Dynamical formation of Gaia BH1 in a young star cluster

Abstract

ABSTRACT Gaia BH1, the first quiescent black hole (BH) detected from Gaia data, poses a challenge to most binary evolution models: its current mass ratio is ≈0.1, and its orbital period seems to be too long for a post-common envelope system and too short for a non-interacting binary system. Here, we explore the hypothesis that Gaia BH1 formed through dynamical interactions in a young star cluster (YSC). We study the properties of BH-main sequence (MS) binaries formed in YSCs with initial mass 3 × 102–3 × 104 M⊙ at solar metallicity, by means of 3.5 × 104 direct N-body simulations coupled with binary population synthesis. For comparison, we also run a sample of isolated binary stars with the same binary population synthesis code and initial conditions used in the dynamical models. We find that BH-MS systems that form via dynamical exchanges populate the region corresponding to the main orbital properties of Gaia BH1 (period, eccentricity, and masses). In contrast, none of our isolated binary systems match the orbital period and MS mass of Gaia BH1. Our best-matching Gaia BH1-like system forms via repeated dynamical exchanges and collisions involving the BH progenitor star, before it undergoes core collapse. YSCs are at least two orders of magnitude more efficient in forming Gaia BH1-like systems than isolated binary evolution.