Black hole masses via spatially resolving the broad line region at redshift 2

I will present a dynamical mass measurement of the black hole in a luminous quasar at z=2.2, achieved by spatially resolving its broad line region (Nature 2024). The observations needed only 4 hours integration with the interferometric instrument VLTI/GRAVITY, which has been used previously to measure black hole masses in the local quasars 3C273 and PDS456 as well as other nearby AGN. The line profile and resolved kinematics data are well reproduced by a thick, moderately inclined disk of gas clouds within the sphere of influence of a black hole with a mass of 3x10^8Msun. With an estimate of the host galaxy dynamical mass from molecular gas data, we find this AGN is an under-massive black hole accreting at a super-Eddington rate. I will describe how we plan to make similar measurements at different epochs in order to probe robustly how the relationship between black holes and galaxies evolves with cosmic time.