The formation of Gaia BH3

TL;DR

This work addresses how Gaia BH3—a $m_{\rm BH}=33\,M_\odot$ black hole in a wide, eccentric orbit around an extremely metal-poor star at $590\,\mathrm{pc}$—formed. It compares quasi-isolated binary evolution against dynamical formation in the ED-2 progenitor cluster by performing $N$-body simulations of plausible ED-2 progenitors, with varying initial masses, densities, and binary content, and tracking dynamical interactions over $13\,\mathrm{Gyr}$. The key finding is that Gaia BH3 most likely originated as an exchange SBH binary formed in the ED-2 progenitor through multiple dynamical encounters, while a quasi-isolated channel is strongly disfavored; dynamics also significantly modify primordial SBH binaries. This highlights the critical role of cluster dynamics in shaping BH binary populations and informs the connection between EM-detected BH binaries and GW-detected BH mergers, with implications for the demographics of binaries with BH components.

Abstract

The Gaia collaboration announced the discovery of a massive black hole (BH) with a low-mass giant star companion, Gaia BH3, located in the ED-2 stellar stream. The properties of Gaia BH3 bridge the gap between known Milky Way BHs and extragalactic BHs found with gravitational waves (GWs). We aim to determine the most likely formation scenario for Gaia BH3 in the progenitor cluster of the ED-2 stream. We perform $N$-body simulations of that progenitor cluster and find that, most likely, Gaia BH3 formed from a stellar binary that formed during cluster formation, which then underwent multiple dynamical interactions that significantly altered its properties, including exchanging the companion star. We highlight the importance of cluster dynamics and discard a formation scenario where it evolved in quasi-isolation.

Figures (2)

• Figure 1: Positions of the stars of the ED-2 stream in Galactocentric cartesian coordinates. In black, Gaia data from Balbinot2023. In blue, a simulation with $M_0=7.7\times 10^3\,\mathrm{{\rm M}\sb{\rm\odot}}$ and $\rho_{h, 0}=760 \,\mathrm{{\rm M}\sb{\rm\odot}}\,\mathrm{pc}^{-3}$, with each star coloured according to its magnitude.
• Figure 2: Average number of S-BH binaries per cluster in our simulations as a function of the logarithm of the period (left), the eccentricity (middle), and the mass ratio (right). In green and yellow, the primordial and exchanged (respectively) S-BH formed in simulations with primordial binaries; in red, the dynamical S-BH formed in simulations without primordial binaries; in blue, the S-BH formed from primordial binaries extracted from the simulations and evolved in isolation; in dashed black, the parameters of Gaia BH3.