"The AGN Channel For LIGO Black Hole Binary Mergers"

Abstract: I will present a model for stellar mass black hole binary (BHB) mergers accelerated by an active galactic nucleus (AGN) accretion disk. This model predicted the existence of overweight' stellar mass BHB mergers, detectable by LIGO (McKernan, Ford, et al. 2014). In more recent work, we find the rate of BHB merger by this channel can span the range 1e-4-1e3 Gpc^-3 yr^-1, depending on a variety of poorly constrained astrophysical parameters. Thus, with LIGO's measured rates (12-213 Gpc^-3 yr^-1), we can already constrain some aspects of AGN physics. In addition, I will argue that despite their relative rarity and small cosmic volume, we should always expect AGNs to dominate over quiescent galactic nuclei (or nuclear star clusters, NSCs) in rates of BHB mergers. If the AGN channel is efficient, it predicts the masses of BHB mergers will be hierarchical, and should range up to IMBH mass. Such systems would produce gravitational wave (GW) signals readily detectable by the planned Laser Interferometer Space Antenna (LISA) mission. LIGO's measured spins for BHB mergers provide an additional important constraint on dominant channels--I will compare current models, each of which has strengths and weaknesses for explaining the observations to date.