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.