- We report on the population properties of the 47 compact binary mergers detected with a false-alarm rate (FAR) < \(1\,\mathrm{yr}^{-1}\) in GWTC-2, including all Advanced LIGO–Virgo observing runs through the most recent observing run O3a. We investigate the mass distribution, spin distribution, and merger rate as a function of redshift. We observe several binary black hole (BBH) population characteristics not discernible until now. First, we find that the primary mass spectrum contains structure beyond a power-law distribution with a sharp high-mass cut-off; it is more consistent with a broken power law with a break at \(39.7^{+20.3}_{-9.1}\,M_\odot\), or a power law with a Gaussian feature peaking at \(33.5^{+4.5}_{-5.5}\,M_\odot\) (90% credible interval). While the primary mass distribution must extend to \(\sim65\,M_\odot\) or beyond, only \(2.9^{+3.4}_{-1.7}\%\) of systems have primary masses greater than \(45\,M_\odot\). At low masses, we find that the primary mass spectrum has a global maximum at \(7.8^{+2.2}_{-2.1}\,M_\odot\), consistent with a gap between \(\sim2.6\,M_\odot\) and \(\sim6\,M_\odot\). Second, we find evidence that a nonzero fraction of BBH systems have component spins misaligned with the orbital angular momentum, giving rise to precession of the orbital plane. Moreover, we infer that 12% to 44% of BBH systems have spins tilted by more than \(90^\circ\) with respect to their orbital angular momentum, giving rise to a negative effective inspiral spin parameter. Third, we provide improved estimates for merger rates using astrophysically motivated mass distributions: for BBH, \(R_\mathrm{BBH} = 23.9^{+14.9}_{-8.6}\,\mathrm{Gpc}^{-3}\,\mathrm{yr}^{-1}\) and for binary neutron stars (BNS), \(R_\mathrm{BNS} = {320}^{+490}_{-240}\,\mathrm{Gpc}^{-3}\,\mathrm{yr}^{-1}\). We constrain the BBH merger rate as a function of redshift and find that the rate likely increases with redshift (85% credibility), but not faster than the star-formation rate (87% credibility). Additionally, we examine recent exceptional events in the context of our population models, finding that the asymmetric masses of GW190412 and the high component masses of GW190521 are consistent with our population models, but the low secondary mass of GW190814 makes it an outlier. We discuss the implications of these results for compact binary formation and for the evolution of massive stars.
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