| Abstract: | We report on the population properties of the 47 compact binary mergers
detected with a false-alarm rate < 1 yr$^{-1}$ in the second LIGO--Virgo
Gravitational-Wave Transient Catalog, GWTC-2. We investigate the binary black
hole (BBH) mass distribution, spin distribution, and merger rate as a function
of redshift, observing several BBH population characteristics not discernible
until now. First, we find the primary mass spectrum contains structure beyond a
power-law 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% crediblity).
While the primary mass distribution must extend past $65,M_odot$, only
$2.9^{+3.4}_{-1.7}\%$ of systems have primary masses greater than
$45,M_odot$. At low masses, the primary mass spectrum is consistent with a
gap between $sim2.6,M_odot$ and $sim6,M_odot$. Second, we find that a
fraction of BBH systems have component spins misaligned with the orbital
angular momentum, giving rise to precession of the orbital plane. Moreover, 12%
to 44% of BBH systems have spins tilted by more than $90^circ$, giving rise to
a negative effective inspiral spin parameter. Third, we estimate merger rates,
finding $R_mathrm{BBH} =
23.9^{+14.9}_{-8.6},mathrm{Gpc}^{-3},mathrm{yr}^{-1}$ for BBH and
$R_mathrm{BNS} = {320}^{+490}_{-240},mathrm{Gpc}^{-3},mathrm{yr}^{-1}$ for
binary neutron stars (BNS). The BBH 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 models, but the low
secondary mass of GW190814 makes it an outlier. |
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