Pilot study of the radio-emitting AGN population: the emerging new class of FR 0 radio-galaxies

¹ SISSA-ISAS, via Bonomea 265, 34136 Trieste, Italy
e-mail: baldi@ph.technion.ac.il
² Physics Department, The Technion, 32000 Haifa, Israel
³ Physics Department, Faculty of Natural Sciences, University of Haifa, 31905 Haifa, Israel
⁴ INAF–Osservatorio Astrofisico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy
⁵ Dipartimento di Fisica e Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁶ INAF–Istituto di Radio Astronomia, via P. Gobetti 101, 40129 Bologna, Italy

Received: 28 November 2014
Accepted: 28 January 2015

Abstract

We present the results of a pilot JVLA project aimed at studying the bulk of the radio-emitting AGN population, that was unveiled by the NVSS/FIRST and SDSS surveys. The key questions are related to the origin of their radio-emission and to its connection with the properties of their hosts. We obtained A-array observations at the JVLA at 1.4, 4.5, and 7.5 GHz for 12 sources, a small but representative subsample. The radio maps reveal compact unresolved or only slightly resolved radio structures on a scale of 1−3 kpc, with the one exception of a hybrid FR I/FR II source extended over ~40 kpc. Thanks to either the new high-resolution maps or to the radio spectra, we isolated the radio core component in most of them. We split the sample into two groups. Four sources have low black hole (BH) masses (mostly ~10⁷ M_⊙) and are hosted by blue galaxies, often showing evidence of a contamination from star formation to their radio emission, and are associated with radio-quiet (RQ) AGN. The second group consists in seven radio-loud (RL) AGN, which are located in red massive (~10¹¹ M_⊙) early-type galaxies, have high BH masses (≳10⁸ M_⊙), and are spectroscopically classified as low excitation galaxies (LEG). These are all characteristics typical of FR I radio galaxies. They also lie on the correlation between radio core power and [O III] line luminosity defined by FR Is. However, they are more core-dominated (by a factor of ~30) than FR Is and show a deficit of extended radio emission. We dub these sources “FR 0” to emphasize their lack of prominent extended radio emission, which is their single distinguishing feature with respect to FR Is. The differences in radio properties between FR 0s and FR Is might be ascribed to an evolutionary effect, with the FR 0 sources undergoing rapid intermittency that prevents the growth of large-scale structures. However, this contrasts with the scenario in which low-luminosity radio-galaxies are fed by continuous accretion of gas from their hot coronae. In our preferred scenario the lack of extended radio emission in FR 0s is due to their lower jet Lorentz Γ factor with respect to FR Is. The slower jets in FR 0s are more subject to instabilities and entrainment, which causes their premature disruption.