Maximum Black Hole mass across Cosmic Time

TL;DR

This paper demonstrates that under certain conditions, massive stars at low metallicity can produce black holes exceeding previous mass limits, challenging existing theories of black hole formation.

Contribution

It introduces a new stellar evolution model considering core overshooting and metallicity-dependent winds, expanding the maximum black hole mass beyond pair-instability constraints.

Findings

Stars with 90-100 Msun at low metallicity can produce black holes over 85 Msun.

Proper physics modeling doubles the maximum black hole mass set by pair instability.

The maximum black hole mass varies with metallicity and cosmic time.

Abstract

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.