were also carried out using PanSTARRS and HST archives. The data sets were based on compact stellar system (CSS) positions, covering the mass / radius space from globular clusters through to dwarf elliptical galaxies, with one data set consolidated from literature CSS positions, and the other from notional GC/CSS populations hosted around massive galaxies at distance 2 ¤ D ¤ 120 Mpc.

We discussed the nature of CSS environments, especially considering the low-mass clusters, or ultra-compact dwarf (UCD), end of what is considered a composite population, with the expectation that any TDEs observed in aged cluster populations would involve IMBH and white dwarfs. The larger mass systems, nominally referred to as compact ellipticals (cEs), derived from tidally threshed larger galaxies, are more likely to host IMBH of higher mass, or even super-massive black holes (SMBH), but no strong evidence was found to preclude TDEs also being observed in these systems.

Despite analysis of the archives covering a period of approximately 42 months, no evidence of optical TDE transient signatures was found through either inspection of light curves and transient alerts at known CSS positions, or alerts in the periphery of host galaxies. A statistical analysis was carried out on the second data set, quantifying the sample size and visibility using the globular cluster luminosity function (GCLF). From this result we conclude that the upper limit for the rate of TDEs in CSS is nTDE,Vis Æ 10_6yr_1gal_1 which is two orders of magnitude below the theoretical TDE rate involving SMBH interacting with solar mass main sequence stars in the nucleus of massive galaxies.