When the black holes align: a subpopulation of aligned massive binary black holes observed via gravitational waves

In this work, we investigate the features present in the joint primary mass and effective spin distribution of binary black holes without relying on specific modelling assumptions. We make use of non-parametric methods, flexible models capable of approximating arbitrary probability densities with minimal mathematical assumptions, applying it to the newly released GWTC-5.0. Our analysis supports the presence of at least two separate sub-population of binary black holes showing different effective spin distributions: one of them, preferring positive ${\chi }_{\mathrm{e}\mathrm{f}\mathrm{f}}$ values, points towards the direction of systems formed in a non-spherically-symmetric, dynamical environment.