Lifetime measurements of young quasars at z~6 from their UV and optical extended nebular emission

The billion-solar-mass black holes observed in the center of quasars within the first Gigayear of cosmic history have posed a challenge to our understanding of supermassive black hole (SMBH) growth. The problem is further exacerbated by a population of young quasars with extremely small proximity zones that have been identified at redshifts z > 6 — the imprint of their ionizing radiation on their rest-frame UV spectra seems to indicate that these quasars have grown their black holes in less than 100,000 years! If continuous, Eddington-limited accretion is assumed, it is extremely challenging to grow a billion-solar-mass black hole in such a short time. Hence, other mechanisms, such as super-Eddington accretion or highly dust-obscured growth, have to be invoked to explain such rapid black hole growth. In this talk, I will present new results on measuring quasar lifetimes using an independent lifetime measurement that promises to disentangle these two scenarios. We use new MUSE as well as JWST/NIRSpec IFU observations to search for the rest-frame UV and optical extended nebular emission around young quasars, and measure their transverse extent to estimate the quasars' lifetimes from their light travel time. This method yields complementary insights into the accretion history of these young quasars and, together with the proximity zone measurements, provides new insights to our understanding of SMBH growth in the early Universe.