Spatially resolving the broad line region and innermost dust continuum in AGN

The broad line region and near-infrared hot dust continuum trace two of the key structures on sub-parsec scales around AGN, and understanding them provides insights into how black holes and their host galaxies evolve. In this presentation, I will introduce the interferometric and spectro-astrometric techniques used in our GRAVITY-AGN programme, and summarize the main results of spatially resolving the BLR in 7 AGN and the hot dust continuum in 17 AGN. Combined with data in the literature, these reveal a tight relation between the BLR size and hot dust continuum size. I will discuss this in the context of (i) the possible mechanisms for the shallower slope of the size-luminosity relations, which we find to be 0.4 rather than the 0.5 expected; (ii) constraints on the dust geometry and composition implied by the systematic offset between the size-luminosity relations from interferometry and reverberation mapping; (iii) implications on inferring black hole masses. Looking ahead, our recent measurement of a black hole mass from spatially resolving the BLR in a quasar at z=2.2 holds promise for observing a large sample of AGNs across cosmic time, thereby enabling us to significantly improve our understanding about the co-evolution of black holes and their host galaxies.