Thanks to the Fermi λ-ray satellite and the current Imaging Atmospheric Cherenkov Telescopes, radio galaxies have arisen as a new class of high- and very-high energy emitters. The favourable orientation of their jets makes radio galaxies extremely relevant in addressing important issues such as: (i) revealing the jet structure complexity; (ii) localising the emitting region(s) of high- and very-high energy radiation; (iii) understanding the physical processes producing these photons. In this review the main results on the λ-ray emission studies of radio galaxies from the MeV to TeV regimes will be presented, and the impact of future Cherenkov Telescope Array observations will be discussed.

We report preliminary results of long-term multi-frequency monitoring observations of S5 0716+714. We conducted observations at 22 and 43 GHz using Korean VLBI Network (KVN) radio telescopes and combined 8 (UMRAO), 15 (OVRO), 95 (CARMA), and 230 GHz (SMA) data to investigate characteristics of radio flares. We identified six flares (P1-P6) from 2010 November to 2014 June. The magnetic field strengths by assuming a synchrotron self-absorption model are in the range of 9 mG - 11 G in P2, 26 mG - 3G in P3, 3 mG - 38 G in P4, and 1 mG - 8 G in P6.

Here we present some preliminary results of our analysis of the combined Chandra observations of the Pictor A radio galaxy. All the available Chandra data for the target, consisting of multiple pointings spanning over 15 years and amounting to the total exposure time of 464 ks, have been included in the analysis. We studied in detail the PSFs of the core region in the individual pointings, as well as the radial profile of the X-ray surface brightness of the source in the combined dataset, in order to discriminate between the radiative output of the unresolved core and the host galaxy. Based on these, we have performed spectral modeling of the active nucleus, constraining its variability.

It is still a mystery why only a small fraction of quasars contain relativistic jets. A strong magnetic field is a necessary ingredient for jet formation. Gas falls from the Bondi radius RB nearly freely to the circularization radius Rc, and a thin accretion disk is formed within Rc We suggest that the external weak magnetic field threading interstellar medium is substantially enhanced in this region, and the magnetic field at Rc can be sufficiently strong to drive outflows from the disk if the angular velocity of the gas is low at RB. In this case, the magnetic field is efficiently dragged in the disk, because most angular momentum of the disk is removed by the outflows that lead to a significantly high radial velocity. The strong magnetic field formed in this way may accelerate jets in the region near the black hole, either by the Blandford-Payne or/and Blandford-Znajek mechanisms. If the angular velocity of the circumnuclear gas is low, the field advection in the thin disk is inefficient, and it will appear as a radio-quiet (RQ) quasar.

The Medicina and Noto radiotelescopes have been employed for over 14 years to monitor the flux density variations of a vast sample of blazars at different radio frequencies. Radio data are essential components of blazar spectral energy distribution (SED, spanning from radio waves to gamma rays), whose trend with luminosity and shape changes provide decisive information on the physics of extra-galactic jets and, eventually, on the mechanism extracting energy from the central black hole in radio-loud AGN. Observations presently carried out at 5, 8 and 24 GHz have taken advantage of the continually evolving control system installed at the antennas. A new, batch-wise analysis tool was also produced, in order to easily handle and reduce the datasets acquired in monthly sessions. We here describe the latest developments and achievements.

It has been controversial for years that the accretion mode is different for bright active galactic nuclei (AGNs) and low-luminosity AGNs (LLAGNs). In this work, we compile from literature a sample of 32 LLAGNs, consisting 18 LINERs and 14 low Eddington ratio (λ) Seyfert galaxies. A strong negative correlation between the radio loudness RUV and the optical to X-ray spectral index αox is reported for the first time. We further demonstrate that this negative correlation can be understood consistently and comprehensively under the truncated accretion — jet model, a model that has been applied successfully to LLAGNs. We argue that the scatter in the observations is mainly due to the spread in the viscosity parameter α of a hot accretion flow, a parameter that can potentially serve as a diagnosis of the strength and/or configuration of magnetic fields in accretion flows.

Black holes at the centers of the galaxies grow mainly by the processes of accretion, mergers, and consumption of stars. In the case of gas accretion with cooling sources, the flow is momentum driven, after which the black hole reaches a saturated mass, and subsequently, it grows only by consumption of stars. In addition, we include the effect of mergers on the growth of black hole spin and mass and study its evolution as a function of redshift in a ΛCDM cosmology using an initial seed mass and spin distribution functions that we have derived. For the stellar ingestion, we have assumed a power-law density profile for the galaxy in our framework of a new relativistic loss cone theory that includes the effect of the black hole spin. We predict the impact of the evolution on the M•−σ relation and compare it with available observations.

Tidal Disruption Events (TDEs) are highly variable high energy phenomena originating from Galactic Nuclei (Komossa & Bade 1999). TDEs are thus powerful tools to study quiescent Galactic Nuclei given their extreme brightness (several times super-Eddington) and the possibility of being seen in non-AGN galaxies. A TDE is the violent disruption of a star passing by a Super Massive Black Hole (SMBH); after the disruption, roughly half of the star mass gains enough energy to escape from the Black Hole, while the other half is bound to the Hole, falls back and eventually accretes onto it. Early works, (Rees 1988), pointed out a t−5/3 behaviour for the light curves of this event and since then such a time dependency became the signature of these events. Strong deviations are however introduced when one considers the internal stellar structure or if one considers partial disruptions. One feature that has never been taken into account is the effect of stellar rotation in the resulting fallback rate, which is the aim of the present work. Firstly, we will show analytical estimates of the impact of stellar rotation on the TDE and we will then present a set of Smoothed Particle Hydrodynamic simulations of the tidal disruption of rotating stars, performed in order to test these analytical estimates.

The Perseus cluster is the X-ray brightest cluster in the sky and with deep Chandra observations we are able to map its central structure on very short spatial scales. In addition, the high quality of X-ray data allows detailed spatially-resolved spectroscopy. In this paper I review what these deep observations have told us about AGN feedback in clusters, sloshing and instabilities, and the metallicity distribution.

Diffuse radio emission from galaxy clusters in the form of radio halos and relics are tracers of the shocks and turbulence in the intra-cluster medium. The imprints of the physical processes that govern their origin and evolution can be found in their radio morphologies and spectra. The role of mildly relativistic population of electrons may be crucial for the acceleration mechanisms to work efficiently. Low frequency observations with telescopes that allow imaging of extended sources over a broad range of low frequencies (<2 GHz) offer the best tools to study these sources. I will review the Giant Metrewave Radio Telescope (GMRT) observations in the past few years that have led to: i) statistical studies of large samples of galaxy clusters, ii) opening of the discovery space in low mass clusters and iii) tracing the spectra of seed relativistic electrons using the Upgraded GMRT.

In our earlier studyDutta (2016a), it has been shown that a number of primordial protostars (the ‘first stars’ in the Universe, also known as Population III or Pop III stars) are being ejected from the cluster of their origin with the velocity exceeding their escape velocity. Hence there is possibility that some of these protostars can enter main sequence and survive till present epoch, even in Milky Way. We ask the question if the protostars can avoid core collapse, and stop accreting before being ejected from the cluster, with the final mass of stars as 0.8 Mȯ.

The hot ISM in early-type galaxies (ETGs) plays a crucial role in understanding their formation and evolution. The structural features of the hot gas identified by Chandra observations point to key evolutionary mechanisms, (e.g., kim12). In our Chandra Galaxy Atlas (CGA) project, taking full advantage of the Chandra capabilities, we systematically analyzed the archival Chandra data of 72 ETGs and produced uniform data products of the hot gas properties. The main data products include spatially resolved 2D spectral maps of the hot gas from individual galaxies. We emphasize that new features can be identified in the spectral maps which are not easily visible in the surface brightness maps. The high-level images can be viewed at the dedicated CGA website, and the CGA data products can be downloaded to compare with other wavelength data and to perform user-specific analyses. Utilizing our data products, we will further address focused science topics.

The growing evidence for energy-conserving outflows in powerful and luminous AGN supports the idea that high-velocity winds launched from the accretion disc evolve systematically after undergoing a shock with the ambient medium and that they are capable to expel enough mass and energy so as to produce feedback. This talk will give an overview of recent results on AGN ultra fast outflows, with focus on grating X-ray spectra of bright sources. I will review how UFO work, their observational properties and their relation with AGN outflows in other bands, what is their impact on the host galaxies and their role in feedback processes.

We analyze the hard X-ray properties of five radio-loud active galactic nuclei with peculiar spectral shape. High-energy exponential cut-offs (HEC) in their hard X-ray 3-500 keV spectra are too high for radio loud AGNs (above 100 keV) or even absent.The probable reason of such visible spectral “peculiarity” can be due to the “jet contamination”, i.e we see some mixture of the jet and nuclear emission, but not pure emission of the innermost nucleus. Here we try to estimate the jet and nuclear components of the spectra for a sample of “peculiar” RL AGNs to find out whether these are real or fake features.

Large-scale, broad outflows are common in active galaxies. In systems where star formation coexists with an AGN, it is unclear yet the role that both play on driving the outflows. In this work we present three-dimensional radiative-cooling MHD simulations of the formation of these outflows, considering the feedback from both the AGN and supernovae-driven winds. We find that a large-opening-angle AGN wind develops fountain structures that make the expanding gas to fallback. Furthermore, it exhausts the gas near the nuclear region, extinguishing star formation and accretion within a few 100.000 yr, which establishes the duty cycle of these outflows. The AGN wind accounts for the highest speed features in the outflow with velocities around 10.000 km s−1 (as observed in UFOs), but these are not as cold and dense as required by observations of molecular outflows. The SNe-driven wind is the main responsible for the observed mass-loading of the outflows.

Very Long Baseline Interferometry (VLBI) at sub-millimeter waves has the potential to image the shadow of the black hole in the Galactic Center, Sagittarius A* (Sgr A*), and thereby test basic predictions of the theory of general relativity. We investigate the imaging prospects of a new Space VLBI mission concept. The setup consists of two satellites in polar or equatorial circular Medium-Earth Orbits with slightly different radii, resulting in a dense spiral-shaped uv-coverage with long baselines, allowing for extremely high-resolution and high-fidelity imaging of radio sources. We simulate observations of a general relativistic magnetohydrodynamics model of Sgr A* for this configuration with noise calculated from model system parameters. After gridding the uv-plane and averaging visibilities accumulated over multiple months of integration, images of Sgr A* with a resolution of up to 4 μ as could be reconstructed, allowing for stronger tests of general relativity and accretion models than with ground-based VLBI.

Observations of low luminosity active galactic nuclei (LLAGNs) and the hard state of black hole X-ray binaries (BHBs) show that the wind exists. Black hole in LLAGNs and hard state of BHBs accretes gas in hot accretion mode. In this paper, we first use magnetohydrodynamic (MHD) simulations of hot accretion flow around a black hole to study the origin of the wind. We find that the wind is driven by the combination of gradients of gas and magnetic pressure and centrifugal forces. Second, we use simulations with focus on the region around Bondi radius to study whether the wind can be generated outside Bondi radius. In the simulation studying hot accretion flow around Bondi radius, in addition to the black hole gravity, we also take into account the gravity of nuclei stars. We find that the wind can not be generated outside Bondi radius. The absence of the wind is due to the change of gravity potential.

We investigate the sample of 16 the youngest radio galaxies with measured kinematic ages and available X-ray data from high-resolution Chandra or XMM-Newton observations. We characterize the accretion properties and derive the jet kinetic luminosities for our sources. We found high accretion rates (>1% Eddington) and very high jet production efficiency for all the sources from our sample.This, along with the fact that the analyzed objects seem over-luminous in radio on the fundamental plane for the black hole activity, implies also that the radiative efficiency of the compact lobes is much higher than in the case of the evolved radio galaxies.

Radio jets are the large-scale and extragalactic footprints of accretion onto supermassive black holes, and are suggested to be the key ingredient controlling the galaxy stellar mass function. Of particular importance is their jet power - the time-averaged energetic feedback into their environment. Hence, the dynamics, energetics and life-cycles of radio-loud AGN (RLAGN) must be understood in order to build a qualitative and quantitative picture of their impact over cosmic time. Here, we present a study of the spectral age of two powerful, cluster-center radio galaxies, and compare with an analytic model to robustly determine their jet powers. We also present some recent LOFAR observations of the different phases of RLAGN activity, namely the remnant and subsequent restarting phases, which are key to understanding the dynamics of RLAGN over their total lifetime.

We analyse archival XMM-Newton observations of the massive galaxy group NGC 4839 falling into the Coma cluster core, which reveal a complex morphology for the merger. By comparing high quality X-ray maps of the merging subcluster with SPH simulations, we propose an infall scenario which qualitatively reproduces the observed structure of the NGC 4839 tail.