The effect of extended radio emission on SMBH accretion rate estimates

ABSTRACT Accretion rates in radio galaxies are typically estimated from optical and total radio flux measurements, incorporating emission from the core, jets, and lobes. These estimates can be used to investigate the link between observed active galactic nuclei (AGN) emission properties and the underlying accretion physics of their supermassive black holes. However, while optical and core radio emission trace the ongoing accretion episode, extended jet and lobe structures may result from past AGN activity. Therefore, accretion rates inferred from spatially unresolved radio observations may be systematically overestimated, a bias whose prevalence and extent have yet to be thoroughly explored. In this study, using a sample of 121 local radio-loud galaxies with spatially resolved radio components, we assess this effect by estimating their Eddington-scaled accretion rates ($\lambda$) using both the common methodology which considers total radio fluxes and a simple but novel approach that treats core and extended emission as signatures of distinct accretion phases. Our results show that the former method systematically overestimates the $\lambda$ by a factor of $\sim 3$, affecting the accretion mode classification in approximately $11{{\ \rm per\ cent}}$ of sources. This discrepancy appears to correlate with radio size, with the most extended galaxies indicating a transition in accretion disc mode. Such a bias could affect AGN classification in unresolved high-redshift radio surveys. Our results motivate re-examining accretion rate calculations from AGN radio surveys and align with the AGN unification model for radio galaxies, revealing a clearer link between accretion disc physics and optical spectral properties.