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The scanning Fabry-Perot interferometer (FPI) - is the oldest method of optical 3D spectroscopy. It is still in use because of the high spectral resolution it provides over a large field of view. The history of the application of this method for the study of extended ob jects (nebulae and galaxies) and the technique of data reduction and analysis are discussed. The paper focuses on the performing observations with the scanning FPI on the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences (SAO RAS). The instrument is currently used as a part of the SCORPIO-2 multimode focal reducer. The results of studies of various galactic and extragalactic objects with the scanning FPI on the 6-m telescope - star-forming regions and young stellar objects, spiral, ring, dwarf and interacting galaxies, ionization cones of active galactic nuclei, galactic winds, etc. are briefly discussed. Further prospects for research with the scanning FPI of the SAO RAS are discussed., Comment: 25 pages, 8 figures, accepted to Astrophysical Bulletin

We present results of our analysis of spectra of the host galaxies of the candidate Tidal Disruption Events (TDEs) XMMSL1 J111527.3+180638 and PTF09axc to determine the nature of these transients. We subtract the starlight component from the host galaxy spectra to determine the origin of the nuclear emission lines. Using a Baldwin-Phillips-Terlevich (BPT) diagram we conclude that the host galaxy of XMMSL1 J111527.3+180638 is classified as a Seyfert galaxy, suggesting this transient is likely to be caused by (extreme) variability in the active galactic nucleus. We find that the host of PTF09axc falls in the 'star-forming' region of the BPT-diagram, implying that the transient is a strong TDE candidate. For both galaxies we find a WISE-colour difference of $W1-W2, 7 pages, 6 figures, 1 table, 1 appendix (2 additional figures, 1 additional table). Accepted for publication in MNRAS

A distinct visual signature occurs in black holes that are surrounded by optically thin and geometrically thick emission regions. This signature is a sharp-edged dip in brightness that is coincident with the black-hole shadow, which is the projection of the black hole's unstable-photon region on the observer's sky. We highlight two key mechanisms responsible for producing the sharp-edged dip: i) the reduction of intensity observed in rays that intersect the unstable-photon region, and thus the perfectly absorbing event horizon, versus rays that do not (blocking), and ii) the increase of intensity observed in rays that travel along extended, horizon-circling paths near the boundary of the unstable-photon region (path-lengthening). We demonstrate that the black-hole shadow is a distinct phenomenon from the photon ring, and that models exist in which the former may be observed, but not the latter. Additionally, we show that the black-hole shadow and its associated visual signature differ from the more model-dependent brightness depressions associated with thin-disk models, because for geometrically thick and optically thin emission regions, the blocking and path-lengthening effects are quite general. Consequentially, the black-hole shadow is a robust and fairly model-independent observable for accreting black holes that are in the deep sub-Eddington regime, such as low-luminosity active galactic nuclei (LLAGN)., Comment: 15 pages, 12 figures, accepted for publication in ApJ (22 July 2021)

This book is intended to be a course about the creation and evolution of the universe at large, including the basic macroscopic building blocks (galaxies) and the overall large-scale structure. This text covers a broad range of topics for a graduate-level class in a physics department where students' available credit hours for astrophysics classes are limited. The sections cover galactic structure, external galaxies, galaxy clustering, active galaxies, general relativity and cosmology.

We described the design and operation principles of a new tunable-filter photometer developed for the 1-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences and the 2.5-m telescope of the Sternberg Astronomical Institute of the Moscow State University. The instrument is mounted on the scanning Fabry-Perot interferometer operating in the tunable-filter mode in the spectral range of 460-800 nm with a typical spectral resolution of about 1.3 nm. It allows one to create images of galactic and extragalactic nebulae in the emission lines having different excitation conditions and to carry out diagnostics of the gas ionization state. The main steps of observations, data calibration, and reduction are illustrated by examples of different emission-line objects: galactic HII regions, planetary nebulae, active galaxies with extended filaments, starburst galaxies, and Perseus galaxy cluster., Comment: Accepted to Experimental Astronomy, 16 pages

Numerical simulations play an essential role in helping us to understand the physical processes behind relativistic jets in active galactic nuclei. The large number of hydrodynamic codes available today enables a variety of different numerical algorithms to be utilized when conducting the simulations. Since many of the simulations presented in the literature use different combinations of algorithms it is important to quantify the differences in jet evolution that can arise due to the precise numerical schemes used. We conduct a series of simulations using the flash (magneto-)hydrodynamics code in which we vary the Riemann solver and spatial reconstruction schemes to determine their impact on the evolution and dynamics of the jets. For highly refined grids the variation in the simulation results introduced by the different combinations of spatial reconstruction scheme and Riemann solver is typically small. A high level of convergence is found for simulations using third-order spatial reconstruction with the Harten–Lax–Van-Leer with contact and Hybrid Riemann solvers.

The class of Double-Double Radio Galaxies (DDRGs) relates to episodic jet outbursts. How various regions and components add to the total intensity in radio images is less well known. In this paper we synthesize synchrotron images for DDRGs based on special relativistic hydrodynamic simulations, making advanced approximations for the magnetic fields. We study the synchrotron images for: Three different radial jet profiles; Ordered, entangled or mixed magnetic fields; Spectral ageing from synchrotron cooling; The contribution from different jet components; The viewing angle and Doppler (de-)boosting; The various epochs of the evolution of the DDRG. To link our results to observational data, we adopt to J1835+6204 as a reference source. In all cases the synthesized synchrotron images show two clear pairs of hotspots, in the inner and outer lobes. The best resemblance is obtained for the piecewise isochoric jet model, for a viewing angle of approximately $\vartheta \sim -71^{\circ}$, i.e. inclined with the lower jet towards the observer, with predominantly entangled ($\gtrsim 70$ per cent of the magnetic pressure) in turbulent, rather than ordered fields. The effects of spectral ageing become significant when the ratio of observation frequencies and cut-off frequency $\nu_{\rm obs}/\nu_{\infty,0} \gtrsim 10^{-3}$, corresponding to $\sim 3 \cdot 10^2$ MHz. For viewing angles $\vartheta \lesssim -30^{\circ}$, a DDRG morphology can no longer be recognized. The second jets must be injected within $\lesssim$ 4 per cent of the lifetime of the first jets for a DDRG structure to emerge, which is relevant for Active Galactic Nuclei feedback constraints., Comment: 24 pages, 8 figures

Tidal Disruption Events (TDEs) are characterized by the emission of a short burst of high-energy radiation. We analyze the cumulative impact of TDEs on galactic habitability using the Milky Way as a proxy. We show that X-rays and extreme ultraviolet (XUV) radiation emitted during TDEs can cause hydrodynamic escape and instigate biological damage. By taking the appropriate variables into consideration, such as the efficiency of atmospheric escape and distance from the Galactic center, we demonstrate that the impact of TDEs on galactic habitability is comparable to that of Active Galactic Nuclei. In particular, we show that planets within distances of $\sim 0.1$-$1$ kpc could lose Earth-like atmospheres over the age of the Earth, and that some of them might be subject to biological damage once every $\gtrsim 10^4$ yrs. We conclude by highlighting potential ramifications of TDEs and argue that they should be factored into future analyses of inner galactic habitability., Comment: MNRAS, in press

We present very long baseline interferometry (VLBI) images and kinematics of water maser emission in three active galaxies: NGC 5728, Mrk 1, and IRAS 08452–0011. IRAS 08452–0011, at a distance of ∼200 Mpc, is a triple-peaked H2O megamaser, consistent with a Keplerian rotating disc, indicating a black hole mass of (3.3$\pm 0.2)\times 10^{7}\, \mathrm{ M}_{\odot }$. NGC 5728 and Mrk 1 display double-peaked spectra, and VLBI imaging reveals complicated gas kinematics that do not allow for a robust determination of black hole mass. We show evidence that the masers in NGC 5728 are in a wind while the Mrk 1 maser system has both disc and outflow components. We also find that disturbed morphology and kinematics are a ubiquitous feature of all double-peaked maser systems, implying that these maser sources may reside in environments where active galactic nucleus (AGN) winds are prominent at ∼1 pc scale and have significant impact on the masing gas. Such AGNs tend to have black hole masses $M_{\rm BH}\, \lt$ 8 × 106 M⊙ and Eddington ratios $\lambda _{\rm Edd}\, \gtrsim$ 0.1, while the triple-peaked megamasers show an opposite trend.

We describe ongoing searches for intermediate-mass black holes with M_BH ~ 100-10^5 M_sun. We review a range of search mechanisms, both dynamical and those that rely on accretion signatures. We find that dynamical and accretion signatures alike point to a high fraction of 10^9-10^10 M_sun galaxies hosting black holes with M_BH, Invited review in Annual Reviews of Astronomy & Astrophysics; an edited final version is to published in 2020; this version corrects an error in Table 1

The radio galaxy 3C 75 is remarkable because it contains a pair of radio-loud active galaxies, each of which produces a two-sided jet, with the jet beams appearing to collide and merge to the west of the galaxies. Motivated by 3C 75, we have conducted three-dimensional hydrodynamic simulations of jet collisions. We have extended previous studies by modelling the physical properties of the cluster atmosphere, including an external wind, and using realistic jet powers obtained from observational data. We are able to produce a morphology similar to that of 3C 75. The simulations imply that direct contact between the bulk jet flows on the west of the source is required to produce a morphology consistent with 3C 75. We quantify how the merging jets decelerate, how the wind deflects the jets and cocoons, the entrainment of intra-cluster material into the cocoons, the cocoon energetics, and how the jet interactions generate enstrophy. By comparing simulations of pairs of two-sided jets with those of single two-sided sources, we determine how the interaction between two bipolar jets changes their evolution. The unprecedented sensitivity and angular resolution of upcoming observatories will lead to the detection of many more complex sources at high redshift, where interacting jets are expected to be more numerous. The morphology of these complex sources can provide significant insight into the conditions in their environments.

During a deep extragalactic Lockman Hole sky survey with an area of 18.5 sq. deg, which was conducted when the SRG observatory was flying to the Lagrange point L2, the eROSITA telescope detected $${\sim}7000$$ X-ray sources. These objects were then provisionally identified and classified using the publicly accessible data of optical and infrared sky surveys by the SRGz machine learning system developed for this purpose at the Space Research Institute of the Russian Academy of Sciences. As a result, a number of new candidates for distant quasars ( $$z\sim 4$$ ) have been selected. The spectroscopic observations of the first two candidates from this list carried out with the 1.6-m AZT-33IK telescope of the Sayan Solar Observatory have confirmed that these objects are actually distant quasars at redshifts 3.878 and 4.116 and are characterized by a high X-ray luminosity $${\sim}10^{45}$$ erg s $${}^{-1}$$ (2–10 keV). The results obtained allow one to count on the detection of a large number of distant quasars during a four-year all-sky survey of the SRG observatory begun in December 2019.

The discussion on the values of redshift, as well as blueshift, is based on a large increase in new evidence that in the whole volume of Universe there are gravitationally-bound objects (galaxies, clusters and super clusters of galaxies) „Using the Chandra and Hubble Space Telescopes we have now observed 72 collisions between galaxy clusters, including both ‘major’ and ‘minor’ mergers” (Harvey, 2015). That adds to a great diversity of galactic movement directions and their diverse appearances to an observer. The accent here is at the point of "clearing the early Universe" and asks questions about how these types of radiation could be measured if all galaxies were created in the early stages of Universe, which had started emitting these types of radiation. At the same time, some questions are asked about. With a redshift of 5.47, (Q0906 + 6930) light from this active galaxy is estimated to have taken around 12.3 billion light-years to reach us.. distance to this galaxy is estimated to be around 26 billion light-years (7961 Mpc). (NASA/IPAC, 2010) i.e., why the actual measured values of redshift are not applied.

We present the results of a multi-wavelength follow up campaign for the luminous nuclear transient Gaia16aax, which was first identified in January 2016. The transient is spatially consistent with the nucleus of an active galaxy at z=0.25, hosting a black hole of mass $\rm \sim6\times10^8M_\odot$. The nucleus brightened by more than 1 magnitude in the Gaia G-band over a timescale of less than one year, before fading back to its pre-outburst state over the following three years. The optical spectra of the source show broad Balmer lines similar to the ones present in a pre-outburst spectrum. During the outburst, the $\rm H\alpha$ and $\rm H\beta$ emission lines develop a secondary peak. We also report on the discovery of two transients with similar light curve evolution and spectra: Gaia16aka and Gaia16ajq. We consider possible scenarios to explain the observed outbursts. We exclude that the transient event could be caused by a microlensing event, variable dust absorption or a tidal encounter between a neutron star and a stellar mass black hole in the accretion disk. We consider variability in the accretion flow in the inner part of the disk, or a tidal disruption event of a star $\geq 1 M_{\odot}$ by a rapidly spinning supermassive black hole as the most plausible scenarios. We note that the similarity between the light curves of the three Gaia transients may be a function of the Gaia alerts selection criteria., Comment: 21 pages, 17 figure - accepted for publication in MNRAS main journal

The history of Very Long Baseline Interferometry (VLBI) observations has been characterized predominantly by an ongoing quest for increasingly high resolution and sensitivity. VLBI monitoring of relatively large samples of Active Galactic Nuclei (AGNs) with uniform quality and linear polarization sensitivity are now available at the moderately high frequencies of 15 and 43 GHz. This has enabled considerable advances in our understanding of the relativistic jets of AGNs, but gaps in the available observational material remain, which must be addressed in future VLBI polarization observations. Linear polarization observations at frequencies above 43 GHz remain non-routine, and the availability of multi-frequency and circular polarization measurements is still limited. It is of interest both to push inward toward the jet base and to study details of the jets themselves on parsec scales, such as magnetic field structures along and across the jets, which are intrinsically related to their formation, launching and propagation. Requirements for future VLBI polarization observations are considered, highlighting the key role that can be played by space VLBI observations. Unique opportunities are offered by relatively low-frequency space VLBI observations that are sensitive to effects such as Faraday rotation, opacity, and low-frequency absorption.

International audience; The Cherenkov Telescope Array (CTA) observatory is the next generation of ground-based imaging atmospheric Cherenkov telescopes (IACTs). Building on the strengths of current IACTs, CTA is designed to achieve an order of magnitude improvement in sensitivity, with unprecedented angular and energy resolution. CTA will also increase the energy reach of IACTs, observing photons in the energy range from 20 GeV to beyond 100 TeV. These advances in performance will see CTA heralding in a new era for high-energy astrophysics, with the emphasis shifting from source discovery, to population studies and precision measurements. In this talk we discuss CTA’s ability to conduct source population studies of $\gamma$-ray bright active galactic nuclei and how this ability will enhance our understanding on the redshift evolution of this dominant $\gamma$-ray source class.

Feedback from Active Galactic Nucleus (AGN) may play a critical role in the coevolution of galaxies and supermassive black holes (SMBH). Specifically, AGN feedback may quench star formation, suppress baryon-to-dark matter mass ratio, shape galaxy morphology, impact the circumgalactic (CGM)/intergalactic medium (IGM), and regulate SMBH accretion. One of the most important forms of AGN feedback is powerful, quasar/AGN-driven outflows. However, the physical details of these outflows, including their driving mechanism and spatial extent, are still not well constrained. In addition, while these outflows are believed to be effective in massive galaxies, their impact on dwarf galaxies (M⋆ ~10^9.5 M⊙) remains largely unknown. To answer these open questions, my thesis focuses on AGN feedback via quasar/AGN-driven outflows in both ultraluminous infrared galaxies (ULIRGs) and dwarf galaxies with four projects.In the first half of my thesis, I examine the outflows in nearby ULIRGs with two objectives: In Chapter 2, I present a dedicated investigation of the highly ionized, likely pc-scale quasar/AGN winds in a sample of 21 nearby ULIRGs through HST/COS far-ultraviolet (FUV) spectroscopy. Blueshifted Lyα emission is prevalent in the sample, which is probably closely related to the outflowing gas and AGN activity in these objects. Additionally, the Lyα escape fractions tend to be slightly larger in objects with stronger AGN and larger outflow velocities. Highly ionized O VI and N V outflows are detected in a coherently selected, AGN-dominated ULIRG sample for the first time. Together with the results from a matched quasar sample, these outflows show higher incidence rates and larger EW and velocities in X-ray weak sources and sources with high X-ray absorbing column densities, implying that these outflows are radiatively-driven; In Chapter 3, I describe a deep, Chandra imaging spectroscopy study of the nearby ULIRG Mrk 273. The data have revealed a ∼40 kpc×40 kpc X-ray nebula, which is relatively hot and has a super-solar α/Fe abundance ratio. This nebula is most likely heated and metal-enriched by outflows over time. Additionally, the existence of a dual AGN is strongly suggested by the data, and extended 1–3 keV emission are detected, likely related to the AGN photoionized gas and/or outflowing gas. In the second half of my thesis, I turn to look at the AGN-driven outflows in dwarf galaxies: In Chapter 4, I report the results from a dedicated optical integral field spectroscopic study of a sample of eight dwarf galaxies with known AGN and suspected outflows. Fast, kpc-scale outflows are detected in seven of them. The outflows show 50-percentile (median) velocity of up to ∼240 km s^−1 and 80-percentile line width reaching ∼1200 km s^−1, in clear contrast with the more quiescent kinematics of the host gas and stellar components. The kinematics and energetics of these outflows suggest that they are primarily driven by the AGN. A small but non-negligible portion of the outflowing material likely escapes the main body of the host galaxy and contributes to the enrichment of the circumgalactic medium. The impact of these outflows on their dwarf host galaxies is similar to those taking place in the more luminous AGN with massive hosts in the low-redshift universe. In Chapter 5, I discuss the results from a pilot HST/COS spectroscopy program to examine three objects studied in Chapter 4. Blueshifted absorption features tracing fast outflows are detected in two of the three objects. For object J0954+47, the outflow is detected in multiple ions and is much faster than those in star-forming galaxies with similar star formation rates. The outflow velocity exceeds the escape velocity of this system, suggesting that a large fraction of the outflowing gas may escape. The outflow carries significant amount of mass, momentum and kinetic energy, which may transport material out of the galaxy more efficiently than the gas consumption by star formation. The ratio of kinetic energy outflow rate to AGN luminosity of this outflow is at least comparable to the expectation from simulations of AGN feedback.Finally, in Chapter 6, I summarize the main results of the whole thesis, and briefly highlight several future works that may lead to a more comprehensive understanding of AGN feedback in ULIRGs and dwarf galaxies.

Dying radio galaxies represent a stage of the evolution of active galactic nuclei (AGN), during which the accreting central black hole has switched off and/or falls to such a low level that the plasma outflow can no longer be sustained. When this happens, the radio source undergoes a period of fading, the dying phase, before it disappears completely. We present the study of three potential dying radio sources using the MeerKAT radio telescope: MKT J072851.2-752743, MKT J001940.4-654722, and ACO 548B. The identification as dying radio sources came from the MeerKAT Galaxy Cluster Legacy Survey (MGCLS). We carry out a multi-wavelength analysis of the sources and derive their energetics. The ages of the sources are ∼30–70 Myr, they have magnetic fields of the order of a few μG, and they have relatively low radio power. Their potential optical counterparts are associated with massive galaxies. We show that ACO 548B, previously classified as two peripheral relic radio sources, is a dying radio galaxy. With its good sensitivity and resolution, MeerKAT is an ideal instrument to detect potential dying radio sources, and contribute to the understanding of the evolution of AGN population.

We review the properties of the established Scaling Relations (SRs) of galaxies and active galactic nuclei (AGN), focusing on their origin and expected evolution back in time, providing a short history of the most important progresses obtained up to now and discussing the possible future studies. We also try to connect the observed SRs with the physical mechanisms behind them, examining to what extent current models reproduce the observational data. The emerging picture clarifies the complexity intrinsic to the galaxy formation and evolution process as well as the basic uncertainties still affecting our knowledge of the AGN phenomenon. At the same time, however, it suggests that the detailed analysis of the SRs can profitably contribute to our understanding of galaxies and AGN., Comment: 116, 15 figures

We present a progress report of a study of FR I and FR II radio galaxies. Several new morphological features in the radio emission are now revealed using the high (μJy) sensitivity reached in the range 550–1712 MHz, more than a factor of three, at the high (∼4″−7″) angular resolution with the upgraded Giant Metrewave Radio Telescope (uGMRT) and MeerKAT. Therefore, the aim of this study is to understand if we need to revise our current classification scheme for classical radio galaxies. In order to address our goals, we have carefully constructed a sample of 14 (6 FR I, 6 FR II and 2 FR 0) radio galaxies. The uGMRT and MeerKAT images of our four target sources revealed a wealth of morphological details, e.g., filamentary structure in the emission from the lobes, misalignments, radio emission beyond the hot-spots in three sources, etc., see Fanaroff et al. (2021). Here, we present preliminary results for two more radio galaxies from our sample using uGMRT, in the light of the local environment. Finally, we are awaiting uGMRT and MeerKAT observations of remaining sample sources. Our results show that for the radio galaxies in this study, the morphological classification scheme for the classical FR I/FR II radio galaxies still holds, even with the improved imaging capabilities of the uGMRT and MeerKAT. Furthermore, we need to be cautious when using automated procedures for classification schemes, e.g., in surveys (with poorer sensitivities and angular resolutions) because of the rich morphological details that are shown in our uGMRT and MeerKAT images.

The Event Horizon Telescope (EHT) collaboration, an Earth-size sub-millimetre radio interferometer, recently captured the first images of the central supermassive black hole in M87. These images were interpreted as gravitationally-lensed synchrotron emission from hot plasma orbiting around the black hole. In the accretion flows around low-luminosity active galactic nuclei such as M87, electrons and ions are not in thermal equilibrium. Therefore, the electron temperature, which is important for the thermal synchrotron radiation at EHT frequencies of 230 GHz, is not independently determined. In this work, we investigate the commonly used parameterised ion-to-electron temperature ratio prescription, the so-called R-$\beta$ model, considering images at 230 GHz by comparing with electron-heating prescriptions obtained from general-relativistic magnetohydrodynamical (GRMHD) simulations of magnetised accretion flows in a Magnetically Arrested Disc (MAD) regime with different recipes for the electron thermodynamics. When comparing images at 230 GHz, we find a very good match between images produced with the R-$\beta$ prescription and those produced with the turbulent- and magnetic reconnection- heating prescriptions. Indeed, this match is on average even better than that obtained when comparing the set of images built with the R-$\beta$ prescription with either a randomly chosen image or with a time-averaged one. From this comparative study of different physical aspects, which include the image, visibilities, broadband spectra, and light curves, we conclude that, within the context of images at 230 GHz relative to MAD accretion flows around supermassive black holes, the commonly-used and simple R-$\beta$ model is able to reproduce well the various and more complex electron-heating prescriptions considered here., Comment: 18 pages, 22 figures, accepted for publication in MNRAS

In the last two decades, very-high-energy gamma-ray astronomy has reached maturity: over 200 sources have been detected, both Galactic and extragalactic, by ground-based experiments. At present, Active Galactic Nuclei (AGN) make up about 40% of the more than 200 sources detected at very high energies with ground-based telescopes, the majority of which are blazars, i.e. their jets are closely aligned with the line of sight to Earth and three quarters of which are classified as high-frequency peaked BL Lac objects. One challenge to studies of the cosmological evolution of BL Lacs is the difficulty of obtaining redshifts from their nearly featureless, continuum- dominated spectra. It is expected that a significant fraction of the AGN to be detected with the future Cherenkov Telescope Array (CTA) observatory will have no spectroscopic redshifts, compromising the reliability of BL Lac population studies, particularly of their cosmic evolution. We started an effort in 2019 to measure the redshifts of a large fraction of the AGN that are likely to be detected with CTA, using the Southern African Large Telescope (SALT). In this contribution, we present two results from an on-going SALT program focused on the determination of BL Lac object redshifts that will be relevant for the CTA observatory., 15 pages, 4 figures, 37th International Cosmic Ray Conference

We analyse optical datacubes of the inner kiloparsec of 30 local ($z\le0.02$) active galactic nuclei (AGN) hosts that our research group, AGNIFS, has collected over the past decade via observations with the integral field units of the Gemini Multi-Object Spectrographs. Spatial resolutions range between $50~{\rm pc}$ and $300~{\rm pc}$ and spectral coverage is from $4800~\mathring{A}$ or $5600~\mathring{A}$ to $7000~\mathring{A}$, at velocity resolutions of $\approx 50~{\rm km~s^{-1}}$. We derive maps of the gas excitation and kinematics, determine the AGN ionisation axis -- which has random orientation relative to the galaxy, and the kinematic major axes of the emitting gas. We find that rotation dominates the gas kinematics in most cases, but is disturbed by the presence of inflows and outflows. Outflows have been found in 21 nuclei, usually along the ionisation axis. The gas velocity dispersion is traced by $W_{80}$ (velocity width encompassing 80 per cent of the line flux), adopted as a tracer of outflows. In 7 sources $W_{80}$ is enhanced perpendicularly to the ionisation axis, indicating lateral expansion of the outflow. We have estimated mass-outflow rates $\dot{M}$ and powers $\dot{E}$, finding median values of $\log\,[\dot{M}/({\rm\,M_\odot\,yr^{-1}})]=-2.1_{-1.0}^{+1.6}$ and $\log\,[\dot{E}/({\rm\,erg\,s^{-1}})]=38.5_{-0.9}^{+1.8}$, respectively. Both quantities show a mild correlation with the AGN luminosity ($L_{\rm AGN}$). $\dot{E}$ is of the order of 0.01 $L_{\rm AGN}$ for 4 sources, but much lower for the majority (9) of the sources, with a median value of $\log\,[\dot{E}/L_{\rm AGN}]=-5.34_{-0.9}^{+3.2}$ indicating that typical outflows in the local Universe are unlikely to significantly impact their host galaxy evolution.

Probing the ultra-high energy cosmic ray (UHECR) spectrum beyond the cutoff at ~40 EeV requires an observatory with large acceptance, which is challenging to implement with ground arrays. We present a concept for radio detection of UHECRs impacting the Moon's regolith from low-lunar orbit called the Zettavolt Askaryan Polarimeter (ZAP). ZAP would observe several thousands of events above the cutoff (~40 EeV) with a full-sky field of view to test whether UHECRs originate from Starburst Galaxies, Active Galactic Nuclei, or other sources associated with the matter distribution of the local universe at a distance $>$ 1 Mpc. The unprecedented sensitivity of ZAP to energies beyond 100 EeV would enable a test of source acceleration mechanisms. At higher energies, ZAP would produce the most stringent limits on super heavy dark matter (SHDM) via limits on neutrinos and gamma rays resulting from self-annihilation or decay.

The nearby Magellanic Clouds system covers more than 200 square degrees on the sky. Much of it has been mapped across the electromagnetic spectrum at high angular resolution and sensitivity X-ray (XMM-Newton), UV (UVIT), optical (SMASH), IR (VISTA, WISE, Spitzer, Herschel), radio (ATCA, ASKAP, MeerKAT). This provides us with an excellent dataset to explore the galaxy populations behind the stellar-rich Magellanic Clouds. We seek to identify and characterise AGN via machine learning algorithms on this exquisite data set. Our project focuses not on establishing sequences and distributions of common types of galaxies and active galactic nuclei (AGN), but seeks to identify extreme examples, building on the recent accidental discoveries of unique AGN behind the Magellanic Clouds., Proceedings paper of the IAU symposium "Nuclear Activity in Galaxies Across Cosmic Time" (Ethiopia) accepted to be published under the Cambridge University Press, eds. M. Povic, P. Marziani, J. Masegosa, H. Netzer, S. H. Negu, and S. B. Tessema

We review the main features and scientific results obtained during the first 10 years of the AGILE space Mission, a scientific program of the Italian Space Agency (ASI) focused on high-energy astrophysics. The Mission was proposed in 1997 for the ASI “Small Mission” Program and the instrument has been developed with the programmatic and technical support of INAF and INFN. The AGILE satellite was launched on April 23, 2007, from the Indian base in Sriharikota. We describe first the characteristics of the instrument and of the satellite. AGILE is a very innovative space Mission for its class, and is capable of reaching an optimal scientific performance for the study of astrophysical sources with detectors operating in the energy ranges 20–60 keV, 400 keV–100 MeV, 50 MeV–30 GeV. We review the main scientific achievements of the Mission with contributions to the study of the most energetic Galactic and extragalactic sources including accreting compact objects (neutron stars and black holes), pulsars, Supernova Remnants, Active Galactic Nuclei (AGN). AGILE made several important discoveries: a novel mechanism of particle acceleration operating in the Crab Nebula, the emission of transient gamma rays at the moment of launching of relativistic jets from Galactic compact sources, the first evidence of hadronic cosmic ray production from Galactic sources, the emission of very powerful gamma-ray flares from accreting super-massive black holes in AGN, sudden spectral transitions in gamma-ray bursts. Furthermore, AGILE is now providing unique contributions for the study of gravitational wave source counterparts and cosmic neutrino emitters. Also terrestrial applications are relevant for the science program: AGILE discovered gamma-ray emission above 20 MeV from Terrestrial gamma-ray flashes produced by powerful lighting and thunderstorms. This discovery has an important impact for environmental and atmospheric studies. In addition, particle resonance effects and time-dependent precipitations in the lower Earth magnetosphere can by addressed by AGILE studying the charged particles detected by the instrument. The AGILE satellite is operating nominally, and continues to observe the sky searching for the most energetic phenomena of our Universe.

AGILE has been coordinating multi-wavelength campaigns on active galactic nuclei with several major observing facilities since its launch in 2007. This effort allowed us to investigate some remarkable sources both on short and on long time-scales, deriving information on the physical mechanisms responsible for the emission in different energy bands. A complete review of the whole set of AGILE results on extra-galactic sources and their theoretical interpretation is well beyond the scope of this paper, therefore I will present an overview on an handful of outstanding objects and describe the most recent observations.

Active Galactic Nuclei are the dominant sources of gamma rays outside our Galaxy and also candidates for being the source of ultra-high energy cosmic rays. In addition to being emitters of broad-band non-thermal radiation throughout the electromagnetic spectrum, their emission is highly variable on timescales from years to minutes. Hence, high-cadence monitoring observations are needed to understand their emission mechanisms. The Africa Millimetre Telescope is planned to be the first mm-wave radio telescope on the African continent and one of few in the Southern hemisphere. Further to contributing to the global mm-VLBI observations with the Event Horizon Telescope, substantial amounts of observation time will be available for monitoring observations of Active Galactic Nuclei. Here we review the scientific scope of the Africa Millimetre Telescope for monitoring of Active Galactic Nuclei at mm-wavelengths., Comment: Accepted for publication in Galaxies, Special Issue "Monitoring the Non-Thermal Universe": https://www.mdpi.com/journal/galaxies/special_issues/non-thermalUniverse

Broad Emission Lines are the most characteristic features of Active Galaxies, but the mechanism of creating a medium able to emit these intense lines is not quite clear. Observations clearly indicate that the motion of the material is predominantly Keplerian, with traces of inflow and clear signatures of outflow, but this still does not point out whether the lines partially come from the disk surface, or exclusively from the circumnuclear material, and whether this material originates from the disk as a wind, or comes, at least partially, from outside. I review the basic scenarios for the formation of the Broad Line Region (BLR), and the recent progress in modelling the physical conditions in the emitting medium. The current state is the the outer radius of the BLR is fixed by the dust sublimation temperature in the medium exposed to the irradiation from the central source, the inner radius is likely fixed by the dust sublimation temperature in the atmosphere of the non-illuminated accretion disk, and the local cloud density is a universal number fixed by the radiation pressure confinement. The time-dependent aspects of the BLR formation, however, still wait for serious modelling effort matching the quality of the observational data., Comment: Open Astronomy, in press. Proceedings of the conference 12th Serbian Conference on Spectral Line Shapes in Astrophysics, Vrdnik, Serbia, June 3-7, 2019

Due to the compactness active galactic nuclei (AGNs) are still unresolved with optical observations. However, structure and physical conditions of the matter in their central parts are especially important to study the processes of the matter accretion to supermassive black holes and eventually these investigations are essential to understand the galaxy evolution. Polarization contains information about the interaction of electromagnetic waves with the environment and provides information about the physical processes in the central regions of the AGNs that could not be found with the help of other optical observations. In this paper, the importance of applying polarimetry methods to the study of geometry, kinematics, and physical processes in active galactic nuclei (AGN) in polarized light is discussed. An overview of the mechanisms of polarization formation, their connection with different structures and scales are provided. Also, we overview the polarimetric investigations based on different assumptions that are done using the observations conducted in Special Astrophysical Observatory of the Russian Academy of Sciences., Proceeding to the XII SCSLSA; published in Open Astronomy

Worldwide very long baseline radio interferometry arrays are expected to obtain horizon-scale images of supermassive black hole candidates as well as of relativistic jets in several nearby active galactic nuclei. This motivates the development of models for magnetohydrodynamic flows in strong gravitational fields. The Black Hole Accretion Code (BHAC) intends to aid with the modelling of such sources by means of general relativistic magnetohydrodynamical (GRMHD) simulations in arbitrary stationary spacetimes. New additions were required to guarantee an accurate evolution of the magnetic field when small and large scales are captured simultaneously. We discuss the adaptive mesh refinement (AMR) techniques employed in BHAC, essential to keep several problems computationally tractable, as well as staggered-mesh-based constrained transport (CT) algorithms to preserve the divergence-free constraint of the magnetic field, including a general class of prolongation operators for face-allocated variables compatible with them. Through several standard tests, we show that the choice of divergence-control method can produce qualitative differences in simulations of scientifically relevant accretion problems. We demonstrate the ability of AMR to reduce the computational costs of accretion simulations while sufficiently resolving turbulence from the magnetorotational instability. In particular, we describe a simulation of an accreting Kerr black hole in Cartesian coordinates using AMR to follow the propagation of a relativistic jet while self-consistently including the jet engine, a problem set up-for which the new AMR implementation is particularly advantageous. The CT methods and AMR strategies discussed here are being employed in the simulations performed with BHAC used in the generation of theoretical models for the Event Horizon Telescope Collaboration., Comment: 21 pages, 16 figures, accepted for publication in Astronomy & Astrophysics. Link to www.bhac.science will soon be active

Active galactic nuclei come in many varieties. A minority of them are radio-loud, and exhibit two opposite prominent plasma jets extending from the proximity of the supermassive black hole up to megaparsec distances. When one of the relativistic jets is oriented closely to the line of sight, its emission is Doppler beamed and these objects show extreme variability properties at all wavelengths. These are called "blazars". The unpredictable blazar variability, occurring on a continuous range of time-scales, from minutes to years, is most effectively investigated in a multi-wavelength context. Ground-based and space observations together contribute to give us a comprehensive picture of the blazar emission properties from the radio to the gamma-ray band. Moreover, in recent years, a lot of effort has been devoted to the observation and analysis of the blazar polarimetric radio and optical behaviour, showing strong variability of both the polarisation degree and angle. The Whole Earth Blazar Telescope (WEBT) Collaboration, involving many tens of astronomers all around the globe, has been monitoring several blazars since 1997. The results of the corresponding data analysis have contributed to the understanding of the blazar phenomenon, particularly stressing the viability of a geometrical interpretation of the blazar variability. We review here the most significant polarimetric results achieved in the WEBT studies., Comment: Review published in "Galaxies" as part of the Special Issue "Polarimetry as a Probe of Magnetic Fields in AGN Jets", Academic Editors: Margo Aller, Jose L. G\'omez and Eric Perlman

Adopting three physically-motivated scales (micro - meso - macro, which refer to mpc - kpc - Mpc, respectively) is paramount for achieving a unified theory of multiphase active galactic nuclei feeding and feedback, and it represents a keystone for astrophysical simulations and observations in the upcoming years. In order to promote this multi-scale idea, we have decided to adopt an interdisciplinary approach, exploring the possible conceptual similarities between supermassive black hole feeding and feedback cycles and the dynamics occurring in human cancer microenvironment., Comment: Multidisciplinary article on Frontiers of Oncology (published on May 11th 2021)

To explain X-ray spectra of active galactic nuclei (AGN), non-thermal activity in AGN coronae such as pair cascade models has been extensively discussed in the past literature. Although X-ray and gamma-ray observations in the 1990s disfavored such pair cascade models, recent millimeter-wave observations of nearby Seyferts establish the existence of weak non-thermal coronal activity. Besides, the IceCube collaboration reported NGC 1068, a nearby Seyfert, as the hottest spot in their 10-yr survey. These pieces of evidence are enough to investigate the non-thermal perspective of AGN coronae in depth again. This article summarizes our current observational understandings of AGN coronae and describes how AGN coronae generate high-energy particles. We also provide ways to test the AGN corona model with radio, X-ray, MeV gamma-ray, and high-energy neutrino observations., Comment: Invited review in Galaxies special issue "Searching for Astrophysical Sources of Cosmic Rays, Gamma-Rays and Neutrinos: Real-Time Multimessenger Programs and Theoretical Models". This paper is based on our previous papers arXiv:1904.00554 & 1909.02239

The field of ground-based γ-ray astronomy has made very significant advances over the last three decades with the extremely successful operations of several atmospheric Cherenkov telescopes worldwide. The advent of the imaging Cherenkov technique for indirect detection of cosmic γ rays has immensely contributed to this field with the discovery of more than 220 γ-ray sources in the Universe. This has greatly improved our understanding of the various astrophysical processes involved in the non-thermal emission at energies above 100 GeV. In this paper, we summarize the important results achieved by the Indian γ-ray astronomers from the GeV-TeV observations using imaging Cherenkov telescopes over the last two decades. We mainly emphasize the results obtained from the observations of active galactic nuclei with the TACTIC (TeV Atmospheric Cherenkov Telescope with Imaging Camera) telescope, which has been operational since 1997 at Mount Abu, India. We also discuss the future plans of the Indian γ-ray astronomy program with special focus on the scientific objectives of the recently installed 21 m diameter MACE (Major Atmospheric Cherenkov Experiment) telescope at Hanle, India.

We present results of spectroscopic monitoring observations of the Ultra-Luminous Infra Red Galaxy F01004-2237. This galaxy was observed to undergo changes in its optical spectrum, detected by comparing a spectrum from 2015 with one from 2000. These changes were coincident with photometric brightening. The main changes detected in the optical spectrum are enhanced He II $\lambda$4686 emission and the appearance of He I $\lambda$3898,$\lambda$5876 emission lines. The favoured interpretation of these changes was that of a tidal disruption event (TDE) happening in 2010. However, subsequent work suggested that these changes are caused by another hitherto unknown reason related to variations in the accretion rate in the active galactic nucleus (AGN). Our optical spectroscopic monitoring observations show that the evolution of the He lines is in line with the evolution seen in TDEs and opposite of what observed from reverberation mapping studies of AGNs, renewing the discussion on the interpretation of the flare as a TDE., Comment: 9 pages, 3 figures, 2 tables. Accepted for publication on ApJ on January 22, 2021

We present the results of a pilot Hubble Space Telescope (HST) imaging study of the host galaxies of ten quasars from the Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project. Probing more than an order of magnitude in BH and stellar masses, our sample is the first statistical sample to study the BH-host correlations beyond z>0.3 with reliable BH masses from reverberation mapping rather than from single-epoch spectroscopy. We perform image decomposition in two HST bands (UVIS-F606W and IR-F110W) to measure host colors and estimate stellar masses using empirical relations between broad-band colors and the mass-to-light ratio. The stellar masses of our targets are mostly dominated by a bulge component. The BH masses and stellar masses of our sample broadly follow the same correlations found for local RM AGN and quiescent bulge-dominant galaxies, with no strong evidence of evolution in the M_BH-M_*,bulge relation to z~0.6. We further compare the host light fraction from HST imaging decomposition to that estimated from spectral decomposition. We found a good correlation between the host fractions derived with both methods. However, the host fraction derived from spectral decomposition is systematically smaller than that from imaging decomposition by ~30%, indicating different systematics in both approaches. This study paves the way for upcoming more ambitious host galaxy studies of quasars with direct RM-based BH masses at high redshift., Comment: 19 pages, 8 figures, submitted to ApJ

The linear polarization of non-thermal emission encodes information about the structure of the magnetic fields, either from the region where the emission is produced (i.e., the intrinsic polarization angle) and/or from the screens of magnetized plasma that may be located on its way towards Earth (i.e., the effect of Faraday rotation). In addition, the variability timescale of the polarized emission, or its Faraday rotation, can be used to estimate the size of the region where the emission (or the Faraday rotation) originates. The observation of polarized emission from active galactic nuclei (AGN) and, in particular, its time evolution, also provides information about the critical role that magnetic fields may play in the process of jet launching and propagation. In this paper, we review some recent results about polarization variability from the cores of AGN jets, including observations at high spatial resolutions and/or at high radio frequencies.

Active galactic nuclei (AGN) at the centres of galaxies can cycle between periods of activity and of quiescence. Characterising the duty-cycle of AGN is crucial for understanding their impact on the evolution of the host galaxy. For radio AGN, their evolutionary stage can be identified from a combination of morphological and spectral properties. We summarise the results we have obtained in the last few years by studying radio galaxies in various crucial phases of their lives, such as remnant and restarted sources. We used morphological information derived from LOw Frequency ARray (LOFAR) images at 150 MHz, combined with resolved spectral indices maps, obtained using recently released images at 1400 MHz from the APERture Tile In Focus (Apertif) phased-array feed system installed on the Westerbork Synthesis Radio Telescope. Our study, limited so far to the Lockman Hole region, has identified radio galaxies in the dying and restarted phases. We found large varieties in their properties, relevant for understanding their evolutionary stage. We started by quantifying their occurrences, the duration of the 'on' (active) and 'off' (dying) phase, and we compared the results with models of the evolution of radio galaxies. In addition to these extreme phases, the resolved spectral index images can also reveal interesting secrets about the evolution of apparently normal radio galaxies. The spectral information can be connected with, and used to improve, the Fanaroff--Riley classification, and we present one example of this, illustrating what the combination of the LOFAR and Apertif surveys now allow us to do routinely., Comment: Accepted for publication in the special issue of Galaxies, proceedings of the conference Radio Galaxies in the Cosmic Web. 19 pages, 6 figures

Active Galactic Nuclei (AGN) are relevant sources of radiation that might have helped reionising the Universe during its early epochs. The super-massive black holes (SMBHs) they host helped accreting material and emitting large amounts of energy into the medium. Recent studies have shown that, for epochs earlier than $z~{\sim}~5$, the number density of SMBHs is on the order of few hundreds per square degree. Latest observations place this value below $300$ SMBHs at $z~{\gtrsim}~6$ for the full sky. To overcome this gap, it is necessary to detect large numbers of sources at the earliest epochs. Given the large areas needed to detect such quantities, using traditional redshift determination techniques -- spectroscopic and photometric redshift -- is no longer an efficient task. Machine Learning (ML) might help obtaining precise redshift for large samples in a fraction of the time used by other methods. We have developed and implemented an ML model which can predict redshift values for WISE-detected AGN in the HETDEX Spring Field. We obtained a median prediction error of $\sigma_{z}^{N} = 1.48 \times (z_{\mathrm{Predicted}} - z_{{\mathrm{True}}}) / (1 + z_{\mathrm{True}}) = 0.1162$ and an outlier fraction of $\eta = 11.58 \%$ at $(z_{\mathrm{Predicted}} - z_{{\mathrm{True}}}) / (1 + z_{\mathrm{True}}) > 0.15$, in line with previous applications of ML to AGN. We also applied the model to data from the Stripe 82 area obtaining a prediction error of $\sigma_{z}^{N} = 0.2501$., Comment: 15 pages, 5 figures. Accepted to the Special Issue "A New Window on the Radio Emission from Galaxies, Galaxy Clusters and Cosmic Web: Current Status and Perspectives" of MDPI Galaxies

Cosmic neutrinos are unique probes of the high energy universe. IceCube has discovered a diffuse astrophysical neutrino flux since 2013, but their origin remains elusive. The potential sources could include, for example, active galactic nuclei, gamma-ray bursts and star burst galaxies. To resolve those scenarios, higher statistics and better angular resolution of astrophysical neutrinos are needed. An optical module with larger photon collection area and more precise timing resolution in a next generation neutrino telescope could help. Silicon photon multipliers (SiPMs), with high quantum efficiency and fast responding time, combining with traditional PMTs, could boost photon detection efficiency and pointing capability. We will present a study on exploring the benefits of combining multiple PMTs and SiPMs in an optical module., Comment: 37th International Cosmic Ray Conference (ICRC2021)

We present the analysis of multi-wavelength observations of seven extragalactic radio sources, listed as unidentified in the Third Cambridge Revised Catalog (3CR). X-ray observations, performed during Chandra Cycle 21, were compared to VLA, WISE and Pan-STARRS observations in the radio, infrared and optical bands, respectively. All sources in this sample lack a clear optical counterpart, and are thus missing their redshift and optical classification. In order to confirm the X-ray and infrared radio counterparts of core and extended components, here we present for the first time radio maps obtained manually reducing VLA archival data. As in previous papers on the Chandra X-ray snapshot campaign, we report X-ray detections of radio cores and two sources, out of the seven presented here, are found to be members of galaxy clusters. For these two cluster sources (namely, 3CR 409 and 3CR 454.2), we derived surface brightness profiles in four directions. For all seven sources, we measured X-ray intensities of the radio sources and we also performed standard X-ray spectral analysis for the four sources (namely, 3CR 91, 3CR 390, 3CR 409 and 3CR 428) with the brightest nuclei (more than 400 photons in the 2'' nuclear region). We also detected extended X-ray emission around 3CR 390 and extended X-ray emission associated with the northern jet of 3CR 158. This paper represents the first attempt to give a multi-wavelength view of the unidentified radio sources listed in the 3CR catalog., Comment: 18 pages, 19 figures, 5 tables, accepted for publication on ApJS

VERITAS is one of the world's most sensitive detectors of astrophysical very high energy (VHE; E > 100 GeV) gamma rays. This observatory has operated for ~14 years, and nearly 7,000 hours of its observations have been targeted on active galactic nuclei (AGN). Approximately 300 AGN were observed with VERITAS, and 40 are detected. These studies are generally accompanied by contemporaneous, broadband observations, which enable detailed probes of the underlying jet-powered processes. Recent scientific results from VERITAS AGN observations are presented., 9 pages; 5 figures; Contribution to the Proceedings of the 37th International Cosmic Ray Conference (ICRC 2021), Berlin, Germany

37th International Cosmic Ray Conference, ICRC 2021, Berlin, online, Germany, 12 Jul 2021 - 23 Jul 2021; Proceedings of Science / International School for Advanced Studies (ICRC2021), 812 (2021). doi:10.22323/1.395.0812, We present a catalog of results of gamma-ray observations made by VERITAS, published from 2008 to 2020. VERITAS is a ground based imaging atmospheric Cherenkov telescope observatory located at the Fred Lawrence Whipple Observatory (FLWO) in southern Arizona, sensitive to gamma-ray photons with energies in the range of $\sim$ 100 GeV - 30 TeV. Its observation targets include galactic sources such as binary star systems, pulsar wind nebulae, and supernova remnants, extragalactic sources like active galactic nuclei, star forming galaxies, and gamma-ray bursts, and some unidentified objects. The catalog includes in digital form all of the high-level science results published in 112 papers using VERITAS data and currently contains data on 57 sources. The catalog has been made accessible via GitHub and at NASA's HEASARC., Published by SISSA, Trieste

Full list of authors: Janssen, Michael; Falcke, Heino; Kadler, Matthias; Ros, Eduardo; Wielgus, Maciek; Akiyama, Kazunori; Baloković, Mislav; Blackburn, Lindy; Bouman, Katherine L.; Chael, Andrew; Chan, Chi-kwan; Chatterjee, Koushik; Davelaar, Jordy; Edwards, Philip G.; Fromm, Christian M.; Gómez, José L.; Goddi, Ciriaco; Issaoun, Sara; Johnson, Michael D.; Kim, Junhan; Koay, Jun Yi; Krichbaum, Thomas P.; Liu, Jun; Liuzzo, Elisabetta; Markoff, Sera; Markowitz, Alex; Marrone, Daniel P.; Mizuno, Yosuke; Müller, Cornelia; Ni, Chunchong; Pesce, Dominic W.; Ramakrishnan, Venkatessh; Roelofs, Freek; Rygl, Kazi L. J.; van Bemmel, Ilse; Event Horizon Telescope Collaboration.--This Open Access article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder., Very-long-baseline interferometry (VLBI) observations of active galactic nuclei at millimetre wavelengths have the power to reveal the launching and initial collimation region of extragalactic radio jets, down to 10–100 gravitational radii (rg ≡ GM/c2) scales in nearby sources1. Centaurus A is the closest radio-loud source to Earth2. It bridges the gap in mass and accretion rate between the supermassive black holes (SMBHs) in Messier 87 and our Galactic Centre. A large southern declination of −43° has, however, prevented VLBI imaging of Centaurus A below a wavelength of 1 cm thus far. Here we show the millimetre VLBI image of the source, which we obtained with the Event Horizon Telescope at 228 GHz. Compared with previous observations3, we image the jet of Centaurus A at a tenfold higher frequency and sixteen times sharper resolution and thereby probe sub-lightday structures. We reveal a highly collimated, asymmetrically edge-brightened jet as well as the fainter counterjet. We find that the source structure of Centaurus A resembles the jet in Messier 87 on ~500 rg scales remarkably well. Furthermore, we identify the location of Centaurus A’s SMBH with respect to its resolved jet core at a wavelength of 1.3 mm and conclude that the source’s event horizon shadow4 should be visible at terahertz frequencies. This location further supports the universal scale invariance of black holes over a wide range of masses5,6. © 2021, The Author(s)., A.C. is an Einstein Fellow of the NASA Hubble Fellowship Program. J.P is an EACOA fellow. Z.Y. is a UKRI Stephen Hawking Fellow. We thank the following organizations and programmes: the Academy of Finland (projects 274477, 284495, 312496, 315721); the Agencia Nacional de Investigacion y Desarrollo (ANID), Chile via NCN19_058 (TITANs) and Fondecyt 3190878; the Alexander von Humboldt Stiftung; an Alfred P. Sloan Research Fellowship; Allegro, the European ALMA Regional Centre node in the Netherlands, the NL astronomy research network NOVA and the astronomy institutes of the University of Amsterdam, Leiden University and Radboud University; the Black Hole Initiative at Harvard University, through a grant (60477) from the John Templeton Foundation; the China Scholarship Council; Consejo Nacional de Ciencia y Tecnologia (CONACYT, Mexico, projects U0004-246083, U0004-259839, F0003-272050, M0037-279006, F0003-281692, 104497, 275201, 263356, 57265507); the Delaney Family via the Delaney Family John A. Wheeler Chair at Perimeter Institute; Direccion General de Asuntos del Personal Academico-Universidad Nacional Autonoma de Mexico (DGAPA-UNAM, projects IN112417 and IN112820); the EACOA Fellowship of the East Asia Core Observatories Association; the European Research Council Synergy Grant "BlackHoleCam: Imaging the Event Horizon of Black Holes" (grant 610058); the Generalitat Valenciana postdoctoral grant APOSTD/2018/177 and GenT Program (project CIDEGENT/2018/021); MICINN Research Project PID2019-108995GB-C22; the Gordon and Betty Moore Foundation (grants GBMF-3561, GBMF-5278); the Istituto Nazionale di Fisica Nucleare (INFN) sezione di Napoli, iniziative specifiche TEONGRAV; the International Max Planck Research School for Astronomy and Astrophysics at the Universities of Bonn and Cologne; Joint Princeton/Flatiron and Joint Columbia/Flatiron Postdoctoral Fellowships, research at the Flatiron Institute is supported by the Simons Foundation; the Japanese Government (Monbukagakusho: MEXT) Scholarship; the Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for JSPS Research Fellowship (JP17J08829); the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (CAS, grants QYZDJ-SSW-SLH057, QYZDJSSW-SYS008, ZDBS-LY-SLH011). We further thank the Leverhulme Trust Early Career Research Fellowship; the Max-Planck-Gesellschaft (MPG); the Max Planck Partner Group of the MPG and the CAS; the MEXT/JSPS KAKENHI (grants 18KK0090, JP18K13594, JP18K03656, JP18H03721, 18K03709, 18H01245, JP19H01943, 25120007); the Malaysian Fundamental Research Grant Scheme (FRGS) FRGS/1/2019/STG02/UM/02/6; the MIT International Science and Technology Initiatives (MISTI) Funds; the Ministry of Science and Technology (MOST) of Taiwan (105-2112-M-001-025-MY3, 106-2112-M-001-011, 106-2119-M-001-027, 107-2119-M-001-017, 107-2119-M-001020, 107-2119-M-110-005, 108-2112-M-001-048, and 109-2124-M-001-005); the National Aeronautics and Space Administration (NASA, Fermi Guest Investigator grant 80NSSC20K1567, NASA Astrophysics Theory Program grant 80NSSC20K0527, NASA grant NNX17AL82G, and Hubble Fellowship grant HST-HF2-51431.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555, and NASA NuSTAR award 80NSSC20K0645); the National Institute of Natural Sciences (NINS) of Japan; the National Key Research and Development Program of China (grant 2016YFA0400704, 2016YFA0400702); the National Science Foundation (NSF, grants AST-0096454, AST-0352953, AST-0521233, AST-0705062, AST-0905844, AST-0922984, AST-1126433, AST-1140030, DGE-1144085, AST-1207704, AST1207730, AST-1207752, MRI-1228509, OPP-1248097, AST-1310896, AST-1337663, AST-1440254, AST-1555365, AST-1615796, AST-1715061, AST-1716327, AST1716536, OISE-1743747, AST-1816420, AST-1903847, AST-1935980, AST-2034306); the Natural Science Foundation of China (grants 11573051, 11633006, 11650110427, 10625314, 11721303, 11725312, 11933007, 11991052, 11991053); a fellowship of China Postdoctoral Science Foundation (2020M671266); the Natural Sciences and Engineering Research Council of Canada (NSERC, including a Discovery Grant and the NSERC Alexander Graham Bell Canada Graduate Scholarships-Doctoral Program); the National Research Foundation of Korea (the Global PhD Fellowship Grant: grants 2014H1A2A1018695, NRF-2015H1A2A1033752, 2015-R1D1A1A01056807, the Korea Research Fellowship Program: NRF-2015H1D3A1066561, Basic Research Support Grant 2019R1F1A1059721); the Netherlands Organization for Scientific Research (NWO) VICI award (grant 639.043. 513) and Spinoza Prize SPI 78-409; the New Scientific Frontiers with Precision Radio Interferometry Fellowship awarded by the South African Radio Astronomy Observatory (SARAO), which is a facility of the National Research Foundation (NRF), an agency of the Department of Science and Innovation (DSI) of South Africa; the South African Research Chairs Initiative of the Department of Science and Innovation and National Research Foundation; the Onsala Space Observatory (OSO) national infrastructure, for the provisioning of its facilities/observational support (OSO receives funding through the Swedish Research Council under grant 2017-00648) the Perimeter Institute for Theoretical Physics (research at Perimeter Institute is supported by the Government of Canada through the Department of Innovation, Science and Economic Development and by the Province of Ontario through the Ministry of Research, Innovation and Science); the Spanish Ministerio de Economia y Competitividad (grants PGC2018-098915-B-C21, AYA2016-80889-P, PID2019-108995GB-C21); the State Agency for Research of the Spanish MCIU through the 'Center of Excellence Severo Ochoa' award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709); the Toray Science Foundation; the Consejeria de Economia, Conocimiento, Empresas y Universidad of the Junta de Andalucia (grant P18-FR-1769), the Consejo Superior de Investigaciones Cientificas (grant 2019AEP112); the US Department of Energy (US DOE) through the Los Alamos National Laboratory (operated by Triad National Security, LLC, for the National Nuclear Security Administration of the US DOE (Contract 89233218CNA000001); the European Union's Horizon 2020 research and innovation programme under grant agreement No 730562 RadioNet; ALMA North America Development Fund; the Academia Sinica; Chandra TM6-17006X and DD7-18089X; the GenT Program (Generalitat Valenciana) Project CIDEGENT/2018/021. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), supported by NSF grant ACI-1548562, and CyVerse, supported by NSF grants DBI-0735191, DBI-1265383, and DBI-1743442. XSEDE Stampede2 resource at TACC was allocated through TG-AST170024 and TG-AST080026N. XSEDE JetStream resource at PTI and TACC was allocated through AST170028. The simulations were performed in part on the SuperMUC cluster at the LRZ in Garching, on the LOEWE cluster in CSC in Frankfurt, and on the HazelHen cluster at the HLRS in Stuttgart. This research was enabled in part by support provided by Compute Ontario (http://computeontario.ca), Calcul Quebec (http://www.calculquebec.ca) and Compute Canada (http://www.computecanada.ca).We thank the staff at the participating observatories, correlation centres, and institutions for their enthusiastic support. This paper makes use of the following ALMA data: ADS/JAO. ALMA#2016.1.01198.V. ALMA is a partnership of the European Southern Observatory (ESO; Europe, representing its member states), NSF, and National Institutes of Natural Sciences of Japan, together with National Research Council (Canada), Ministry of Science and Technology (MOST; Taiwan), Academia Sinica Institute of Astronomy and Astrophysics (ASIAA; Taiwan), and Korea Astronomy and Space Science Institute (KASI; Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, Associated Universities, Inc. (AUI)/NRAO, and the National Astronomical Observatory of Japan (NAOJ). The NRAO is a facility of the NSF operated under cooperative agreement by AUI. APEX is a collaboration between the Max-Planck-Institut fur Radioastronomie (Germany), ESO, and the Onsala Space Observatory (Sweden). The SMA is a joint project between the SAO and ASIAA and is funded by the Smithsonian Institution and the Academia Sinica. The JCMT is operated by the East Asian Observatory on behalf of the NAOJ, ASIAA, and KASI, as well as the Ministry of Finance of China, Chinese Academy of Sciences, and the National Key R&D Program (No. 2017YFA0402700) of China. Additional funding support for the JCMT is provided by the Science and Technologies Facility Council (UK) and participating universities in the UK and Canada. The LMT is a project operated by the Instituto Nacional de Astrofisica, Optica, y Electronica (Mexico) and the University of Massachusetts at Amherst (USA), with financial support from the Consejo Nacional de Ciencia y Tecnologia and the National Science Foundation. The IRAM 30-m telescope on Pico Veleta, Spain is operated by IRAM and supported by CNRS (Centre National de la Recherche Scientifique, France), MPG (Max-Planck-Gesellschaft, Germany) and IGN (Instituto Geografico Nacional, Spain). The SMT is operated by the Arizona Radio Observatory, a part of the Steward Observatory of the University of Arizona, with financial support of operations from the State of Arizona and financial support for instrumentation development from the NSF. The SPT is supported by the National Science Foundation through grant PLR-1248097. Partial support is also provided by the NSF Physics Frontier Center grant PHY-1125897 to the Kavli Institute of Cosmological Physics at the University of Chicago, the Kavli Foundation and the Gordon and Betty Moore Foundation grant GBMF 947. The SPT hydrogen maser was provided on loan from the GLT, courtesy of ASIAA. The EHTC has received generous donations of FPGA chips from Xilinx Inc., under the Xilinx University Program. The EHTC has benefited from technology shared under open-source license by the Collaboration for Astronomy Signal Processing and Electronics Research (CASPER). The EHT project is grateful to T4Science and Microsemi for their assistance with Hydrogen Masers. This research has made use of NASA's Astrophysics Data System. We gratefully acknowledge the support provided by the extended staff of the ALMA, both from the inception of the ALMA Phasing Project through the observational campaigns of 2017 and 2018. The grants listed above collectively fund the Event Horizon Telescope project.

The past decade has experienced an explosive increase of optically-discovered tidal disruption events (TDEs) with the advent of modern time-domain surveys. However, we still lack a comprehensive observational view of their infrared (IR) echoes in spite of individual detections. To this end, we have conducted a statistical study of IR variability of the 23 optical TDEs discovered between 2009 and 2018 utilizing the full public dataset of Wide-field Infrared Survey Explorer. The detection of variability is performed on the difference images, yielding out 11 objects with significant (>$3\sigma$) variability in at least one band while dust emission can be only fitted in 8 objects. Their peak dust luminosity is around $10^{41}$-$10^{42}$ erg/s, corresponding to a dust covering factor $f_c\sim0.01$ at scale of sub-parsec. The only exception is the disputed source ASASSN-15lh, which shows an ultra-high dust luminosity ($\sim10^{43.5}$ erg/s) and make its nature even elusive. Other non-detected objects show even lower $f_c$, which could be one more order of magnitude lower. The derived $f_c$ is generally much smaller than those of dusty tori in active galactic nuclei (AGNs), suggesting either a dearth of dust or a geometrically thin and flat disk in the vicinity of SMBHs. Our results also indicate that the optical TDE sample (post-starburst galaxies overrepresented) is seriously biased to events with little dust at sub-pc scale while TDEs in dusty star-forming systems could be more efficiently unveiled by IR echoes., Comment: ApJ accepted, 5 figures, 3 tables