Beyond the far side: Observing black hole mergers beyond the pair-instability mass gap with next-generation gravitational wave detectors

Stellar evolution predicts the existence of a mass gap for black hole remnants produced by pair-instability supernova dynamics, whose lower and upper edges are very uncertain. We study the possibility of constraining the location of the upper end of the pair-instability mass gap, which is believed to appear around m_min∼130M_⊙, using gravitational wave observations of compact binary mergers with next-generation ground-based detectors. While high metallicity may not allow for the formation of first-generation black holes on the "far side" beyond the gap, metal-poor environments containing Population III stars could lead to such heavy black hole mergers. We show that, even in the presence of contamination from other merger channels, next-generation detectors will measure the location of the upper end of the mass gap with a relative precision close to Δm_min/m_min≃ 4% (N_det/100 )^-1/2 at 90 C.L., where N_det is the number of detected mergers with both members beyond the gap. These future observations could reduce current uncertainties in nuclear and astrophysical processes controlling the location of the gap.