  Low ionization emission line regions (LINERs) are a heterogeneous collection
of up to $1/3$ of galactic nuclei in the local Universe. It is unclear whether
LINERs are simply the result of low accretion rates onto supermassive black
holes or whether they include a large number of optically thick radiatively
inefficient but super-Eddington accretion flows (RIAFs). Optically thick RIAFs
are typically disks 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
black holes, 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 black holes can be greatly
accelerated over the field rate. Here we show that the upper limits on the rate
of black hole mergers observed with LIGO demonstrate that most LINERs cannot be
optically thick RIAFs.
