N -body modelling of the ED-2 stream progenitor shows Gaia BH3’s formation involved dynamical interactions

Context . The Gaia collaboration announced the discovery of a binary of a massive black hole (33 M ⊙ ) with a low-mass giant star ( Gaia BH3) in the ED-2 stellar stream. The properties of this binary, as well as its position in the stream, challenge a formation scenario invoking only isolated binary evolution. Aims . We aim to quantify the importance of cluster dynamics in the formation of Gaia BH3 in the progenitor cluster of the ED-2 stream. Methods . We performed detailed N -body simulations of the progenitor cluster of the ED-2 stream, including the effects of single and binary stellar evolution. We compared these simulations to observations of the ED-2 stream and the properties of Gaia BH3. Results . We determine that Gaia BH3 most likely formed as an exchange binary that underwent multiple strong dynamical interactions. We highlight the importance of cluster dynamics in assembling Gaia BH3 and disfavour a formation scenario where it evolved unperturbed by dynamical interactions. Conclusions . The role of dynamics should be considered when interpreting the properties of the population of star–black hole binaries found in the next Gaia data release.