Central Engine of AGNs through Optical Spectroscopy

Abstract: Supermassive black holes (SMBHs) are known to co-evolve with their host galaxies, yet the central regions that regulate this co-evolution, specifically the accretion disc and the broad-line region (BLR), are beyond the spatial resolution limits of current telescopes. Reverberation mapping (RM) provides a powerful indirect method for studying unresolved regions by measuring the time delay between variations in the accretion disc continuum and the response of broad emission lines from the BLR. This time delay determines the characteristic size of the BLR and underlies the empirical BLR size–luminosity (R_BLR–L) relation, which is widely used to estimate black hole masses from single-epoch spectra. However, this calibration is weakest at the extremes of the AGN population. At low luminosities, intermediate-mass black holes (IMBHs) are poorly sampled and may not follow the canonical R–L relation. At high luminosities, especially in radio-loud quasars, relativistic jet emission contaminates the optical continuum, biasing luminosity estimates and therefore BLR radius measurements. Furthermore, classical reverberation mapping campaigns require long-term spectroscopic monitoring, making them observationally demanding.

I will present a set of targeted RM studies designed to address these limitations. These include short-timescale RM campaigns of nearby IMBHs, jet–disc decomposition analyses in radio-loud AGNs to quantify how jets bias BLR measurements, and pilot spectrophotometric campaigns to develop geometric distance measurements using AGNs. Together, these efforts improve the calibration of the R_BLR–L relation across a wide luminosity range and explore the potential of AGN-based distance indicators to measure the Hubble constant H₀ with better than 3% precision, providing an independent probe of the ongoing H₀ tension.