Populating the Black Hole Mass-Gap with Stellar Collisions in Dense Star Clusters

Over the past few years, the groundbreaking detections of gravitational wave signals from merging binary black holes (BHs) by LIGO/Virgo have ignited immense interest in how these sources form. Dynamical formation of binary BHs in dense stellar environments like globular clusters has emerged as an important formation channel. Dynamical interactions in globular clusters present opportunities to form binary BHs with properties that cannot be produced through classic evolution channels. For example, in a cluster environment, heavy BHs can form through runaway collisions of massive stars before BH formation. This motivates us to examine formation pathways for heavy BHs in globular clusters. In this study, we build a detailed collisional history of BH progenitor stars in globular clusters through the use of an N-body simulation code. We find that collisions of massive stars can indeed lead to the formation of BHs at or above the single stellar evolution limit, including intermediate-mass BHs. Such heavy BHs in star clusters likely acquire a companion BH, possibly becoming loud and detectable sources of GWs.