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

Abstract

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 85 M ⊙ and 66 M ⊙ (GW190521). Motivated by recent observations of massive stars in the 30 Doradus cluster in the Large Magellanic Cloud ( M ⋆  ≳ 200 M ⊙ ;  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  ∼ 90 M ⊙ ), 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 (180 M ⊙ + 150 M ⊙ ) 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  < 50 M ⊙ ) is of the order of 10–100Gpc −3 yr −1 , while GW190521 indicates that the rate of heavier mergers is 0.02–0.43Gpc −3 yr −1 . Our model (with standard assumptions about input physics), but extended to include 200 M ⊙ stars and allowing for the possibility of stellar cores collapsing to 90 M ⊙ BHs, produces the following rates: 63Gpc −3 yr −1 for light BH–BH mergers and 0.04Gpc −3 yr −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.