LIGO tells us LINERs are not optically thick RIAFs

Abstract

Low ionization nuclear emission-line regions (LINERs) are a heterogeneous collection of up to one-third of galactic nuclei in the local Universe. It is unclear whether LINERs are simply the result of low accretion rates onto supermassive black holes (BHs) or whether they include a large number of optically thick radiatively inefficient but super-Eddington accretion flows (RIAFs). Optically thick RIAFs are typically discs of large-scale height or quasi-spherical gas flows. These should be dense enough to trap and merge a large number of the stellar mass BHs, which we expect to exist in galactic nuclei. Electromagnetic observations of photospheres of accretion flows do not allow us to break model degeneracies. However, gravitational wave observations probe the interior of accretion flows where the merger of stellar mass BHs can be greatly accelerated over the field rate. Here, we show that the upper limits on the rate of BH mergers observed with LIGO demonstrate that most LINERs cannot be optically thick RIAFs.