Powerful Active Galactic Nuclei Research Articles

Modeling AGN outbursts from supermassive black hole binaries

When galaxies merge to assemble more massive galaxies, their nuclear supermassive black holes (SMBHs) should form bound binaries. As these interact with their stellar and gaseous environments, they will become increasingly compact, culminating in inspiral and coalescence through the emission of gravitational radiation. Because galaxy mergers and interactions are also thought to fuel star formation and nuclear black hole activity, it is plausible that such binaries would lie in gas-rich environments and power active galactic nuclei (AGN). The primary difference is that these binaries have gravitational potentials that vary - through their orbital motion as well as their orbital evolution - on humanly tractable timescales, and are thus excellent candidates to give rise to coherent AGN variability in the form of outbursts and recurrent transients. Although such electromagnetic signatures would be ideally observed concomitantly with the binary's gravitational-wave signatures, they are also likely to be discovered serendipitously in wide-field, high-cadence surveys; some may even be confused for stellar tidal disruption events. I discuss several types of possible smoking gun AGN signatures caused by the peculiar geometry predicted for accretion disks around SMBH binaries.

Galaxy-scale outflows driven by active galactic nuclei

We present hydrodynamical simulations of major mergers of galaxies and study the effects of winds produced by active galactic nuclei (AGN) on interstellar gas in the AGN’s host galaxy. We consider winds with initial velocities ∼10 000 km s−1 and an initial momentum (energy) flux of ∼τw L/c (∼ 0.01 τw L), with ⁠. The AGN wind sweeps up and shock heats the surrounding interstellar gas, leading to a galaxy-scale outflow with velocities ∼1000 km s−1, peak mass outflow rates comparable to the star formation rate and a total ejected gas mass of ∼3 × 109 M⊙. Large momentum fluxes, τw≳ 3, are required for the AGN-driven galactic outflow to suppress star formation and accretion in the black hole’s host galaxy. Less powerful AGN winds (τw≲ 3) still produce a modest galaxy-scale outflow, but the outflow has little global effect on the ambient interstellar gas. We argue that this mechanism of AGN feedback can plausibly produce the high-velocity outflows observed in post-starburst galaxies and the massive molecular and atomic outflows observed in local ultraluminous infrared galaxies. Moreover, the outflows from local ultraluminous infrared galaxies are inferred to have τw∼ 10, comparable to what we find is required for AGN winds to regulate the growth of black holes and set the MBH - σ relation. We conclude by discussing theoretical mechanisms that can lead to AGN wind mass loading and momentum/energy fluxes large enough to have a significant impact on galaxy formation.

The Galaxy Zoo survey for giant AGN-ionized clouds: past and present black hole accretion events

Some active galactic nuclei (AGN) are surrounded by extended emission-line regions (EELRs), which trace both the illumination pattern of escaping radiation and its history over the light-travel time from the AGN to the gas. From a new set of such EELRs, we present evidence that the AGN in many Seyfert galaxies undergo luminous episodes 20,000-200,000 years in duration. Motivated by the discovery of the spectacular nebula known as Hanny's Voorwerp, ionized by a powerful AGN which has apparently faded dramatically within ~ 100,000 years, Galaxy Zoo volunteers have carried out both targeted and serendipitous searches for similar emission-line clouds around low-redshift galaxies.We present the resulting list of candidates and describe spectroscopy identifying 19 galaxies with AGN-ionized regions at projected radii > 10 kpc. This search recovered known EELRs and identified additional previously unknown cases, one with detected emission to r = 37 kpc. At least 14/19 are in interacting or merging systems; tidal tails are a prime source of extraplanar ionized gas. We see a mix of one- and two-sided structures, with observed cone angles from 23-112 degrees. We consider the energy balance in the ionized clouds, with lower and upper bounds on ionizing luminosity from recombination and ionization-parameter arguments, and estimate the luminosity of the core from the far-infrared data. The implied ratio of ionizing radiation seen by the clouds to that emitted by the nucleus, for a constant nuclear source, ranges from 0.02 to > 12; 7/19 exceed unity. Small values imply heavily obscured AGN. However, large values may require that the AGN has faded over tens of thousands of years, giving us several examples of systems in which such dramatic long-period variation has occurred; this is the only current technique for addressing these timescales in AGN history. (Abridged)

THE VLA SURVEY OF CHANDRA DEEP FIELD SOUTH. V. EVOLUTION AND LUMINOSITY FUNCTIONS OF SUB-MILLIJANSKY RADIO SOURCES AND THE ISSUE OF RADIO EMISSION IN RADIO-QUIET ACTIVE GALACTIC NUCLEI

We present the evolutionary properties and luminosity functions of the radio sources belonging to the Chandra Deep Field South VLA survey, which reaches a flux density limit at 1.4 GHz of 43 microJy at the field center and redshift ~5, and which includes the first radio-selected complete sample of radio-quiet active galactic nuclei (AGN). We use a new, comprehensive classification scheme based on radio, far- and near-IR, optical, and X-ray data to disentangle star-forming galaxies from AGN and radio-quiet from radio-loud AGN. We confirm our previous result that star-forming galaxies become dominant only below 0.1 mJy. The sub-mJy radio sky turns out to be a complex mix of star-forming galaxies and radio-quiet AGN evolving at a similar, strong rate; non-evolving low-luminosity radio galaxies; and declining radio powerful (P > 3 10^24 W/Hz) AGN. Our results suggest that radio emission from radio-quiet AGN is closely related to star formation. The detection of compact, high brightness temperature cores in several nearby radio-quiet AGN can be explained by the co-existence of two components, one non-evolving and AGN-related and one evolving and star-formation-related. Radio-quiet AGN are an important class of sub-mJy sources, accounting for ~30% of the sample and ~60% of all AGN, and outnumbering radio-loud AGN at < 0.1 mJy. This implies that future, large area sub-mJy surveys, given the appropriate ancillary multi-wavelength data, have the potential of being able to assemble vast samples of radio-quiet AGN by-passing the problems of obscuration, which plague the optical and soft X-ray bands.

Molecular accretion in the core of the galaxy cluster 2A 0335+096

We present adaptive optics-assisted K-band integral field spectroscopy of the central cluster galaxy in 2A 0335+096 (z= 0.0349). The H2 v=1–0 S(1) emission is concentrated in two peaks within 600 pc of the nucleus and fainter but kinematically active emission extends towards the nucleus. The H2 is in a rotating structure which aligns with, and appears to have been accreted from, a stream of Hα emission extending over 14 kpc towards a companion galaxy. The projected rotation axis aligns with the 5 GHz radio lobes. This H2 traces the known 1.2 × 109 M⊙ CO-emitting reservoir; limits on the Brγ emission confirm that the H2 emission is not excited by star formation, which occurs at a rate of less than 1 M⊙ yr−1 in this gas. If its accretion on to the black hole can be regulated whilst star formation remains suppressed, the reservoir could last for at least 1 Gyr; the simultaneous accretion of just ∼5 per cent of the gas could drive a series of active galactic nucleus (AGN) outbursts which offset X-ray cooling in the cluster core for the full ∼1 Gyr. Alternatively, if the regulation is ineffective and the bulk of the H2 accretes within a few orbital periods (25–100 Myr), the resulting 1062 erg outburst would be among the most powerful cluster AGN outbursts known. In either case, these observations further support cold feedback scenarios for AGN heating.

Violent interaction between the active galactic nucleus and the hot gas in the core of the galaxy cluster Sérsic 159−03

We present a multiwavelength study of the energetic interaction between the central active galactic nucleus (AGN), the intracluster medium (ICM) and the optical emission-line nebula in the galaxy cluster Sérsic 159−03. We use X-ray data from Chandra, high-resolution X-ray spectra and ultraviolet (UV) images from XMM–Newton, Hα images from the Southern Astrophysics Research Telescope, Hubble Space Telescope optical imaging, and Very Large Array and Giant Metrewave Radio Telescope radio data. The cluster centre displays signs of powerful AGN feedback, which has cleared the central regions (r < 7.5 kpc) of a dense, X-ray-emitting ICM. X-ray spectral maps reveal a high-pressure ring surrounding the central AGN at a radius of r∼ 15 kpc, indicating an AGN-driven weak shock. The cluster harbours a bright, 44 kpc long Hα+[N ii] filament extending from the centre of the cD galaxy to the north. Along the filament, we see low-entropy, high-metallicity, cooling X-ray gas. The gas in the filament has most likely been uplifted by ‘radio mode’ AGN activity and subsequently stripped from the galaxy due to its relative southward motion. Because this X-ray gas has been removed from the direct influence of the AGN jets, part of it cools and forms stars as indicated by the observed dust lanes, molecular and ionized emission-line nebulae and the excess UV emission.

A POWERFUL AGN OUTBURST IN RBS 797

Utilizing $\sim 50$ ks of Chandra X-ray Observatory imaging, we present an analysis of the intracluster medium (ICM) and cavity system in the galaxy cluster RBS 797. In addition to the two previously known cavities in the cluster core, the new and deeper X-ray image has revealed additional structure associated with the active galactic nucleus (AGN). The surface brightness decrements of the two cavities are unusually large, and are consistent with elongated cavities lying close to our line-of-sight. We estimate a total AGN outburst energy and mean jet power of $\approx 3 - 6 \times 10^{60}$ erg and $\approx 3 - 6 \times 10^{45}$ erg s$^{-1}$, respectively, depending on the assumed geometrical configuration of the cavities. Thus, RBS 797 is apparently among the the most powerful AGN outbursts known in a cluster. The average mass accretion rate needed to power the AGN by accretion alone is $\sim 1 M_{\odot}$ yr$^{-1}$. We show that accretion of cold gas onto the AGN at this level is plausible, but that Bondi accretion of the hot atmosphere is probably not. The BCG harbors an unresolved, non-thermal nuclear X-ray source with a bolometric luminosity of $\approx 2 \times 10^{44}$ erg s$^{-1}$. The nuclear emission is probably associated with a rapidly-accreting, radiatively inefficient accretion flow. We present tentative evidence that star formation in the BCG is being triggered by the radio jets and suggest that the cavities may be driving weak shocks ($M \sim 1.5$) into the ICM, similar to the process in the galaxy cluster MS 0735.6+7421.

The production of ultra high energy cosmic rays during the early epochs of radio-loud AGN

Powerful radio-loud active galactic nuclei (AGN) with large Mpc-scale jets have been theoretically motivated as emitters of high-energy cosmic rays. Recent radio observations have established a populous class of young radio-loud galaxies with compact (<1 kpc) symmetric jets that are morphologically similar to large-scale AGNs. We show that these compact AGNs, so-called compact symmetric objects (CSOs), can accelerate protons up to 10 20 eV at their hot spots via a Fermi type mechanism on the assumption of efficient acceleration. The required magnetic field strengths are comparable to those derived from the minimum energy condition. We further show that the accelerated protons can escape through the photon fields of the cocoon without significant energy loss. However, the local number density of powerful CSOs is insufficient for CSOs to power the entire observed flux of ultra-high-energy cosmic rays, providing maximally only a few percent. A heavy composition of UHECRs allows more CSOs to accelerate particles to UHECR energies, but escaping the cocoon is difficult. We comment on a method that may test CSOs as UHECR sources.

UHECRs from magnetic reconnection in relativistic jets

Abstract Ultra-high-energy cosmic rays (UHECRs) may be produced in active galactic nuclei (AGNs) or gamma-ray burst (GRB) jets. I argue that magnetic reconnection in jets can accelerate UHECRs rather independently of physical processes in the magnetic dissipation region. First-order Fermi acceleration can efficiently take place in the region where the unreconnected (upstream) magnetized fluid converges into the reconnection layer. I find that protons can reach energies up to E ∼ 1020 eV in GRB and powerful AGN jets while iron nuclei can reach similar energies in AGN jets of more moderate luminosity.

Feedback and the structure of simulated galaxies at redshift z= 2

We study the properties of simulated high-redshift galaxies using cosmological N-body/gasdynamical runs from the OverWhelmingly Large Simulations (OWLS) project. The runs contrast several feedback implementations of varying effectiveness: from no feedback, to supernova-driven winds to powerful active galactic nucleus (AGN)-driven outflows. These different feedback models result in large variations in the abundance and structural properties of bright galaxies at z= 2. In agreement with earlier work, models with inefficient or no feedback lead to the formation of massive compact galaxies collecting a large fraction (upwards of 50 per cent) of all available baryons in each halo. Increasing the efficiency of feedback reduces the baryonic mass and increases the size of simulated galaxies. A model that includes supernova-driven gas outflows aided by the energetic output of AGNs reduces galaxy masses by roughly a factor of ∼10 compared with the no-feedback case. Other models give results that straddle these two extremes. Despite the large differences in galaxy formation efficiency, the net specific angular momentum of a galaxy is, on average, roughly half that of its surrounding halo, independent of halo mass (in the range probed) and of the feedback scheme. Feedback thus affects the baryonic mass of a galaxy much more severely than its spin. Feedback induces strong correlations between angular momentum content and galaxy mass that leave their imprint on galaxy scaling relations and morphologies. Encouragingly, we find that galaxy discs are common in moderate-feedback runs, making up typically ∼50 per cent of all galaxies at the centres of haloes with virial mass exceeding ∼1011 M⊙. The size, stellar masses and circular speeds of simulated galaxies formed in such runs have properties in between those of large star-forming discs and of compact early-type galaxies at z= 2. Once the detailed abundance and structural properties of these rare objects are well established, it may be possible to use them to gauge the overall efficacy of feedback in the formation of high-redshift galaxies.

REMOVING COOL CORES AND CENTRAL METALLICITY PEAKS IN GALAXY CLUSTERS WITH POWERFUL ACTIVE GALACTIC NUCLEUS OUTBURSTS

Recent X-ray observations of galaxy clusters suggest that cluster populations are bimodally distributed according to central gas entropy and are separated into two distinct classes: cool core (CC) and non-cool core (NCC) clusters. While it is widely accepted that active galactic nucleus (AGN) feedback plays a key role in offsetting radiative losses and maintaining many clusters in the CC state, the origin of NCC clusters is much less clear. At the same time, a handful of extremely powerful AGN outbursts have recently been detected in clusters, with a total energy ∼1061–1062 erg. Using two-dimensional hydrodynamic simulations, we show that if a large fraction of this energy is deposited near the centers of CC clusters, which is likely common due to dense cores, these AGN outbursts can completely remove CCs, transforming them to NCC clusters. Our model also has interesting implications for cluster abundance profiles, which usually show a central peak in CC systems. Our calculations indicate that during the CC to NCC transformation, AGN outbursts efficiently mix metals in cluster central regions and may even remove central abundance peaks if they are not broad enough. For CC clusters with broad central abundance peaks, AGN outbursts decrease peak abundances, but cannot effectively destroy the peaks. Our model may simultaneously explain the contradictory (possibly bimodal) results of abundance profiles in NCC clusters, some of which are nearly flat, while others have strong central peaks similar to those in CC clusters. A statistical analysis of the sizes of central abundance peaks and their redshift evolution may shed interesting insights on the origin of both types of NCC clusters and the evolution history of thermodynamics and AGN activity in clusters.

MID-INFRARED SPECTROSCOPY OF HIGH-REDSHIFT 3CRR SOURCES

Using the Spitzer Space Telescope, we have obtained rest frame 9-16mu spectra of 11 quasars and 9 radio galaxies from the 3CRR catalog at redshifts 1.0<z<1.4. This complete flux-limited 178MHz-selected sample is unbiased with respect to orientation and therefore suited to study orientation-dependent effects in the most powerful active galactic nuclei (AGN). The mean radio galaxy spectrum shows a clear silicate absorption feature (tau_9.7mu = 1.1) whereas the mean quasar spectrum shows silicates in emission. The mean radio galaxy spectrum matches a dust-absorbed mean quasar spectrum in both shape and overall flux level. The data for individual objects conform to these results. The trend of the silicate depth to increase with decreasing core fraction of the radio source further supports that for this sample, orientation is the main driver for the difference between radio galaxies and quasars, as predicted by AGN unification. However, comparing our high-z sample with lower redshift 3CRR objects reveals that the absorption of the high-z radio galaxy MIR continuum is lower than expected from a scaled up version of lower luminosity sources, and we discuss some effects that may explain these trends.

Spectral energy distribution of hyperluminous infrared galaxies

The relationship between star formation and super-massive black hole growth is central to our understanding of galaxy formation and evolution. Hyper-Luminous Infrared Galaxies (HLIRGs) are unique laboratories to investigate the connection between starburst (SB) and Active Galactic Nuclei (AGN), since they exhibit extreme star formation rates, and most of them show evidence of harbouring powerful AGN. Our previous X-ray study of a sample of 14 HLIRGs shows that the X-ray emission of most HLIRGs is dominated by AGN activity. To improve our estimate of the relative contribution of the AGN and SB emission to its total bolometric output, we have built broad band spectral energy distributions (SEDs) for these HLIRGs, and we have fitted empirical AGN and SB templates to these SEDs. In broad terms, most sources are well fitted using this method, and we found AGN and SB contributions similar to those obtained by previous studies of HLIRGs. We have classified the HLIRGs SEDs in two groups, named class A and class B. Class A HLIRGs show a flat SED from the optical to the infrared energy range. Three out of seven class A sources can be modelled with a pure luminosity-dependent QSO template, while the rest of them require a type 1 AGN template and a SB template. The SB component is dominant in three out of four class A objects. Class B HLIRGs show SEDs with a prominent and broad IR bump. These sources can not trivially be modelled with a combination of pure AGN and pure SB, they require templates of composite objects, suggesting that >50% of their emission comes from stellar formation processes. We propose that our sample is actually composed by three different populations: very luminous QSO, young galaxies going through their maximal star formation period and the high luminosity tail of ULIRG population distribution.

MID-INFRARED SPECTROSCOPY OF CANDIDATE ACTIVE GALACTIC NUCLEI-DOMINATED SUBMILLIMETER GALAXIES

Spitzer spectroscopy has revealed that ~80% of submm galaxies (SMGs) are starburst (SB) dominated in the mid-infrared. Here we focus on the remaining ~20% that show signs of harboring powerful active galactic nuclei (AGN). We have obtained Spitzer-IRS spectroscopy of a sample of eight SMGs which are candidates for harboring powerful AGN on the basis of IRAC color-selection (S8/S4.5>2; i.e. likely power-law mid-infrared SEDs). SMGs with an AGN dominating (>50%) their mid-infrared emission could represent `missing link' sources in an evolutionary sequence involving a major merger. First of all, we detect PAH features in all of the SMGs, indicating redshifts from 2.5-3.4, demonstrating the power of the mid-infrared to determine redshifts for these optically faint dusty galaxies. Secondly, we see signs of both star-formation (from the PAH features) and AGN activity (from continuum emission) in our sample: 62% of the sample are AGN-dominated in the mid-infrared with a median AGN content of 56%, compared with <30% on average for typical SMGs, revealing that our IRAC color selection has successfully singled out sources with proportionately more AGN emission than typical SB-dominated SMGs. However, we find that only about 10% of these AGN dominate the bolometric emission of the SMG when the results are extrapolated to longer infrared wavelengths, implying that AGN are not a significant power source to the SMG population overall, even when there is evidence in the mid-infrared for substantial AGN activity. When existing samples of mid-infrared AGN-dominated SMGs are considered, we find that S8/S4.5>1.65 works well at selecting mid-infrared energetically dominant AGN in SMGs, implying a duty cycle of ~15% if all SMGs go through a subsequent mid-infrared AGN-dominated phase in the proposed evolutionary sequence.

A MULTIWAVELENGTH STUDY OF A SAMPLE OF 70 μm SELECTED GALAXIES IN THE COSMOS FIELD. I. SPECTRAL ENERGY DISTRIBUTIONS AND LUMINOSITIES

We present a large robust sample of 1503 reliable and unconfused 70microm selected sources from the multiwavelength data set of the Cosmic Evolution Survey (COSMOS). Using the Spitzer IRAC and MIPS photometry, we estimate the total infrared luminosity, L_IR (8--1000 microns), by finding the best fit template from several different template libraries. The long wavelength 70 and 160 micron data allow us to obtain a reliable estimate of L_IR, accurate to within 0.2 and 0.05 dex, respectively. The 70 micron data point enables a significant improvement over the luminosity estimates possible with only a 24 micron detection. The full sample spans a wide range in L_IR, L_IR ~ 10^8-10^14 L_sun, with a median luminosity of 10^11.4 L_sun. We identify a total of 687 luminous, 303 ultraluminous, and 31 hyperluminous infrared galaxies (LIRGs, ULIRGs, and HyLIRGs) over the redshift range 0.01<z<3.5 with a median redshift of 0.5. Presented here are the full spectral energy distributions for each of the sources compiled from the extensive multiwavelength data set from the ultraviolet (UV) to the far-infrared (FIR). Using SED fits we find possible evidence for a subset of cooler ultraluminous objects than observed locally. However, until direct observations at longer wavelengths are obtained, the peak of emission and the dust temperature cannot be well constrained. We use these SEDs, along with the deep radio and X-ray coverage of the field, to identify a large sample of candidate active galactic nuclei (AGN). We find that the fraction of AGN increases strongly with L_IR, as it does in the local universe, and that nearly 70% of ULIRGs and all HyLIRGs likely host a powerful AGN.

Influence of AGN jets on the magnetized ICM

Abstract Galaxy clusters are the largest structures for which there is observational evidence of a magnetized medium. Central cores seem to host strong magnetic fields ranging from a few 0.1 μG up to several 10 μG in cooling flow clusters. Numerous clusters harbour central powerful active galactic nuclei (AGN), which are thought to prevent cooling flows in some clusters. The influence of such feedback on the magnetic field remains unclear: does the AGN-induced turbulence compensate for the loss of magnetic amplification within a cool core? And how is this turbulence sustained over several Gyr? Using high-resolution magnetohydrodynamical simulations of the self-regulation of a radiative cooling cluster, we study for the first time the evolution of the magnetic field within the central core in the presence of a powerful AGN jet. It appears that the jet-induced turbulence strongly amplifies the magnetic amplitude in the core beyond the degree to which it would be amplified by pure compression in the gravitational field of the cluster. The AGN produces a non-cooling core and increases the magnetic field amplitude in good agreement with μG field observations.

COSMIC EVOLUTION OF RADIO SELECTED ACTIVE GALACTIC NUCLEI IN THE COSMOS FIELD

We explore the cosmic evolution of luminous active galactic nuclei (AGNs) with low powers (L_(1.4 GHz) ≾ 5 × 10^(25) W Hz^(–1)) out to z = 1.3 using to date the largest sample of ~600 low-luminosity AGN at intermediate redshift drawn from the VLA-COSMOS survey. We derive the radio-luminosity function for these AGNs, and its evolution with cosmic time assuming two extreme cases: (1) pure luminosity and (2) pure density evolution. The former and latter yield L_* α (1 + z)^(0.8 ± 0.1), and Φ_* α (1 + z)^(1.1 ± 0.1), respectively, both implying a fairly modest change in properties of low-radio-power AGNs since z = 1.3. We show that this is in stark contrast with the evolution of powerful (L_(1.4 GHz) > 5 × 10^25 W Hz^(–1)) AGN over the same cosmic time interval, constrained using the 3CRR, 6CE, and 7CRS surveys by Willot et al. We demonstrate that this can be explained through differences in black hole fueling and triggering mechanisms, and a dichotomy in host galaxy properties of weak and powerful AGNs. Our findings suggest that high- and low-radio-power AGN activities are triggered in different stages during the formation of massive red galaxies. We show that weak AGN occur in the most massive galaxies already at z ~ 1, and they may significantly contribute to the heating of their surrounding medium and thus inhibit gas accretion onto their host galaxies, as recently suggested for the radio mode in cosmological models.

GIANT AGN FLARES AND COSMIC RAY BURSTS

We predict a new class of very intense, short-duration active galactic nucleus (AGN) flares capable of accelerating the highest energy cosmic rays, resulting from the tidal disruption of a star or from a disk instability. The rate and power of these flares readily explains the observed flux and density statistics of ultrahigh energy cosmic ray. The photon bursts produced by the predicted AGN flares are discussed; they may soon be detectable. Observations are shown to exclude that continuous jets of powerful AGNs are the sole source of ultrahigh energy cosmic rays; the stringent requirements for gamma-ray bursts to be the source are delineated.

THE LOPSIDEDNESS OF PRESENT-DAY GALAXIES: CONNECTIONS TO THE FORMATION OF STARS, THE CHEMICAL EVOLUTION OF GALAXIES, AND THE GROWTH OF BLACK HOLES

We have used the Sloan Digital Sky Survey (SDSS) to undertake an investigation of lopsidedness in a sample of ~25,000 nearby galaxies (z < 0.06). We use the m=1 azimuthal Fourier mode between the 50% and 90% light radii as our measure of lopsidedness. The SDSS spectra are used to measure the properties of the stars, gas, and black hole in the central-most few-kpc-scale region. We show that there is a strong link between lopsidedness in the outer parts of the galactic disk and the youth of the stellar population in the central region. This link is independent of the other structural properties of the galaxy. These results provide a robust statistical characterization of the connections between accretion/interactions/mergers and the resulting star formation. We also show that residuals in the galaxy mass-metallicity relation correlate with lopsidedness (at fixed mass, the more metal-poor galaxies are more lopsided). This suggests that the events causing lopsidedness and enhanced star formation deliver lower metallicity gas into the galaxy's central region. Finally, we find that there is a trend for the more powerful active galactic nuclei to be hosted by more lopsided galaxies (at fixed galaxy mass, density, or concentration). However if we compare samples matched to have both the same structures and central stellar populations, we then find no difference in lopsidedness between active and non-active galaxies. This leads to the following picture. The presence of cold gas in the central region of a galaxy (irrespective of its origin) is essential for both star-formation and black hole growth. The delivery of cold gas is aided by the processes that produce lopsidedness. Other processes on scales smaller than we can probe with our data are required to transport the gas to the black hole.

GAMMA-RAY EMISSION OF RELATIVISTIC JETS AS A SUPERCRITICAL PROCESS

Supercriticality of the same kind as that in a nuclear pile can take place in high-energy astrophysical objects producing a number of impressive effects. For example, it could cause an explosive release of the energy of a cloud of ultrarelativistic protons into radiation. More certainly, supercriticality should be responsible for energy dissipation of very energetic relativistic fluids such as ultrarelativistic shocks in gamma-ray bursts and jets in active galactic nuclei (AGNs). In this case, the photon breeding process operates. It is a kind of converter mechanism with the high-energy photons and e+e- pairs converting into each other via pair production and inverse Compton scattering. Under certain conditions, which should be satisfied in powerful AGNs, the photon breeding mechanism becomes supercritical: the high-energy photons breed exponentially until their feedback on the fluid changes its velocity pattern. Then the system comes to a self-adjusting near-critical steady state. Monte-Carlo simulations with detailed treatment of particle propagation and interactions demonstrate that a jet with a Lorentz factor Γ ≈ 20 can radiate away up to a half of its total energy, and for Γ = 40 the radiation efficiency can be up to 80 per cent. Outer layers of the jet decelerate down to a moderate Lorentz factor 2–4, while the spine of the jet has a final Lorentz factor in the range 10–20 independent of the initial Γ. Such sharp deceleration under the impact of radiation must cause a number of interesting phenomena such as formation of internal shocks and an early generation of turbulence.