The eMERLIN and EVN View of FR 0 Radio Galaxies

Ranieri D Baldi,Gabriele Giovannini,Alessandro Capetti

doi:10.3390/galaxies9040106

Abstract

We present the results from high-resolution observations carried out with the eMERLIN UK-array and the European VLBI network (EVN) for a sample of 15 FR 0s, i.e., compact core-dominated radio sources associated with nearby early-type galaxies (ETGs), which represent the bulk of the local radio galaxy population. The 5 GHz eMERLIN observations available for five objects exhibit sub-mJy core components and reveal pc-scale twin jets for four out of five FR 0s once the eMERLIN and JVLA archival visibilities data are combined. The 1.66 GHz EVN observations available for 10 FR 0s display one- and two-sided jetted morphologies and compact cores. The pc-scale core emission contributes, on average, to about one tenth of the total extended radio emission, although we noted an increasing core contribution for flat-/inverted-spectrum sources. We found an unprecedented linear correlation between the pc-scale core luminosity (∼1021.3–1023.6 W Hz−1) and [O III] line luminosity, generally considered as proxy of the accretion power, for a large sample of LINER-type radio-loud low-luminosity active nuclei, all hosted in massive ETGs, which include FR 0s and FR Is. This result represents further evidence of a common jet–disc coupling in FR 0s and FR Is, despite then differing in kpc-scale radio structure. For our objects and for other FR 0 samples reported in the literature, we estimated the jet brightness sidedness ratios, which typically range between one and three. This parameter roughly gauges the jet bulk Lorentz factor Γ, which turns out to range from 1 to 2.5 for most of the sample. This corroborates the scenario that FR 0s are characterized by mildly relativistic jets, possibly as a result of lower-spinning black holes (BHs) than the highly spinning BHs of relativistic-jetted radio galaxies, FR Is.

Highlights

The presence of accreting supermassive black holes (BHs) at the center of local massive early-type galaxies, ETGs) has been largely reported by large-area surveys (e.g., [1,2,3,4,5,6]) and supported by theoretical studies (e.g., [7,8,9])
The central core, which pinpoints the location of the putative BH, represents the jet base, where we can explore the mechanisms of jet launching and propagation at the parsec scale
The fraction of the pc-scale core flux density with respect to the JVLA component gradually increases with the radio frequencies

Summary

Introduction

The presence of accreting supermassive black holes (BHs) (active galactic nuclei, AGNs) at the center of local massive early-type galaxies, ETGs) has been largely reported by large-area surveys (e.g., [1,2,3,4,5,6]) and supported by theoretical studies (e.g., [7,8,9]). From the most powerful to the weakest, RGs encompass a large range of jet properties, such as luminosities, morphologies, duty cycles, and speeds, but all sharing a single type of evolved host [11,12]. This capability of massive ETGs to be frequently associated with radio AGNs has been recently the object of several observational and theoretical studies and contextualized in the framework of AGN feedback, since RGs, even at low powers, can continuously inject energy in the central kpc of galaxies (e.g., [8,13,14,15,16])

Results

Discussion

Conclusion