Prior to the detection of black holes ( BHs ) via the gravitational waves ( GWs ) they generate at merger , the presence of BHs was inferred in X-ray binaries , mostly via dynamical measurements , with masses in the range between \sim 5 - 20 ~ { } M _ { \odot } .
The LIGO discovery of the first BHs via GWs was surprising in that the two BHs that merged had masses of 35.6 ^ { +4.8 } _ { -3.0 } and 30.6 ^ { +3.0 } _ { -4.4 } M _ { \odot } , which are both above the range inferred from X-ray binaries .
With 20 BH detections from the O1/O2 runs , the distribution of masses remains generally higher than the X-ray inferred one , while the effective spins are generally lower , suggesting that , at least in part , the GW-detected population might be of dynamical origin rather than produced by the common evolution of field binaries .
Here we perform high-resolution N-body simulations of a cluster of isolated BHs with a range of initial mass spectra and upper mass cut-offs , and study the resulting binary mass spectrum resulting from the dynamical interactions .
Our clusters have properties similar to those of the massive remnants in an OB association \sim 10 \mathrm { Myr } after formation .
We perform a likelihood analysis for each of our dynamically-formed binary population against the data from the O1 and O2 LIGO/Virgo runs .
We find that an initial mass spectrum M _ { BH } \propto M ^ { -2.35 } with an upper mass cutoff M _ { max } \sim 50 M _ { \odot } is favored by the data , together with a slight preference for a merger rate that increases with redshift .