Age determination of Gaia BH2. Panel (a): temperature–radius diagram with Gaia BH2 marked by red error ellipses of 1, 2, and 3σ, with PARSEC isochrones for [M/H] = −0.0244 in black. Panel (b): age distribution estimated from Monte Carlo sampling. Panel (c): distribution of original mass of Gaia BH2, which is the current mass divided by (1 + z)³ according to Eq. (2). The vertical dashed line indicates 2.2 M_⊙, below which a BH is unlikely to form.