Driver behind the bimodal distribution of Eddington-scaled radio luminosity in nearby early-type galaxies

¹ Department of Theoretical Physics and Astrophysics, Faculty of Science, Masaryk University, Kotláršká 2, Brno 61137, Czechia
² Astronomical Observatory, Jagiellonian University, ul. Orla 171, PL-30244 Kraków, Poland
³ Space Science Division, Naval Research Laboratory, Washington, DC 20375, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 19 December 2025
Accepted: 4 February 2026

Abstract

Context. Early-type galaxies host low-luminosity active galactic nuclei, as indicated by radio emission spanning up to several orders of magnitude in terms of the physical scale, ranging from compact parsec-scale to extended kiloparsec-scale radio morphology.

Aims. We investigate the Eddington-scaled radio luminosity distribution of nearby early-type galaxies in a large sample of 117 sources to confirm whether this distribution is bimodal, as previously inferred for a smaller sample of 62 galaxies. We also consider whether the bimodality can be attributed to specific host galaxy properties.

Methods. We compiled a sample of early-type galaxies with black hole masses measured using direct methods, as well as the radio flux densities at 1.4 GHz and 3 GHz. We used statistical tests to confirm whether the Eddington-scaled radio luminosities are bimodal. We investigated the properties of radio-dim and -bright sources, assessed the presence of extended jets with VLASS imaging, and examined host galaxy kinematics and central stellar structure.

Results. We confirm, using a twice larger sample of all known 1.4 GHz-detected early-type galaxies with directly measured black hole masses (117 galaxies), that the distribution of L_1.4 GHz/L_Edd is bimodal, with a characteristic antimode at L_1.4 GHz/L_Edd ≈ −8.6, which disappears when considering the subset of sources with black hole masses obtained using the M_BH − σ_★ relation. The radio-bright peak is dominated by galaxies hosting resolved jets, while radio-dim systems show compact nuclear emission with many showing excess radio emission relative to that expected from star formation as indicated by the well-known far-infrared and radio (FIR-radio) correlation. Moreover, we find that radio-bright galaxies are primarily slow rotators with depleted stellar cores, whereas radio-dim galaxies are predominantly fast rotators.

Conclusions. We show that nearby early-type galaxies with direct black hole mass measurements exhibit a clear bimodality in Eddington-scaled radio luminosity, separating radio-dim nuclei with compact emission from radio-bright systems hosting extended jets. The dichotomy correlates strongly with host-galaxy kinematics and central structure, suggesting that the ability to sustain jet production is governed primarily by galaxy assembly history and feeding mode, rather than by black hole mass or accretion rate alone. In this term, the radio output of radio-dim sources likely reflects modest, intermittent supplies of magnetized gas delivered to the accretion flow through stochastic processes and, most plausibly, the tidal disruption of giant-branch stars passing through the immediate vicinity of the supermassive black hole (SMBH).