  We use the gravitational wave signals from binary black hole merger events
observed by LIGO and Virgo to reconstruct the underlying mass and spin
distributions of the population of merging black holes. We reconstruct the
population using the mixture model framework VAMANA (Tiwari 2020) using
observations in GWTC-2 occurring during the first two observing runs and the
first half of the third run (O1, O2, and O3a). Our analysis identifies a
structure in the chirp mass distribution of the observed population.
Specifically, we identify peaks in the chirp mass distribution at 8, 14, 26,
and 45 M and a complementary structure in the component mass distribution with
an excess of black holes at masses of 9, 16, 30 and 57 M_. Intriguingly, for
both the distributions, the location of subsequent peaks are separated by a
factor of around two and there is a lack of mergers with chirp masses of 10-12
M. The appearance of multiple peaks is a feature of a hierarchical merger
scenario when, due to a gap in the black-hole mass spectrum, a pile-up occurs
at the first peak followed by mergers of lower mass black-holes to
hierarchically produce higher mass black-holes. However, cross-generation
merger peaks and observations with high spins are also predicted to occur in
such a scenario that we are not currently observing. The results presented are
limited in measurement accuracy due to small numbers of observations but if
corroborated by future gravitational wave observations these features have
far-reaching implications.
