Radio Activity of Supermassive Black Holes with Extremely High Accretion Rates

Journal article, 2020

Radio emission from the high- A nd super-Eddington accreting active galactic nuclei (AGNs) has various origins: A persistent jet, the magnetized corona, and the wind-like outflows. It is still unclear which is the leading mechanism responsible for the observed radio emission and how the radio emission is related to other characteristic parameters such as the Eddington ratio and black hole mass. In this paper, we present the 5 GHz Very Large Array (VLA) observational results of a sample of 25 extremely high Eddington accreting supermassive black holes (EESBHs, the Eddington ratio λ Edd close to or above 1) in narrow-line Seyfert 1 galaxies, among which 22 sources are detected. Most of the EESBHs show a compact radio structure from a few hundred parsecs to 1 kpc scale. We estimated the lowest star formation rate surface density required for producing the observed radio emission and found that it is higher than the largest value previously detected in circumnuclear starburst galaxies, implying that the radio emission is from the AGN activity. Along with a comparison sample, we find an overall inverse-λ Edd correlation ranging from sub-to super-Eddington ratios. The high-Eddington and mildly super-Eddington AGNs (-0.5 < log) have a radio-to-X-ray luminosity ratio L R/L X ∼ 10-5-10-4 and a steep radio spectrum, supporting that the radio emission is from transient ejecta (outflows) of corona; however, the jet contribution cannot be entirely ruled out. Our highly super-Eddington sources (log) have a flatter radio spectrum, along with its low radio luminosity: ; their radio emission is likely dominated by a magnetized corona, and a radiation-pressure-caused jet is also proposed in this paper.