Maximum Black Hole mass across Cosmic Time
  At the end of its life, a very massive star is expected to collapse into a
black hole. The recent detection of an 85 Msun black hole from the
gravitational wave event GW 190521 appears to present a fundamental problem as
to how such heavy black holes exist above the approximately 50 Msun
pair-instability limit where stars are expected to be blown to pieces with no
remnant left. Using MESA, we show that for stellar models with non-extreme
assumptions, 90..100 Msun stars at reduced metallicity (Z/Zsun < 0.1) can
produce blue supergiant progenitors with core masses sufficiently small to
remain below the fundamental pair-instability limit, yet at the same time lose
an amount of mass via stellar winds that is small enough to end up in the range
of an "impossible" 85 Msun black hole. The two key points are the proper
consideration of core overshooting and stellar wind physics with an improved
scaling of mass loss with iron (Fe) contents characteristic for the host galaxy
metallicity. Our modelling provides a robust scenario that not only doubles the
maximum black hole mass set by pair instability, but also allows us to probe
the maximum stellar black hole mass as a function of metallicity and Cosmic
time in a physically sound framework.
