Radio AGN in the local universe: unification, triggering and evolution

Abstract

Associated with one of the most important forms of active galactic nucleus (AGN) feedback, and showing a strong preference for giant elliptical host galaxies, radio AGN (L_1.4GHz > 10^24 W Hz^-1) are a key sub-class of the overall AGN population. Here I review our current state of understanding of the population of radio AGN at low and intermediate redshifts (z < 0.7), concentrating on their AGN and host galaxy properties, and covering three interlocking themes: the classification of radio AGN and its interpretation; the triggering and fuelling of the jet and AGN activity; and the evolution of the host galaxies. I show that much of the observed diversity in the AGN properties of radio AGN can be explained in terms of a combination of orientation/anisotropy, mass accretion rate, and variability effects. The detailed morphologies of the host galaxies are consistent with the triggering of strong-line radio galaxies (SLRG) in galaxy mergers. However, the star formation properties and cool ISM contents suggest that the triggering mergers are relatively minor in terms of their gas masses in most cases, and would not lead to major growth of the supermassive black holes and stellar bulges; therefore, apart from a minority (<20%) that show evidence for higher star formation rates and more massive cool ISM reservoirs, the SLRG represent late-time re-triggering of activity in mature giant elliptical galaxies. In contrast, the host and environmental properties of weak-line radio galaxies (WLRG) with Fanaroff-Riley class I (FRI) radio morphologies are consistent with more gradual fuelling of the activity via gas accretion at low rates onto the supermassive black holes.Comment: Accepted for publication in Astronomy & Astrophysics Review, 67 pages, 17 figure

Composite Spectral Energy Distributions and Infrared–Optical Colors of Type 1 and Type 2 Quasars

“…We select quasars as RL based on a monochromatic 1.4 GHz luminosity threshold of 10 24 W Hz −1 . This is the approximate luminosity limit for the FIRST survey at z=0.8, is characteristic of moderately powerful radio AGNs (e.g., Hickox et al 2009;Tadhunter 2016), and is above the luminosity for typical star-forming galaxies at low redshift (Kauffmann et al 2008). Among our samples, 456 (13%) of the QSO 1s and 600 (21%) of the QSO 2s are identified as RL.…”

“…This contrast suggests a weak connection between the mechanical power of the quasar (as measured by radio luminosity) and the radiative power, with implications for the launching mechanisms of relativistic jets (e.g., Sikora et al 2007;Tadhunter 2016;Padovani et al 2017).…”

Restarting activity in the nucleus of PBC J2333.9-2343

“…Treister et al 2012). In particular for radio galaxies, modest mergers have been proposed to explain the triggering mechanism of FRII galaxies, whereas hot accretion from the intra cluster medium or the interstellar medium has been proposed to explain the triggering mechanism of FRI galaxies (Tadhunter 2016). PBC J2333.9-2343 is intermediate between the FRI and FRII classification, therefore we cannot discard any of these possibilities as the triggering mechanism.…”

Why do extremely massive disc galaxies exist today?

“…These theoretical predictions appear similar to what is seen in observations. For example, Tadhunter (2016) show that a minority of the hosts of radio AGN at high stellar masses (M * > 10 11 M ), that have clearly discy morphologies, show the same patterns. They exhibit similarly high BH masses as their spheroidal counterparts (M BH > 10 8 M ), with broadly similar accretion rates.…”