What Drives the Bimodal Distribution of Eddington-Scaled Radio Luminosity in Nearby Early-Type Galaxies?

TL;DR

This study tests whether the Eddington-normalized radio luminosity in nearby early-type galaxies is bimodal by enlarging the sample to 117 galaxies with directly measured black-hole (BH) masses and 1.4 GHz fluxes. Using Gaussian-mixture analysis and maximum-likelihood fits, the authors find a robust bimodality in log(L_{1.4GHz}/L_{Edd}) with an antimode near ≈ -8.6 when BH masses are dynamical, but the bimodality disappears when BH masses are inferred from M_BH-σ_*; radio-bright hosts exhibit extended jets while radio-dim systems are compact and frequently radio-excess relative to the FIR–radio correlation. Infrared diagnostics (WISE colors and FIR data) show that many radio-bright galaxies are AGN-dominated in the mid-IR, whereas a large fraction of radio-dim systems resemble star-forming ISM in MIR-FIR properties, though some still show AGN-like radio excess. The results reveal a strong link between the radio dichotomy and host-galaxy kinematics and central structure: radio-bright systems are typically slow rotators with depleted cores, while radio-dim galaxies are fast rotators, supporting a scenario where sustained jet production is governed by assembly history and feeding mode rather than BH mass or accretion rate alone; a plausible fueling mechanism for the radio-dim population is intermittent delivery of magnetized gas from stochastic events such as tidal disruptions of giant-branch stars near the BH.

Abstract

{Early-type galaxies host low-luminosity active galactic nuclei, traced by radio emission spanning parsec- to kiloparsec scales.} {We investigate the Eddington-scaled radio luminosity distribution of 117 nearby early-type galaxies to test for bimodality and assess the role of host-galaxy properties, extending results from a 62-galaxy sample \cite{Wojtowicz2023}.} {We compile galaxies with directly measured black hole masses and 1.4,GHz and 3,GHz flux densities. Statistical tests assess bimodality, while VLASS imaging, host-galaxy kinematics, and central stellar structure characterize radio-dim and -bright sources.} {Using the 117-galaxy sample, we confirm that $L_{\rm 1.4,GHz}/L_{\mathrm{Edd}}$ is bimodal, with an antimode at $\approx -8.6$, which disappears when black hole masses are inferred from the $M_{\rm BH}$-$σ_\star$ relation. Radio-bright galaxies host resolved jets, while radio-dim systems show compact nuclear emission often exceeding that expected from star formation (FIR-radio correlation). Radio-bright galaxies are mainly slow rotators with depleted cores; radio-dim galaxies are predominantly fast rotators.} {Nearby early-type galaxies show a clear bimodality in Eddington-scaled radio luminosity, separating compact, radio-dim nuclei from extended, radio-bright systems. The dichotomy correlates with host-galaxy kinematics and central structure, indicating that sustained jet production depends primarily on galaxy assembly history and feeding mode rather than black hole mass or accretion rate alone. Radio-dim emission likely reflects intermittent, stochastic delivery of magnetized gas, plausibly via tidal disruption of giant-branch stars near the SMBH.}

Figures (5)

• Figure 1: The comparison of source properties presented in this work and in previous studies. The upper-right histogram shows the distribution of $\log_{10} M_{\rm BH}$ for sources presented in this work (red), Wojtowicz2023 (white), and early-type sources from Bosch2016 (green). The upper-left panel shows the distribution of source distances (in Mpc), plotted on a logarithmic scale using the same color scheme. The bottom-right and bottom-left histograms show, respectively, the distributions of $\log_{10} L_{1.4}$ and $L_{1.4\,\mathrm{GHz}}/L_{\rm Edd}$ for sources from Wojtowicz2023 (white) and this work (red).
• Figure 2: Histogram of integrated 1.4 GHz radio luminosity expressed in Eddington units, estimated using BH masses determined via direct methods. The position of the antimode at $\log L_{\mathrm{1.4\,GHz}}/L_{\mathrm{Edd}} =-8.6$, identified using the locmodes function, is indicated by a black vertical line. Left: Same as the right panel, but using BH masses estimated indirectly from the $M_{\rm BH}-\sigma_{*}$ relation of Graham13; the vertical line indicates the position of the antimode at $\log L_{\mathrm{1.4\,GHz}}/L_{\mathrm{Edd}} = -8.6$.
• Figure 3: WISE color-color diagram for the analyzed early-type galaxies. Sources belonging to the radio-dim and radio-bright classes are denoted as open blue diamonds and filled red circles respectively. The dashed line indicates the star formation sequence as given in Equation 2.
• Figure 4: The FIR--radio correlation for the analyzed early-type galaxies. The symbols denote the same classes of sources as in Figure \ref{['fig:wise']}, except for objects with only upper-limit measurements in the far-infrared, which are marked with stars. Grey shaded area denotes the q-values consistent with the range corresponding $q = 2.40 \pm 2\sigma_q$ with $\sigma_q$ = 0.24 Ivison10.
• Figure 5: Left panel shows the relation between black hole mass ($\log_{10} M_{\rm BH}$) and stellar velocity dispersion ($\log_{10}(\sigma_*/200\,\mathrm{km\,s^{-1}})$) for the early-type galaxies in our sample. Radio-dim sources are shown as blue diamonds and radio-bright sources as red circles, and their corresponding regression lines plotted as solid lines with shaded areas indicating 95% credibility intervals. The black dashed line shows the regression obtained by Graham13 for their full sample of 77 galaxies. Right panel shows the scatter of the radio-dim and radio-bright sources from the mean regression line obtained by Graham13, as a function of velocity dispersion. The projected histograms of the source distributions along both axes are presented with the same color scheme for the two radio sub-types.