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 (
 $$L_{1.4\,\mathrm{GHz}} > 10^{24}$$
  W $$\hbox {Hz}^{-1}$$
 ) are a key sub-class of the overall AGN population. Recently their study has benefitted dramatically from the availability of high-quality data covering the X-ray to far-IR wavelength range obtained with the current generation of ground- and space-based telescope facilities. Reflecting this progress, 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 nuclear 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 radio morphologies are consistent with more gradual fuelling of the activity via gas accretion at low rates onto the supermassive black holes.

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

We present results from the first twelve months of operation of Radio Galaxy
Zoo, which upon completion will enable visual inspection of over 170,000 radio
sources to determine the host galaxy of the radio emission and the radio
morphology. Radio Galaxy Zoo uses $1.4\,$GHz radio images from both the Faint
Images of the Radio Sky at Twenty Centimeters (FIRST) and the Australia
Telescope Large Area Survey (ATLAS) in combination with mid-infrared images at
$3.4\,\mu$m from the {\it Wide-field Infrared Survey Explorer} (WISE) and at
$3.6\,\mu$m from the {\it Spitzer Space Telescope}. We present the early
analysis of the WISE mid-infrared colours of the host galaxies. For images in
which there is $>\,75\%$ consensus among the Radio Galaxy Zoo
cross-identifications, the project participants are as effective as the science
experts at identifying the host galaxies. The majority of the identified host
galaxies reside in the mid-infrared colour space dominated by elliptical
galaxies, quasi-stellar objects (QSOs), and luminous infrared radio galaxies
(LIRGs). We also find a distinct population of Radio Galaxy Zoo host galaxies
residing in a redder mid-infrared colour space consisting of star-forming
galaxies and/or dust-enhanced non star-forming galaxies consistent with a
scenario of merger-driven active galactic nuclei (AGN) formation. The
completion of the full Radio Galaxy Zoo project will measure the relative
populations of these hosts as a function of radio morphology and power while
providing an avenue for the identification of rare and extreme radio
structures. Currently, we are investigating candidates for radio galaxies with
extreme morphologies, such as giant radio galaxies, late-type host galaxies
with extended radio emission, and hybrid morphology radio sources.

/pdf/radio-galaxy-zoo-host-galaxies-and-radio-morphologies-3kebeiudkg.pdf

Radio Galaxy Zoo : host galaxies and radio morphologies derived from visual inspection

/pdf/giant-radio-galaxies-in-the-lofar-two-metre-sky-survey-i-1q8zs6b89b.pdf

Giant radio galaxies in the LOFAR Two-metre Sky Survey-I

Double-lobe radio galaxies in the local Universe have traditionally been found to be hosted in elliptical or lenticular galaxies. We report the discovery of four spiral-host double-lobe radio galaxies (J0836+0532, J1159+5820, J1352+3126 and J1649+2635) that are discovered by cross-matching a large sample of 187005 spiral galaxies from SDSS DR7 to the full catalogues of FIRST and NVSS. J0836+0532 is reported for the first time. The host galaxies are forming stars at an average rate of 1.7 $-$ 10 M$_{\odot}$ yr$^{-1}$ and possess Super Massive Black Holes (SMBHs) with masses of a few times 10$^{8}$ M$_{\odot}$. Their radio morphologies are similar to FR-II radio galaxies with total projected linear sizes ranging from 86 kpc to 420 kpc, but their total 1.4 GHz radio luminosities are only in the range 10$^{24}$ $-$ 10$^{25}$ W Hz$^{-1}$. We propose that the formation of spiral-host double-lobe radio galaxies can be attributed to more than one factor, such as the occurrence of strong interactions, mergers, and the presence of unusually massive SMBHs, such that the spiral structures are not destroyed. Only one of our sources (J1649+2635) is found in a cluster environment, indicating that processes other than accretion through cooling flows e.g., galaxy-galaxy mergers or interactions could be plausible scenarios for triggering radio-loud AGN activity in spiral galaxies.

Discovery of rare double-lobe radio galaxies hosted in spiral galaxies

We present the first results of a project called SAGAN, which is dedicated solely to the studies of relatively rare megaparsec-scale radio galaxies in the Universe, called giant radio galaxies (GRGs). We have identified 162 new GRGs primarily from the NRAO VLA Sky Survey with sizes ranging from ∼0.71 Mpc to ∼2.82 Mpc in the redshift range of ∼0.03−0.95, of which 23 are hosted by quasars (giant radio quasars). As part of the project SAGAN, we have created a database of all known GRGs, the GRG catalogue, from the literature (including our new sample); it includes 820 sources. For the first time, we present the multi-wavelength properties of the largest sample of GRGs. This provides new insights into their nature. Our results establish that the distributions of the radio spectral index and the black hole mass of GRGs do not differ from the corresponding distributions of normal-sized radio galaxies (RGs). However, GRGs have a lower Eddington ratio than RGs. Using the mid-infrared data, we classified GRGs in terms of their accretion mode: either a high-power radiatively efficient high-excitation state, or a radiatively inefficient low-excitation state. This enabled us to compare key physical properties of their active galactic nuclei, such as the black hole mass, spin, Eddington ratio, jet kinetic power, total radio power, magnetic field, and size. We find that GRGs in high-excitation state statistically have larger sizes, stronger radio power, jet kinetic power, and higher Eddington ratio than those in low-excitation state. Our analysis reveals a strong correlation between the black hole Eddington ratio and the scaled jet kinetic power, which suggests a disc-jet coupling. Our environmental study reveals that ∼10% of all GRGs may reside at the centres of galaxy clusters, in a denser galactic environment, while the majority appears to reside in a sparse environment. The probability of finding the brightest cluster galaxy as a GRG is quite low and even lower for high-mass clusters. We present new results for GRGs that range from black hole mass to large-scale environment properties. We discuss their formation and growth scenarios, highlighting the key physical factors that cause them to reach their gigantic size.

/pdf/search-and-analysis-of-giant-radio-galaxies-with-associated-43o5zjln5s.pdf

Search and analysis of giant radio galaxies with associated nuclei (SAGAN) I. New sample and multi-wavelength studies

The radio galaxy phenomenon is directly connected to mass-accreting, spinning supermassive black holes found in the active galactic nuclei. It is still unclear how the collimated jets of relativistic plasma on hundreds to thousands of kiloparsec scales form and why they are nearly always launched from the nuclei of bulge-dominated elliptical galaxies and not flat spirals. Here we present the discovery of the giant radio source J2345–0449 (z = 0.0755), a clear and extremely rare counterexample where relativistic jets are ejected from a luminous and massive spiral galaxy on a scale of ∼1.6 Mpc, the largest known so far. Extreme physical properties observed for this bulgeless spiral host, such as its high optical and infrared luminosity, large dynamical mass, rapid disk rotation, and episodic jet activity, are possibly the results of its unusual formation history, which has also assembled, via gas accretion from a disk, its central black hole of mass >2 × 10{sup 8} M {sub ☉}. The very high mid-IR luminosity of the galaxy suggests that it is actively forming stars and still building a massive disk. We argue that the launch of these powerful jets is facilitated by an advection-dominated, magnetized accretion flow at a low Eddingtonmore » rate onto this unusually massive (for a bulgeless disk galaxy) and possibly fast spinning central black hole. Therefore, J2345–0449 is an extremely rare, unusual galactic system whose properties challenge the standard paradigms for black hole growth and the formation of relativistic jets in disk galaxies. Thus, it provides fundamental insight into accretion disk-relativistic jet coupling processes.« less

http://iopscience.iop.org/article/10.1088/0004-637X/788/2/174/pdf

Megaparsec relativistic jets launched from an accreting supermassive black hole in an extreme spiral galaxy

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Megaparsec relativistic jets launched from an accreting supermassive black hole in an extreme spiral galaxy