Mass and effective spin distributions of binary black hole events: Evidence for a population of hierarchical black hole mergers in active galactic nuclei disks

Abstract

The origins of the coalescing binary black holes (BBHs) detected by the advanced LIGO/Virgo are still in debate and clues may present in the mass and effective spin (${\chi }_{\mathrm{eff}}$) distributions of these merger events. Here we analyze the GWTC-1 and GWTC-2.1 BBH events, and find evidence for a primary mass dependent ${\chi }_{\mathrm{eff}}$ distribution. Below (above) the mass of $\sim 50M_{\odot }$, a nearly zero (relatively high) effective spin is typical. In the joint analysis of the primary mass and ${\chi }_{\mathrm{eff}}$ distributions, we identify a sub-population of BBHs characterized by both a bump-like primary mass distribution extending to $\sim 100M_{\odot }$ and a relatively high ${\chi }_{\mathrm{eff}}\sim 0.3$, consistent with the expectation for the $\ge 2$g merger events in the disks of active galactic nuclei (AGNs). Our results suggest that the merger rate in AGN disks is around $1.7{\mathrm{Gpc}}^{-3}{\mathrm{yr}}^{-1}$.

7 pages, 3 figures

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