hide
Free keywords:
ACTIVE GALACTIC NUCLEI; DIGITAL SKY SURVEY; ADVECTION-DOMINATED
ACCRETION; TIDAL DISRUPTION EVENT; QUASAR HOST GALAXIES; DEPTH SURVEY
EFEDS; X-RAY VARIABILITY; BARE AGN; MULTIWAVELENGTH CAMPAIGN;
INSTABILITY MODELAstronomy & Astrophysics; accretion, accretion discs; black hole physics; galaxies: active;
Abstract:
The structure of the accretion flow on to supermassive black holes is not well understood. Standard disc models match to zeroth-order in predicting substantial energy dissipation within optically thick material producing a characteristic strong blue/ultraviolet continuum. However, they fail at reproducing more detailed comparisons to the observed spectral shapes along with their observed variability. Based on stellar mass black holes within our Galaxy, accretion discs should undergo a transition into an X-ray hot, radiatively inefficient flow, below a (mass scaled) luminosity of similar to 0.02LEdd. While this has been seen in limited samples of nearby low-luminosity active galactic nuclei (AGN) and few rare changing-look AGN, it is not at all clear whether this transition is present in the wider AGN population across cosmic time. A key issue is the difficulty in disentangling a change in spectral state from increased dust obscuration and/or host galaxy contamination, effectively drowning out the AGN emission. Here, we use the new eROSITA eFEDS Survey to identify unobscured AGN from their X-ray emission, matched to excellent optical imaging from Subaru's Hyper Suprime-Cam; allowing the subtraction of the host galaxy contamination. The resulting, uncontaminated, AGN spectra reveal a smooth transition from a strongly disc-dominated state in bright AGN, to the collapse of the disc into an inefficient X-ray plasma in the low-luminosity AGN, with the transition occurring at similar to 0.02LEdd; revealing fundamental aspects of accretion physics in AGN.
