The most ordinary formation of the most unusual double black hole merger

TL;DR

This paper demonstrates that the most massive black hole mergers, like GW190521, can be formed through classical isolated binary evolution of very massive stars, challenging the need for exotic formation scenarios.

Contribution

It shows that massive black hole mergers observed by LIGO/Virgo can be explained by standard binary evolution models including stars up to 200 solar masses.

Findings

Massive black hole mergers can be formed via classical binary evolution.

Predicted merger rates align with observed rates for both light and heavy BH-BH mergers.

Isolated binary evolution could account for GW190521-like events.

Abstract

LIGO/Virgo Collaboration reported the detection of the most massive black hole - black hole (BH-BH) merger up to date with component masses of 85 Msun and 66 Msun (GW190521). Motivated by recent observations of massive stars in the 30 Doradus cluster in the Large Magellanic Cloud (>200 Msun; e.g. R136a) and employing newly estimated uncertainties on pulsational pair-instability mass-loss (that allow for possibility of forming BHs with mass up to 90Msun) 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+150 Msun) Population II stars (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: <50 Msun) is of the order of 10-100 Gpc^-3 yr^-1, while GW190521 indicates that the rate of heavier mergers is 0.02-0.43 Gpc^-3 yr^-1. Our model (with standard assumptions about input physics) but extended to include 200 Msun stars and allowing for the possibility of stellar cores collapsing to 90 Msun BHs produces the following rates: 63 Gpc^-3 yr^-1 for light BH-BH mergers and 0.04 Gpc^-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 can not be excluded.