Radio Emission of Nearby Early-type Galaxies in the Low and Very Low Radio Luminosity Range

Abstract We analyze radio continuum emission of early-type galaxies with dynamical measurements of central supermassive black hole (SMBH) masses and well-characterized large-scale environments, but regardless of the exact level of the nuclear activity. The 1.4 GHz radio fluxes collected with ∼arcmin resolution for 62 nearby targets (distances ≲153 Mpc) correspond to low and very low monochromatic luminosities of L r ∼ 1035–1041 erg s−1. We quantify possible correlations between the radio properties with the main parameters of SMBHs, host galaxies, and hot gaseous halos, finding a general bimodality in the radio luminosity distribution, with the borderline between radio-bright and radio-dim populations at log L r / L Edd ≃ − 8.5 . We analyze the far-infrared data for the targets, finding that all radio-bright and over a half of the radio-dim sources are overluminous in radio wavelengths with respect to the far-infrared–radio correlation. High-resolution radio maps reveal that the overwhelming majority of radio-dim sources is unresolved on the arcsecond scale, while the bulk of radio-bright sources display extended jets and lobes of low- and intermediate-power radio galaxies; these jets dominate the radio emission of radio-bright objects. Regarding the origin of the radio emission of radio-dim sources, we discuss two main possibilities. One possibility is the advection-dominated accretion flow model, in which the radio and nuclear X-ray radiative outputs at very low accretion rates are both dominated by unresolved jets. The other possibility is that the radio-dim sources, unlike the radio-bright ones, are characterized by low values of SMBH spins, so that their radio emission is not related to the jets, but instead is due to a combination of star-forming processes and previous nuclear outbursts.