On minor black holes in galactic nuclei

Abstract

Small- and intermediate-mass black holes should be expected in galactic nuclei as a result of stellar evolution, minor mergers and gravitational dynamical friction. If these minor black holes accrete as X-ray binaries or ultraluminous X-ray sources (ULXs) and are associated with star formation, they could account for observations of many low-luminosity active galactic nuclei (AGN) or low-ionization nuclear emission regions (LINERs). Accreting and inspiralling intermediate-mass black holes could provide a crucial electromagnetic counterpart to strong gravitational wave signatures, allowing tests of strong gravity. Here we discuss observational signatures of minor black holes in galactic nuclei, and we demonstrate that optical line ratios observed in LINERs or transition-type objects can be produced by an ionizing radiation field from ULXs. We conclude by discussing constraints from existing observations as well as candidates for future study. © 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS.