Is GW190521 the merger of black holes from the first stellar generations?

Understand

GW190521 challenges our understanding of the late-stage evolution of massive stars and the effects of the pair-instability in particular.

• We discuss the possibility that stars at low or zero metallicity could retain most of their hydrogen envelope until the pre-supernova stage, avoid the pulsational pair-instability regime and produce a black hole with a mass in the mass gap by fallback.
• We present a series of new stellar evolution models at zero and low metallicity computed with the Geneva and MESA stellar evolution codes and compare to existing grids of models.
• Models with a metallicity in the range 0-0.0004 have three properties which favour higher BH masses as compared to higher metallicity models.