Low-mass black holes as the remnants of primordial black hole formation

Bridging the gap between the approximately ten solar mass `stellar mass' black holes and the `supermassive' black holes of millions to billions of solar masses are the elusive `intermediate-mass' black holes. Their discovery is key to understanding whether supermassive black holes can grow from stellar-mass black holes or whether a more exotic process accelerated their growth soon after the Big Bang. Currently, tentative evidence suggests that the progenitors of supermassive black holes were formed as ~10⁴-10⁵M_solar black holes via the direct collapse of gas. Ongoing searches for intermediate-mass black holes at galaxy centres will help shed light on this formation mechanism.