The effect of extended radio emission on SMBH accretion rate estimates

TL;DR

This study addresses biases in SMBH accretion-rate estimates caused by using unresolved total radio flux, which blends current accretion with relic jet/lobe power. It introduces a two-episode framework that separately evaluates current ($L_{ m bol,curr}$, $L_{ m mech,core}$) and past ($L_{ m bol,past}$, $L_{ m mech,past}$) accretion, and compares these to literature estimates based on total radio output. Applying this to 121 local radio galaxies, the authors find that traditional methods overestimate the Eddington-scaled rate by a factor of about $3$, misclassifying accretion modes in ~11% of sources, with the bias increasing for more extended systems. The results highlight the importance of core-focused measurements for reliable accretion-state inferences and have implications for AGN unification, high-redshift surveys, and studies of episodic accretion, while recognizing substantial uncertainties in the normalization and scaling relations used. Overall, the work provides a framework to derive systematic accretion-rate corrections when high-resolution data enable separation of core and extended radio components.

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 Super-Massive Black Holes (SMBHs). 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 \textit{Eddington}-scaled accretion rates ($λ$) 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 $λ$ by a factor of $\sim 3$, affecting the accretion mode classification in approximately $11\%$ of sources. This discrepancy appears to correlate with radio size, with the most extended galaxies indicating a transition in accretion disk 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 disk physics and optical spectral properties.

Figures (3)

• Figure 1: Left panel: comparison of past (consideration of $L_{\rm 1.4GHz,jets+lobes}$; equation \ref{['equation_past']}) and current (using solely $L_{\rm 1.4GHz,core}$; equation \ref{['equation_curr']}) Eddington-scaled accretion rates ($\lambda$) for the sample of 121 radio galaxies. Galaxies classified as LERGs and HERGs are shown in blue with circular and square markers, respectively, while black and red edges denote FRI and FRII galaxies. The intensity of each data point's colour reflects the $L_{\rm mech,core}/L_{\rm bol,curr}$ ratio (i.e., brighter red or blue colours indicate higher ratios), while the marker's size corresponds to the length of the radio jet. Error bars on the x-axis represent the lower and upper bounds of $L_{\rm bol, past}$. Right panel: comparison of the $\lambda_{\rm curr}$ with $\lambda_{\rm liter}$ (integrating the total radio flux; see equation \ref{['equation0']}), with the error-bars following the $W_0$ range. The dotted grey lines correspond to the accretion limit that separates quasar from radio accretion disks.
• Figure 2: Left panel: the number density histograms of the current $\lambda$ (empty rectangles) and literature (filled rectangles) for the LERGs and HERGs presented with blue and orange colour, respectively. The dashed lines correspond to the $\lambda$ distribution excluding the radio/mechanical luminosity (i.e., $\lambda=L_{\rm bol}/L_{\rm Edd}$), and are colored accordingly. The dotted grey line depicts the transition limit between quasar and radio accretion mode. Right panel: the difference in $\%$ between the current and literature $\lambda$ values over an estimation of the time that each jet will require to extend assuming a relativistic speed of 0.5c. Galaxies classified as LERG and HERG are shown as blue circles and orange squares, respectively, with X-shaped markers indicating sources for which their accretion rate has shifted above/below the accretion mode limit. A linear regression fit (grey shaded area) with its $3 \sigma$ interval is also included, as well as the $\Delta \lambda$ distributions for H/LERGs and FRI/IIs, annotated with their mean values and corresponding colour.
• Figure 3: The absolute difference between $\lambda_{\mathrm{liter}}$ and $\lambda_{\mathrm{curr}}$ as a function of the Core Prominence (CP) for our sample. The colour intensity of each point represents the jet length of the corresponding radio galaxy, while the markers layout follows the same scheme as in figure \ref{['z_comparison']}. The dotted grey vertical line indicates the threshold for the restarted AGN phase (CP = 0.1), as proposed by 2020AA...638A..34J.