Sometime ago, Gregoire and I argued for the possible evidence of a spin magnitude/mass correlation for binary black holes observed via Gravitational Waves. In a recent preprint, with colleagues from the University of Rome Sapienza, CERN and Johns Hopkins University, we tried to better understand this correlation by modelling the astrophysical formation channels that can create it.

We looked at possible spin/mass correlations introduced by black holes formed from isolated binary stars, dynamical formation in dense star clusters or in Active Galactic Nuclei (AGN), and primordial black holes. The main result of this study is that we still find this spin magnitude/mass correlation, but now we know a bit

We have found that Primordial black holes alone can definitely not be the only formation channel of black holes; instead, we need at least two formation channels to describe the data. Unfortunately, we can not say who these channels are at the moment.

Figure: Top row – spin magnitude distribution as a function of the source mass (colorbar). Bottom row – distribution of the cosine of the tilt angle of the black holes from the orbital angular momentum.