Existence and characterization of a distinct subpopulation of binary black holes with unequal masses and large spin–orbit misalignments

Determine whether a distinct subpopulation of merging binary black holes exists—characterized by low primary masses, unequal mass ratios, and a wide range of spin–orbit misalignment angles as suggested by the events GW241011 and GW241110—and, if it exists, characterize this subpopulation within the broader LIGO–Virgo–KAGRA binary black hole population.

Background

The paper reports two binary black hole coalescences, GW241011 and GW241110, each with low primary masses, unequal mass ratios, and markedly misaligned primary spins. These properties may indicate hierarchical formation in dense stellar environments and suggest the possibility of a distinct subpopulation within the overall binary black hole population.

Population-level inferences from previous catalogs indicate that black hole spins are often small and tend to align with the orbital angular momentum, but selection effects and parameter degeneracies can mask rare subpopulations. The authors note that, while these two events are not clear outliers, they could be representatives of an emerging subpopulation with specific mass and spin–orbit characteristics.

Confirming the existence of such a subpopulation requires additional observations and a formal population characterization, motivating a targeted investigation into whether this subgroup can be robustly identified and described with current and future data.