Where are Gaia's small black holes?

Abstract

Gaia has recently revealed a population of over 20 compact objects in wide astrometric binaries, while LIGO-Virgo-KAGRA (LVK) have observed around 100 compact object binaries as gravitational-wave (GW) mergers. Despite belonging to different systems, the compact objects discovered by both Gaia and the LVK follow a multimodal mass distribution, with a global maximum at neutron star (NS) masses ($\sim 1$-$2\,M_\odot$) and a secondary local maximum at black hole (BH) masses $\sim10\,M_\odot$. However, the relative dearth of objects, or ``mass gap," between these modes is more pronounced among the wide binaries observed by Gaia compared to the GW population, with $9^{+10}_{-6}\%$ of GW component masses falling between $2.5$--$5\,M_\odot$ compared to $\lesssim5\%$ of Gaia compact objects. We explore whether this discrepancy can be explained by the natal kicks received by low-mass BHs. GW progenitor binaries may be more likely to survive natal kicks, because the newborn BH has a more massive companion and/or is in a tighter binary than Gaia progenitor binaries. We compare the survival probabilities of Gaia and GW progenitor binaries as a function of natal kick strength and pre-supernova binary parameters, and map out the parameter space and kick strength required to disrupt the progenitor binaries leading to low-mass BHs in Gaia systems more frequently than those in GW systems.