# LIGO tells us LINERs are not optically thick RIAFs

## TL;DR

This paper uses LIGO gravitational wave data to show that most LINERs are unlikely to be optically thick RIAFs, clarifying the nature of these galactic nuclei.

## Contribution

It introduces a novel method of constraining LINER accretion flow models using gravitational wave merger rates.

## Key findings

- LIGO merger rate limits exclude most LINERs as optically thick RIAFs.
- Gravitational wave observations can probe the interior of accretion flows.
- Most LINERs are consistent with low accretion rate models, not thick RIAFs.

## Abstract

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.