Where are LIGO's Big Black Holes?

Abstract

In LIGO's O1 and O2 observational runs, the detectors were sensitive to stellar mass binary black hole coalescences with component masses up to $100M_{\odot }$, with binaries with primary masses above $40M_{\odot }$ representing $\gtrsim 90\%$ of the total accessible sensitive volume. Nonetheless, of the 5.9 detections (GW150914, LVT151012, GW151226, GW170104, GW170608, GW170814) reported by LIGO-Virgo, the most massive binary detected was GW150914 with a primary component mass of $\sim 36M_{\odot }$, far below the detection mass limit. Furthermore, there are theoretical arguments in favor of an upper mass gap, predicting an absence of black holes in the mass range $50\lesssim M\lesssim 135M_{\odot }$. We argue that the absence of detected binary systems with component masses heavier than $\sim 40M_{\odot }$ may be preliminary evidence for this upper mass gap. By allowing for the presence of a mass gap, we find weaker constraints on the shape of the underlying mass distribution of binary black holes. We fit a power-law distribution with an upper mass cutoff to real and simulated BBH mass measurements, finding that the first 3.9 BBHs favor shallow power law slopes $\hat{I}\pm \lesssim 3$ and an upper mass cutoff $M_{\max }\sim 40M_{\odot }$. This inferred distribution is entirely consistent with the two recently reported detections, GW170608 and GW170814. We show that with $\sim 10$ additional LIGO-Virgo BBH detections, fitting the BH mass distribution will provide strong evidence for an upper mass gap if one exists.

includes more detailed analysis of mass ratio distribution; accepted for publication in ApJL