The Most Ordinary Formation of the Most Unusual Double Black Hole Merger

The Laser Interferometer Gravitational-Wave Observatory (LIGO)/Virgo Collaboration reported the detection of the most massive black hole–black hole (BH–BH) merger to date with component masses of 85M ⊙ and 66M ⊙ (GW190521). Motivated by recent observations of massive stars in the 30 Doradus cluster in the Large Magellanic Cloud (M ⋆ ≳ 200M ⊙; e.g., R136a) and employing newly estimated uncertainties on pulsational pair-instability mass loss (that allow for the possibility of forming BHs with mass up to M BH ∼ 90M ⊙), we show that it is trivial to form such massive BH–BH mergers through the classical isolated binary evolution (with no assistance from either dynamical interactions or exotica). A binary consisting of two massive (180M ⊙ + 150M ⊙) Population II stars (metallicity: Z ≈ 0.0001) evolves through a stable Roche lobe overflow and common envelope episode. Both exposed stellar cores undergo direct core collapse and form massive BHs while avoiding pair-instability pulsation mass loss or total disruption. LIGO/Virgo observations show that the merger rate density of light BH–BH mergers (both components: M BH < 50M ⊙) is of the order of 10–100Gpc−3yr−1, while GW190521 indicates that the rate of heavier mergers is 0.02–0.43Gpc−3yr−1. Our model (with standard assumptions about input physics), but extended to include 200M ⊙ stars and allowing for the possibility of stellar cores collapsing to 90M ⊙ BHs, produces the following rates: 63Gpc−3yr−1 for light BH–BH mergers and 0.04Gpc−3yr−1 for heavy BH–BH mergers. We do not claim that GW190521 was formed by an isolated binary, but it appears that such a possibility cannot be excluded.