Speaker
Description
Quasars (QSOs) efficiently turn gravitational energy into huge luminous outputs, as matter spirals towards a supermassive black hole (SMBH) through an accretion disc. The spectral energy distributions (SEDs) of QSOs are mainly governed by few parameters, namely the SMBH mass ($M_{\rm BH}$), the accretion rate ($\dot{M}$) and the efficiency (i.e. the SMBH spin). Despite the general agreement on these very general grounds, puzzling observational findings urge a comprehensive justification. For instance, the SEDs predicted by emission line based $M_{\rm BH}$, estimated via the single epoch (SE) method, often do not agree with observations. Furthermore, extremely tight relations between broad lines and continuum luminosities (i.e. the Baldwin effect) still lack a compelling, unified explanation. In my talk, leveraging observations of some 100,000 QSOs, on a wide redshift range, I will show how these, and other inconsistencies can be naturally solved if blue optically-selected QSOs are powered by a classic geometrically thin and optically thick accretion disc with a narrow Eddington ratio distribution. Within this simple framework: 1) the observed slopes of the Baldwin effect are successfully predicted, 2) the continuum luminosity itself provides more accurate $M_{\rm BH}$ estimates than SE prescriptions, 3) super-Eddington QSOs are extremely rare, posing tight constraints on the burstiness of the super-critical accretion phases.