Merging Black Holes in the Low-mass and High-mass Gaps from 2+2 Quadruple Systems

Understand

The origin of the black hole (BH) binary mergers observed by LIGO-Virgo is still uncertain, as are the boundaries of the stellar BH mass function.

• Stellar evolution models predict a dearth of BHs both at masses $\gtrsim 50$ M$_\odot$ and $\lesssim 5$ M$_\odot$, thus leaving low- and high-mass gaps in the BH mass function.
• A natural way to form BHs of these masses is through mergers of neutron stars (NSs; for the low-mass gap) or lower-mass BHs (for the high-mass gap); the low- or high-mass-gap BH produced as a merger product can then be detected by LIGO-Virgo if it merges again with a new companion.
• We show that the evolution of a 2+2 quadruple system can naturally lead to BH mergers with component masses in the low- or high-mass gaps.