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Observations at submillimetre and X-ray wavelengths show that rapid star formation was common in the host galaxies of active galactic nuclei when the Universe was 2–6 Gyr old, but that the most vigorous star formation is not observed around powerful black holes, thereby confirming a key prediction of models in which an active galactic nucleus expels the interstellar medium of its host galaxy. Star formation blocked by powerful black holes Radiation from active galactic nuclei (AGNs) outshines that produced by star formation at most wavelengths, but in the far-infrared to millimetre waveband AGNs emit comparatively little radiation in comparison with strongly star-forming galaxies. A combination of deep X-ray observations from the Chandra catalogue and submillimetre observations from the SPIRE instrument on the Herschel Space Observatory shows that rapid star formation was common in the host galaxies of AGNs when the Universe was between two billion and six billion years old, but that vigorous star formation is not seen around the more luminous black holes. This suppression of star formation in galaxies that host a powerful AGN is a key prediction of models in which the AGN expels the interstellar medium of its host galaxy when it becomes sufficiently powerful. The old, red stars that constitute the bulges of galaxies, and the massive black holes at their centres, are the relics of a period in cosmic history when galaxies formed stars at remarkable rates and active galactic nuclei (AGN) shone brightly as a result of accretion onto black holes. It is widely suspected, but unproved, that the tight correlation between the mass of the black hole and the mass of the stellar bulge 1 results from the AGN quenching the surrounding star formation as it approaches its peak luminosity 2 , 3 , 4 . X-rays trace emission from AGN unambiguously 5 , whereas powerful star-forming galaxies are usually dust-obscured and are brightest at infrared and submillimetre wavelengths 6 . Here we report submillimetre and X-ray observations that show that rapid star formation was common in the host galaxies of AGN when the Universe was 2–6 billion years old, but that the most vigorous star formation is not observed around black holes above an X-ray luminosity of 10 44 ergs per second. This suppression of star formation in the host galaxy of a powerful AGN is a key prediction of models in which the AGN drives an outflow 7 , 8 , 9 , expelling the interstellar medium of its host and transforming the galaxy’s properties in a brief period of cosmic time.

L-line route to black holes The emission line arising from a transition of an electron from the iron K shell to the ground state (the K line) is prominent in the reflection spectrum of the hard X-ray continuum irradiating dense accreting matter around a black hole. The corresponding iron L-line emission should be detectable when iron abundance is high. That's the theory, and now broad iron L-line emission has been observed, together with the broad K line in the narrow-line Seyfert galaxy 1H0707. There is a reverberation lag of about 30 s between the direct X-ray continuum and its reflection from matter falling into the hole, a timescale comparable to the light-crossing time of the innermost radii around a supermassive black hole. This discovery opens a window on events close to the black hole event horizon in these objects. Emission arising from a transition of an electron from the iron K shell to the ground state (the K line) is prominent in the reflection spectrum created by the hard X-ray continuum irradiating the dense accreting matter around a black hole. Here the presence of both iron K and L emission is reported in the spectrum of the active galaxy 1H 0707-495. There is a 'reverberation lag' with a timescale comparable to the light-crossing time of the innermost radii around a supermassive black hole. Since the 1995 discovery of the broad iron K-line emission from the Seyfert galaxy MCG–6-30-15 (ref. 1 ), broad iron K lines have been found in emission from several other Seyfert galaxies 2 , from accreting stellar-mass black holes 3 and even from accreting neutron stars 4 . The iron K line is prominent in the reflection spectrum 5 , 6 created by the hard-X-ray continuum irradiating dense accreting matter. Relativistic distortion 7 of the line makes it sensitive to the strong gravity and spin of the black hole 8 . The accompanying iron L-line emission should be detectable when the iron abundance is high. Here we report the presence of both iron K and iron L emission in the spectrum of the narrow-line Seyfert 1 galaxy 9 1H 0707-495. The bright iron L emission has enabled us to detect a reverberation lag of about 30 s between the direct X-ray continuum and its reflection from matter falling into the black hole. The observed reverberation timescale is comparable to the light-crossing time of the innermost radii around a supermassive black hole. The combination of spectral and timing data on 1H 0707-495 provides strong evidence that we are witnessing emission from matter within a gravitational radius, or a fraction of a light minute, from the event horizon of a rapidly spinning, massive black hole.

This review summarises what we have learnt in the last two decades based on H i 21 cm absorption observations about the cold interstellar medium (ISM) in the central regions of active galaxies and about the interplay between this gas and the active nucleus (AGN). H i absorption is a powerful tracer on all scales, from the parsec-scales close to the central black hole to structures of many tens of kpc tracing interactions and mergers of galaxies. Given the strong radio continuum emission often associated with the central activity, H i absorption observations can be used to study the H i near an active nucleus out to much higher redshifts than is possible using H i emission. In this way, H i absorption has been used to characterise in detail the general ISM in active galaxies, to trace the fuelling of radio-loud AGN, to study the feedback occurring between the energy released by the active nucleus and the ISM, and the impact of such interactions on the evolution of galaxies and of their AGN. In the last two decades, significant progress has been made in all these areas. It is now well established that many radio loud AGN are surrounded by small, regularly rotating gas disks that contain a significant fraction of H i. The structure of these disks has been traced down to parsec scales by very long baseline interferometry observations. Some groups of objects, and in particular young and recently restarted radio galaxies, appear to have a particularly high detection rate of H i. This is interesting in connection with the evolution of these AGN and their impact on the surrounding ISM. This is further confirmed by an important discovery, made thanks to technical upgrades of radio telescopes, namely the presence of fast, AGN-driven outflows of cold gas which give a direct view of the impact of the energy released by AGN on the evolution of galaxies (AGN feedback). In addition, evidence has been collected that clouds of cold gas can play a role in fuelling the nuclear activity. This review ends by briefly describing the upcoming large, blind H i absorption surveys planned for the new radio telescopes which will soon become operational. These surveys will allow to significantly expand existing work, but will also allow to explore new topics, in particular, the evolution of the cold ISM in AGN.

The relationship between radio jet and black hole Radio jets from active galactic nuclei, such as the nearby galaxy M87, are thought to be powered by the accretion of material into a supermassive black hole. The relative position of this 'central engine' and the bright radio core that marks the base of the jet remain the subject of much speculation. New observations of M87 at six frequencies have been used to determine the position of the radio core to an accuracy of ∼ 20 microarcseconds. The data reveal that the central engine is located close to the radio core, within a distance of 14–23 Schwarzschild radii at 43 GHz. Powerful radio jets from active galactic nuclei are thought to be powered by the accretion of material onto the supermassive black hole (the ‘central engine’) 1 , 2 . M87 is one of the closest examples of this phenomenon, and the structure of its jet has been probed on a scale of about 100 Schwarzschild radii ( R s , the radius of the event horizon) 3 . However, the location of the central black hole relative to the jet base (a bright compact radio ‘core’) remains elusive 4 , 5 . Observations of other jets indicate that the central engines are located about 10 4 –10 6 R s upstream from the radio core 6 , 7 , 8 , 9 . Here we report radio observations of M87 at six frequencies that allow us to achieve a positional accuracy of about 20 microarcseconds. As the jet base becomes more transparent at higher frequencies, the multifrequency position measurements of the radio core enable us to determine the upstream end of the jet. The data reveal that the central engine of M87 is located within 14–23 R s of the radio core at 43 GHz. This implies that the site of material infall onto the black hole and the eventual origin of the jet reside in the bright compact region seen on the image at 43 GHz.

Active galactic nuclei: on the hour Although active galactic nuclei (AGNs) are thought — like quasars — to be scaled-up versions of binary black holes, quasiperiodic oscillations have proved elusive in the supermassive black holes found in AGNs. This is surprising, since these oscillations are generally so well defined in black holes. That anomaly has now been straightened out with the observation of X-ray emissions with a periodicity of about 1 hour in the bright active galaxy RE J1034+396. The X-ray modulation arises from the direct vicinity of the black hole. The study of this and similar phenomena should reveal more about accretion flows around black holes. Active galactic nuclei and quasars are thought to be scaled-up versions of Galactic black hole binaries, powered by accretion onto supermassive black holes with masses of 10 6 –10 9   , as opposed to the ∼10  in binaries (here is the solar mass). One example of the similarities between these two types of systems is the characteristic rapid X-ray variability seen from the accretion flow 1 . The power spectrum of this variability in black hole binaries consists of a broad noise with multiple quasi-periodic oscillations superimposed on it. Although the broad noise component has been observed in many active galactic nuclei 2 , 3 , there have hitherto been no significant detections of quasi-periodic oscillations 4 , 5 , 6 . Here we report the discovery of an ∼1-hour X-ray periodicity in a bright active galaxy, RE J1034+396. The signal is highly statistically significant (at the 5.6 σ level) and very coherent, with quality factor Q  > 16. The X-ray modulation arises from the direct vicinity of the black hole.

Due to its proximity, M87 is a prime target for next-generation high-resolution VLBI at short millimeter wavelengths, by which the jet launching region and the black hole shadow are expected to be resolved and imaged sometime soon. Along with this situation, high-quality VLBI imaging and monitoring at lower frequencies play an important role in complementing the high-frequency data. Here, we present our recent and ongoing observational studies of the M87 jet on pc-to-subpc scales based on ultra-deep VLBI imaging programs at 86 GHz and 15 GHz. The high-dynamic-range images have allowed us to obtain some remarkably improved views on this jet. We also introduce the KVN and VERA Array (KaVA), a new regularly-operating VLBI network in East Asia, which is quite suitable for studying the structure and propagation of relativistic jets. Some early results from our pilot study for M87—including the detection of superluminal motions near the jet base—implying an efficient magnetic-to-kinetic conversion at these scales, are reported.

We investigate the differences in the stellar population properties, the structure, and the environment between massive compact star-forming galaxies (cSFGs) with or without active galactic nucleus (AGN) at 2 < z < 3 in the five 3D-Hubble Space Telescope/CANDELS fields. In a sample of 221 massive cSFGs, we constitute the most complete AGN census so far, identifying 66 AGNs by the X-ray detection, the mid-infrared color criterion, and/or the spectral energy distribution fitting, while the rest (155) are non-AGNs. Further dividing these cSFGs into two redshift bins, i.e., 2 < z < 2.5 and 2.5 ≤ z < 3, we find that in each redshift bin the cSFGs with AGNs have similar distributions of the stellar mass, the specific star formation rate, and the ratio of LIR to LUV to those without AGNs. After having performed a two-dimensional surface brightness modeling for those cSFGs with X-ray-detected AGNs (37) to correct for the influence of the central point-like X-ray AGN on measuring the structural parameters of its host galaxy, we find that in each redshift bin the cSFGs with AGNs have comparable distributions of all concerned structural parameters, i.e., the Sérsic index, the 20%-light radius, the Gini coefficient, and the concentration index, to those without AGNs. With a gradual consumption of available gas and dust, the structure of cSFGs, indicated by the above structural parameters, seem to be slightly more concentrated with decreasing redshift. At 2 < z < 3, the similar environment between cSFGs with and without AGNs suggests that their AGN activities are potentially triggered by internal secular processes, such as gravitational instabilities or/and dynamical friction.

A key question in the formation of the relativistic jets in active galactic nuclei (AGNs) is the collimation process of their energetic plasma flow launched from the central supermassive black hole (SMBH). Recent observations of nearby low-luminosity radio galaxies exhibit a clear picture of parabolic collimation inside the Bondi accretion radius. On the other hand, little is known of the observational properties of jet collimation in more luminous quasars, where the accretion flow may be significantly different due to much higher accretion rates. In this paper, we present preliminary results of multi-frequency observations of the archetypal quasar 3C 273 with the Very Long Baseline Array (VLBA) at 1.4, 15, and 43 GHz, and Multi-Element Radio Linked Interferometer Network (MERLIN) at 1.6 GHz. The observations provide a detailed view of the transverse structure resolved on a broad range of spatial scales from sub-parsec to kilo parsecs, allowing us to profile the jet width as a function of the distance from the core for the first time in jets of bright quasars. We discovered a transition from a parabolic stream to a conical stream, which has been seen in much lower-luminosity radio galaxies. The similarity in the profile to the much lower-powered radio galaxy M87 suggests the universality of jet collimation among AGNs with different accretion rates.

Super-massive black holes cut down to size? Active galactic nuclei (AGN) are the most luminous objects in the Universe. They are powered by super-massive black holes of between 10 million and 10 billion times the mass of the Sun, and are surrounded by a broad emission line region probably associated with an accretion disk. The kinematics and structure of the central broad-line region are poorly understood despite intensive studies over more than 30 years. Wolfram Kollatschny and Matthias Zetzl now show that there is a fundamental relationship between observed emission line width and emission line shape in the spectra of AGN, from which they infer that the geometry of the inner region is flattest for the fast-rotating broad-line objects, while slow-rotating narrow-line AGN have a more spherical structure. Knowing the rotational velocities, it is possible to derive more accurate estimates for the masses of the central black holes — which turn out to be 2–10 times smaller than previously estimated. The super-massive black holes of 10 6 to 10 9 solar masses that reside in the nuclei of active galaxies (AGN) are surrounded by a region emitting broad emission lines, probably associated with an accretion disk, which cannot be resolved spatially. The relative significance of inflow, outflow, rotational, or turbulent motions in the broad-line region as well as their structure (spherical and/or thin/thick accretion disk) are unknown. This study reports a fundamental relation between the observed emission line width and shape in AGN spectra, from which it is inferred that the geometry of the inner region is flattest for the fast-rotating broad-line objects whereas slow-rotating narrow-line AGN have a more spherical structure. Knowing the rotational velocities one can derive more accurately the central black hole masses, which are two to ten times smaller than previously estimated. The super-massive black holes of 10 6 M ⊙ to 10 9 M ⊙ that reside in the nuclei of active galaxies 1 (AGN) are surrounded by a region emitting broad lines, probably associated with an accretion disk. The diameters of the broad-line regions range from a few light-days to more than a hundred light-days 1 , and cannot be resolved spatially. The relative significance of inflow, outflow, rotational or turbulent motions in the broad-line regions as well as their structure (spherical, thin or thick accretion disk) are unknown despite intensive studies over more than thirty years 2 , 3 . Here we report a fundamental relation between the observed emission linewidth full-width at half-maximum (FWHM) and the emission line shape FWHM/ σ line in AGN spectra. From this relation we infer that the predominant motion in the broad-line regions is Keplerian rotation in combination with turbulence. The geometry of the inner region varies systematically with the rotation velocity: it is flattest for the fast-rotating broad-line objects, whereas slow-rotating narrow-line AGN have a more spherical structure. Superimposed is the trend that the line-emitting region becomes geometrically thicker towards the centre within individual galaxies. Knowing the rotational velocities, we can derive the central black-hole masses more accurately; they are two to ten times smaller than has been estimated previously.

Particle acceleration in relativistic jets, to very high levels of energy, occurs at the expense of the dissipation of magnetic or kinetic energy. Therefore, understanding the processes that can trigger this dissipation is key to the characterization of the energy budgets and particle acceleration mechanisms in action in active galaxies. Instabilities and entrainment are two obvious candidates to trigger dissipation. On the one hand, supersonic, relativistic flows threaded by helical fields, as expected from the standard formation models of jets in supermassive black-holes, are unstable to a series of magnetohydrodynamical instabilities, such as the Kelvin–Helmholtz, current-driven, or possibly the pressure-driven instabilities. Furthermore, in the case of expanding jets, the Rayleigh–Taylor and centrifugal instabilities may also develop. With all these destabilizing processes in action, a natural question is to ask how can some jets keep their collimated structure along hundreds of kiloparsecs. On the other hand, the interaction of the jet with stars and clouds of gas that cross the flow in their orbits around the galactic centers provides another scenario in which kinetic energy can be efficiently converted into internal energy and particles can be accelerated to non-thermal energies. In this contribution, I review the conditions under which these processes occur and their role both in jet evolution and propagation and energy dissipation.