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 greater than or similar to 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 similar to 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 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 200M stars and allowing for the possibility of stellar cores collapsing to 90M 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.