Where Are LIGO's Big Black Holes?

In LIGO's O1 and O2 observational runs, the detectors were sensitive to stellar-mass binary black hole (BBH) coalescences with component masses up to 100 M-circle dot, with binaries with primary masses above 40 M-circle dot representing greater than or similar to 90% of the total accessible sensitive volume. Nonetheless, of the 5.9 detections (GW150914, LVT151012, GW151226, GW170104, GW170608, and GW170814) reported by LIGO-Virgo, the most massive binary detected was GW150914 with a primary component mass of similar to 36 M-circle dot, 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 less than or similar to M less than or similar to 135 M-circle dot. We argue that the absence of detected binary systems with component masses heavier than similar to 40 M-circle dot 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 BBHs. 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 alpha less than or similar to 3 and an upper mass cutoff M-max similar to 40M(circle dot). This inferred distribution is entirely consistent with the two recently reported detections, GW170608 and GW170814. We show that with similar to 10 additional LIGO-Virgo BBH detections, fitting the BH mass distribution will provide strong evidence for an upper mass gap if one exists.