The light echo of a high-redshift quasar mapped with Lyα tomography

Ultraviolet (UV) radiation from accreting black holes ionizes the intergalactic gas around early quasars, carving out highly ionized bubbles in their surroundings. Any changes in a quasar’s luminosity are therefore predicted to produce outward-propagating ionization gradients, affecting the Lyα absorption opacity near the quasar’s systemic redshift. This “proximity effect” is well-documented in rest-UV quasar spectra but only provides a one-dimensional probe along our line of sight. Here we present deep spectroscopic observations with the James Webb Space Telescope (JWST) of galaxies in the background of a superluminous quasar at zQSO ≈ 6.3, which reveal the quasar’s “light echo” with Lyα tomography in the transverse direction. This transverse proximity effect is detected for the first time toward multiple galaxy sightlines, allowing us to map the extent and geometry of the quasar’s ionization cone. We obtain constraints on the orientation and inclination of the cone, as well as an upper limit on the obscured solid angle fraction of fobsc < 91%. Additionally, we find a timescale of the quasar’s UV radiation of tqso = 10^5.6+0.1-0.3 yr, which is significantly shorter than would be required to build up the central supermassive black hole (SMBH) with conventional growth models, but is consistent with independent measurements of the quasars’ duty cycle. Our inferred obscured fraction disfavors a scenario where short quasar lifetimes can be explained exclusively by geometric obscuration, and instead supports the idea that radiatively inefficient accretion or growth in initially heavily enshrouded cocoons plays a pivotal role in early SMBH growth. Our results pave the way for novel studies of quasars’ ionizing geometries and radiative histories at early cosmic times.