Title: Distant Quasars: The Observational Key to the Formation of Structure Across Scales

Abstract: The most distant luminous quasars, now found up to redshifts z~7.5, are in many ways the most robust tools used by astronomers to study the first billion years of our Universe.
On the gigaparsec scales of the cosmic web, the Lyman-alpha forest in front of the first quasars gives us the most precise measurement of reionisation’s end at z=5.3. Soon, advances in ground-based instrumentation will also yield an accurate value for the process’ mid-point, a crucial boundary condition on Cosmic Dawn.
On galactic scales, quasar spectra capture metal absorbers from the circum-galactic medium of faint early galaxies which cannot be detected in emission. Using this approach, I will present recent record-breaking observations of the most metal-poor galaxy detected to date: 11,000 times more metal-poor than the Sun, it is the first “ultra” metal-poor galaxy, and gives a fascinating glimpse of the properties of the first metal-poor stars.
Finally, the existence of quasars at z>7 is a tremendous mystery in itself: the supermassive black holes powering them cannot easily grow from the remnants of the first stars. I will present cutting-edge observations from JWST which help elucidate this mystery.