Description
In this talk, I will present an overview of the population properties of merging compact objects, based on 158 newly detected events from the first part of the fourth observing run of the LIGO, Virgo, and KAGRA observatories. These events are included in the fourth Gravitational-Wave Transient Catalog (GWTC-4), which comprises binary neutron star (BNS), neutron star–black hole (NSBH), and binary black hole (BBH) mergers.
I will highlight the presence of distinct features in the black hole mass distribution, with prominent excesses around primary masses of 10 M$_\odot$ and 35 M$_\odot$, and possible additional structure near $\sim$20 M$_\odot$. These features deviate from an underlying power-law-like distribution that becomes steeper above 35 M$_\odot$. Systems with primary masses around 10 M$_\odot$ tend to be paired with lighter companions, with the mass ratio distribution peaking at $q = 0.74 ^{+0.13}_{-0.13}$, which may point to stable mass transfer during binary evolution.
Spin measurements indicate that black holes are typically far from maximal rotation, with 90% having $\chi < 0.57$, and tend to be aligned with the orbital angular momentum, supporting formation in isolated binary systems. At the same time, a non-negligible fraction (0.24–0.42) of systems exhibit negative effective inspiral spins, suggesting that a substantial subset may originate from dynamical formation channels.
The BBH merger rate is found to evolve with redshift as $\propto (1 + z)^\kappa$, with $\kappa = 3.2^{+0.94}_{-1.00}$, in agreement with the trend of cosmic star formation. While no significant redshift dependence is observed in the mass distribution, the spread of the effective inspiral spin appears to increase with redshift up to $z \approx 1$. Finally, the inferred local merger rates (at $z = 0$) are 7.6–250 Gpc$^{-3}$ yr$^{-1}$ for BNSs, 9.1–84 Gpc$^{-3}$ yr$^{-1}$ for NSBH systems, and 14–26 Gpc$^{-3}$ yr$^{-1}$ for BBHs, where all intervals correspond to 90% credible ranges.