The origin of the most massive, high-redshift quasars

The discovery of billion solar mass quasars at redshifts of 6–7 challenges our understanding of the early Universe; how did such massive objects form in the first billion years? Observational constraints and numerical simulations increasingly favour the "direct collapse" scenario, in which rapid accretion in a primordial halo leads to the formation of an initially-massive (~100,000 solar mass) black hole from the collapse of a supermassive "seed." In this talk, I'll present a systematic study of this channel under a variety of formation conditions, and propose observational diagnostics to reveal the origin of high-z quasars, and understand their co-evolution with their surrounding environment. In particular, we outline a search strategy wherein the Nancy Grace Roman Space Telescope, together with follow-up from other next-generation facilities, may be able to provide a unique, direct test of the formation of massive black hole seeds in the early Universe. I'll also discuss the broader picture of massive and supermassive star formation at high redshift, as well as further observational prospects for understanding the first stars in the next decade.