Radio Feedback Research Articles

Molecular flows in contemporary active galaxies and the efficacy of radio-mechanical feedback

ABSTRACT Molecular gas flows are analysed in 14 cluster galaxies (BCGs) centred in cooling hot atmospheres. The BCGs contain $10^{9}\!-\!10^{11}~\rm M_\odot$ of molecular gas, much of which is being moved by radio jets and lobes. The molecular flows and radio jet powers are compared to molecular outflows in 45 active galaxies within z < 0.2. We seek to understand the relative efficacy of radio, quasar, and starburst feedback over a range of active galaxy types. Molecular flows powered by radio feedback in BCGs are ∼10–1000 times larger in extent compared to contemporary galaxies hosting quasar nuclei and starbursts. Radio feedback yields lower flow velocities but higher momenta compared to quasar nuclei, as the molecular gas flows in BCGs are usually ∼10–100 times more massive. The product of the molecular gas mass and lifting altitude divided by the AGN or starburst power – a parameter referred to as the lifting factor – exceeds starbursts and quasar nuclei by 2–3 orders of magnitude, respectively. When active, radio feedback is generally more effective at lifting gas in galaxies compared to quasars and starburst winds. The kinetic energy flux of molecular clouds generally lies below and often substantially below a few per cent of the driving power. We find tentatively that star formation is suppressed in BCGs relative to other active galaxies, perhaps because these systems rarely form molecular discs that are more impervious to feedback and are better able to promote star formation.

On the environments of giant radio galaxies

ABSTRACT We test the hypothesis that environments play a key role in enabling the growth of enormous radio structures spanning more than 700 kpc, an extreme population of radio galaxies called giant radio galaxies (GRGs). To achieve this, we explore (1) the relationships between the occurrence of GRGs and the surface number density of surrounding galaxies, including satellite galaxies and galaxies from neighbouring haloes, and (2) the GRG locations towards large-scale structures. The analysis is done by making use of a homogeneous sample of 110 GRGs detected from the LOFAR Two-metre Sky Survey in combination with photometric galaxies from the DESI Legacy Imaging Surveys and a large-scale filament catalogue from the Sloan Digital Sky Survey. Our results show that the properties of galaxies around GRGs are similar with that around the two control samples, consisting of galaxies with optical colours and luminosity matched to the properties of the GRG host galaxies. Additionally, the properties of surrounding galaxies depend on neither their relative positions to the radio jet/lobe structures nor the sizes of GRGs. We also find that the locations of GRGs and the control samples with respect to the nearby large-scale structures are consistent with each other. These results demonstrate that there is no correlation between the GRG properties and their environments traced by stars, indicating that external galaxy environments are not the primary cause of the large sizes of the radio structures. Finally, regarding radio feedback, we show that the fraction of blue satellites does not correlate with the GRG properties, suggesting that the current epoch of radio jets has minimal influence on the nature of their surrounding galaxies.

Perceptions of why Malawians engage in concurrent sexual partnerships among a select population of radio listeners: findings from an exploratory study

Concurrent sexual partnerships have been identified as a potential driver in the HIV epidemic in Southern Africa. This study utilised an innovative approach to explore perceptions of why Malawians may engage in these relationships, and their suggestions for reducing the practice among a select population of radio listeners. Using radio listener feedback in the form of text messages, we analysed approximately 1 000 text messages sent by individuals who listened to a reality radio programme that included real stories, told by Malawians, on topics related to HIV/AIDS. Listeners suggested that lack of satisfaction with one’s partner was the overarching reason why individuals had concurrent sexual partnerships. Within the context of lack of satisfaction, listeners cited alcohol use, poor communication and gendered norms as factors related to satisfaction. Listeners suggested that couple communication could increase satisfaction, which, in turn, could reduce concurrent sexual partnerships. Prevention efforts should consider how to utilise couple communication to improve satisfaction as an approach to reduce HIV risk in Southern Africa.

GALAXIES ON DIET: FEEDBACK SIGNATURES IN RADIO-AGN HOST GALAXIES

There exists strong evidence supporting the co-evolution of central supermassive black holes and their host galaxies; however it is still under debate how such a relation comes about and whether it is relevant for all or only a subset of galaxies. An important mechanism connecting AGN to their host galaxies is AGN feedback, potentially heating up or even expelling gas from galaxies. AGN feedback may hence be responsible for the eventual quenching of star formation and halting of galaxy growth. A rich multi-wavelength dataset ranging from the X-ray regime (Chandra), to far-IR (Herschel), and radio (WSRT) is available for the North Ecliptic Pole field, most notably surveyed by the AKARI infrared space telescope, covering a total area on the sky of 5.4 sq. degrees. We investigate the star formation properties and possible signatures of radio feedback mechanisms in the host galaxies of 237 radio sources below redshift z

Through the kaleidoscope: star formation the host galaxies of radio-AGN

AbstractThere exist strong evidence supporting the co-evolution of central supermassive black holes and their host galaxies; however it is still under debate how such a relation comes about and whether it is relevant for all or only a subset of galaxies. A rich multi-wavelength dataset is available for the North Ecliptic Pole field, most notably surveyed by the AKARI infrared space telescope. We investigate the star-formation properties of the host galaxies of radio-AGN together with the radio feedback mechanism, potentially responsible for the eventual quenching of star formation. Using broadband SED modelling, the nuclear and host galaxy components of these sources are studied as a function of their radio luminosity. Here we present results concerning the AGN content of the radio sources in this field, while offering evidence supporting a “maintenance” type of feedback from powerful radio-jets.

On the radio luminosity distribution of active galactic nuclei and the black hole fundamental plane

We have studied the dependence of the nuclear radio (1.4 GHz) luminosity of active galactic nuclei (AGN) on both 2–10 keV X-ray and host-galaxy K-band luminosities. A complete sample of 1268 X-ray-selected AGN (both type 1 and type 2) has been used, which is the largest catalogue of AGN belonging to statistically well-defined samples where radio, X-ray and K-band information exists. At variance with previous studies, radio upper limits have been taken into account statistically using a Bayesian maximum-likelihood fitting method. A good fit is obtained assuming a plane in 3D LR–LX–LK space, namely log LR = ξX log LX + ξK log LK + ξ0, with a ∼1 dex wide (1σ) spread in radio luminosity. As already shown by La Franca, Melini & Fiore, no evidence of bimodality in the radio luminosity distribution was found and therefore any definition of radio-loudness in AGN is arbitrary. Using scaling relations between the black hole (BH) mass and the host galaxy K-band luminosity, we have also derived a new estimate of the BH fundamental plane (in L5 GHz–LX–MBH space). Our analysis shows that previous measures of the BH fundamental plane are biased by ∼0.8 dex in favour of the most luminous radio sources. Therefore, many AGN studies, where the BH fundamental plane is used to investigate how AGN regulate their radiative and mechanical luminosity as a function of the accretion rate, and many AGN/galaxy co-evolution models, where radio feedback is computed using the AGN fundamental plane, should revise their conclusions.

The jet-cloud interacting radio galaxy PKS B2152−699 - I. Structures revealed in new deep radio and X-ray observations

PKS B2152-699 has radio power characteristic of sources that dominate radio feedback. We present new deep ATCA, Chandra and optical observations, and test the feedback model. We report the first high-resolution observations of the radio jet. The inner jet extends ~8.5 kpc towards an optical emission-line High Ionization Cloud (HIC) before taking a zig-zag path to an offset position. Jet X-ray synchrotron radiation is seen. The HIC is associated with 0.3 keV X-ray gas of anomalously low metallicity. On larger scales the radio galaxy displays all three X-ray features that together confirm supersonic expansion of the lobes into the external medium: gas cavities, inverse-Compton emission showing excess internal lobe pressure, and high-contrast arms of temperature above the ~1 keV ambient medium. The well-formed S lobe on the counterjet side is expanding with a Mach number 2.2-3. We estimate a cavity power ~3x10^43 ergs/s, which falls well below previously reported correlations with radio power. The total inferred time-averaged jet power, ~4x10^44 ergs/s, is dominated by the kinetic and thermal energy of shocked gas, and if used instead would bring the source into better agreement with the correlations. The S hotspot is the more complex, with a spiral polarization structure. Its bright peak emits synchrotron X-rays. The fainter N hotspot is particularly interesting, with X-rays offset in the direction of the incoming jet by ~1 arcsec relative to the radio peak. Here modest (delta ~ 6) relativistic beaming and a steep radio spectrum cause the jet to be X-ray bright through inverse-Compton scattering before it decelerates. With such beaming, a modest proton content or small departure from minimum energy in the jet will align estimates of the instantaneous and time-averaged jet power. The hotspots suggest acceleration of electrons to a maximum energy ~10^13 eV in the jet termination shocks.

Efficient Selective Feedback Design for Multicell Cooperative Networks

Multicell cooperative processing (MCP) has been recognized as a promising technique for increasing the spectral efficiency of future wireless systems. Unfortunately, the provided benefits of MCP come at the cost of increased radio feedback and backhaul overhead; for downlink transmission in frequency-division duplexing (FDD) systems, users need to feed back their channel-state information (CSI) to the MCP scheduler, and user data need to be exchanged between all cooperating base stations (BSs) through the backhaul network. In the context of conventional noncooperative networks, it has been suggested that radio feedback load could be reduced by preventing users with low-quality channels from feeding back their CSI (concept of selective feedback), at the cost of a small fraction of the multiuser diversity gain. In this paper, we investigate the translation of this selective feedback concept to MCP systems. According to this technique, users with weak interference links are prevented from feeding back their full CSI to the MCP scheduler. Although efficient, this technique alone cannot mitigate the backhaul overhead related to routing user data possibly to several BSs. To overcome this condition, we propose two schemes: one scheme based on Media Access Control (MAC)-layer scheduling and the other scheme based on physical-layer precoding. These schemes, combined with feedback load reduction, allow for a substantial mitigation of the MCP overheads related to feedback and backhaul load. We analyze the improvements in terms of both user-to-BS feedback reduction and backhaul load reduction, and we show that this framework results in a good tradeoff between performance and overhead for multicell cooperative networks.

On Active Learning and Supervised Transmission of Spectrum Sharing Based Cognitive Radios by Exploiting Hidden Primary Radio Feedback

This paper studies the wireless spectrum sharing between a pair of distributed primary radio (PR) and cognitive radio (CR) links. Assuming that the PR link adapts its transmit power and/or rate upon receiving an interference signal from the CR and such transmit adaptations are observable by the CR, this results in a new form of feedback from the PR to CR, refereed to as hidden PR feedback, whereby the CR learns the PR's strategy for transmit adaptations without the need of a dedicated feedback channel from the PR. In this paper, we exploit the hidden PR feedback to design new learning and transmission schemes for spectrum sharing based CRs, namely active learning and supervised transmission. For active learning, the CR initiatively sends a probing signal to interfere with the PR, and from the observed PR transmit adaptations the CR estimates the channel gain from its transmitter to the PR receiver, which is essential for the CR to control its interference to the PR during the subsequent data transmission. This paper proposes a new transmission protocol for the CR to implement the active learning and the solutions to deal with various practical issues for implementation, such as time synchronization, rate estimation granularity, power measurement noise, and channel variation. Furthermore, with the acquired knowledge from active learning, the CR designs a supervised data transmission by effectively controlling the interference powers both to and from the PR, so as to achieve the optimum performance tradeoffs for the PR and CR links. Numerical results are provided to evaluate the effectiveness of the proposed schemes for CRs under different system setups.

EVERY BCG WITH A STRONG RADIO AGN HAS AN X-RAY COOL CORE: IS THE COOL CORE-NONCOOL CORE DICHOTOMY TOO SIMPLE?

Radio AGN feedback in X-ray cool cores has been proposed as a crucial ingredient in the evolution of baryonic structures. However, it has long been known that strong radio AGN also exist in "noncool core" clusters, which brings up the question whether an X-ray cool core is always required for radio feedback. In this work, we present a systematic analysis of BCGs and strong radio AGN in 152 groups and clusters from the Chandra archive. All 69 BCGs with radio AGN more luminous than 2x10^23 W Hz^-1 at 1.4 GHz are found to have X-ray cool cores. The BCG cool cores can be divided into two classes, the large-cool-core (LCC) class and the corona class. Small coronae, easily overlooked at z>0.1, can trigger strong heating episodes in groups and clusters, long before large cool cores are formed. Strong radio outbursts triggered by coronae may destroy embryonic large cool cores and thus provide another mechanism to prevent formation of large cool cores. However, it is unclear whether coronae are decoupled from the radio feedback cycles as they have to be largely immune to strong radio outbursts. Our sample study also shows the absence of groups with a luminous cool core while hosting a strong radio AGN, which is not observed in clusters. This points to a greater impact of radio heating on low-mass systems than clusters. As examples of the corona class, we also present detailed analyses of a BCG corona associated with a strong radio AGN (ESO 137-006 in A3627) and one of the faintest coronae known (NGC 4709 in the Centaurus cluster). Our results suggest that the traditional cool core / noncool core dichotomy is too simple. A better alternative is the cool core distribution function, with the enclosed X-ray luminosity or gas mass.

Feedback from AGN: The Kinetic/Radio Luminosity Function

We have measured the probability distribution function of the ratio RX = log L1.4/LX, where L1.4/LX = ν Lν(1.4 GHz)/LX(2–10 keV), between the 1.4 GHz and the unabsorbed 2–10 keV luminosities and its dependence on LX and z. We have used a complete sample of ~1800 hard X-ray selected AGN, observed in the 1.4 GHz band, cross-correlated in order to exclude FR II-type objects, and thus obtain a contemporaneous measure of the radio and X-ray emission. The distribution P(RX|LX,z) is shown in Figure 1. Convolution of the distribution P(RX|LX,z) with the 2–10 keV X-ray AGN luminosity function from La Franca et al. (2005) and the relations between radio power and kinetic energy from Best et al. (2006) and Willott et al. (1999) allows us to derive the AGN kinetic power and its evolution. As shown in Figure 1, our results are in good agreement with the predictions of the most recent models of galaxy formation and evolution (e.g., Croton et al. 2006), where AGN radio feedback is required to quench the star formation.

Obscured and powerful AGN and starburst activities at z ~ 3.5

Aims. Short phases of coeval powerful starburst and AGN activity during the lifetimes of the most massive galaxies are predicted by various models of galaxy formation and evolution. In spite of their recurrence and high luminosity, such events are rarely observed. Finding such systems, understanding their nature, and constraining their number density can provide key constraints to galaxy evolutionary models and insights into the interplay between starburst and AGN activities. Methods. We report the discovery of two sources at z = 3.867 and z = 3.427 that exhibit both powerful starburst and AGN activities. They benefit from multi-wavelength data from radio to X rays from the CFHTLS-D1/SWIRE/XMDS surveys. Follow-up optical and near-infrared spectroscopy, and millimeter IRAM/MAMBO observations are also available. We performed a multi-wavelength analysis of their spectral energy distributions with the aim of understanding the origin of their emission and constraining their luminosities. A comparison with other composite systems at similar redshifts from the literature is also presented. Results. The AGN and starburst bolometric luminosities are ∼10 13 L� . The AGN emission dominates at X ray, optical, mid-infrared wavelengths, and probably also in the radio. The starburst emission dominates in the far-infrared. The estimated star formation rates range from 500 to 3000 M� /yr. The AGN near-infrared and X ray emissions are heavily obscured in both sources with an estimated dust extinction AV ≥ 4, and Compton-thick gas column densities. The two sources are the most obscured and most luminous AGNs detected at millimeter wavelengths currently known. Conclusions. The sources presented in this work are heavily obscured QSOs, but their properties are not fully explained by the standard AGN unification model. In one source, the ultraviolet and optical spectra suggest the presence of outflowing gas and shocks, and both sources show emission from hot dust, most likely in the vicinity of the nucleus. Evidence of moderate, AGN-driven radio activity is also found in both sources. Based on the estimated stellar and black hole masses, the two sources lie on the local MBH−Mbulge relation. To remain on this relation as they evolve, their star formation rate has to decrease or stop. Our results support evolutionary models that invoke radio feedback such as the star formation quenching mechanism, and suggest that such a mechanism might play a major role also in powerful AGNs.

The impact of radio feedback from active galactic nuclei in cosmological simulations: formation of disc galaxies

In this paper, we present a new implementation of feedback due to active galactic nuclei (AGN) in cosmological simulations of galaxy formation. We assume that a fraction of jet energy, which is generated by an AGN, is transferred to the surrounding gas as thermal energy. Combining a theoretical model of mass accretion onto black holes with a multiphase description of star-forming gas, we self-consistently follow evolution of both galaxies and their central black holes. The novelty in our model is that we consider two distinct accretion modes: standard radiatively efficient thin accretion disks and radiatively inefficient accretion flows which we will generically refer to as RIAFs; motivated by theoretical models for jet production in accretion disks, we assume that only the RIAF is responsible for the AGN feedback. We find that, after an initial episode of bursting star formation, the accretion rate onto the central black hole drops so that the accretion disk switches to a RIAF structure. At this point, the feedback from the AGN becomes efficient and slightly suppresses star formation in the galactic disk and almost completely halts star formation in the bulge. As a result, the nucleus becomes a stochastically fuelled low-luminosity AGN (Seyfert galaxy) with recurrent short-lived episodes of activity after the star bursts. Our model predicts several properties of the low-luminosity AGN including the bolometric luminosity, jet powers, the effect on kpc-scale of the radio jet and the AGN lifetime, which are in broad agreement with observations of Seyfert galaxies and their radio activity. We also find that the mass ratios between the central black hole and the the host spheroid at z = 0 are ~10^{-3} regardless of the strength of either supernova feedback or AGN feedback. (abridged)

Galaxy formation and evolution

We take a multi-faceted approach to study galaxy populations in the local universe, using the completed Two Degree Field Galaxy Redshift Survey (2dFGRS), the ``Millennium Run'' LCDM N-body simulation, and a semi-analytic model of galaxy formation. Our investigation covers both small and large scale aspects of the galaxy distribution. This work can be broken into three sections, outlined below. Using the 2dFGRS we explore the higher-order clustering properties of local galaxies to quantify both (i) the linear and non-linear bias of the distribution relative to the underlying matter field, and (ii) the nature of hierarchical scaling in the clustering moments of the galaxy distribution. This last point is the expected signature of an initially Gaussian distribution of matter density fluctuations that evolved under the action of gravitational instability. We show in Chapters 2, 3, and 4 that the 2dFGRS higher-order clustering moments are indeed hierarchical, which we measure up to sixth order for galaxies brighter than M_bJ-5log10 h=-17 and which sample the survey volume out to z~0.3. The moments are found to be well described by the negative binomial probability distribution function, and we rule out, at high significance, other models of galaxy clustering, such as the lognormal distribution. This result holds in redshift space on all scales where we obtain a good statistical signal, typically 0.5< R (h^-1 Mpc) < 30 (i.e. from strongly non-linear to quasi-linear regimes). Interestingly, we find that the moments on larger scales can be significantly altered by two massive superclusters present in the 2dFGRS. The skewness of the galaxy distribution is found to have a weak dependence on galaxy luminosity. We show that a simple linear biasing model provides an inadequate description of the higher order results, suggesting that non-linear biasing is present in the clustering moments of the 2dFGRS. The large-scale distribution of structure within the 2dFGRS allows us to study the properties of the galaxy population as a function of local environment. In Chapter 5 we measure the luminosity function of early and late-types galaxies in survey regions ranging from sparse voids to dense clusters to reveal the dominant population in each. Fitting each luminosity function with a Schechter function allows us to quantify how the bright and faint populations transform with changing density contrast. We find that (i) the population in voids is dominated by late types, with a noticeable deficit of intermediate and bright galaxies relative to the mean, and (ii) cluster regions have an excess of very bright early-type galaxies relative to the mean. When directly comparing faint early and late type galaxies in void and cluster regions, the cluster population shows comparable abundances of both types, whereas in voids the late types dominate by almost an order of magnitude. Of interest to many galaxy formation models is our measurement that reveals that the faint-end slope of the overall luminosity function depends at most weakly on density environment. Finally, in Chapter 6, we develop a self-consistent model of galaxy formation and couple this to the Millennium Run LCDM N-body simulation. This simulation represents a significant step forward in both size and resolution, allowing us to follow the the complete evolutionary histories of approximately 20 million galaxies down to luminosities as faint as the Small Magellanic Cloud in a volume comparable to that sampled by the 2dFGRS. In our galaxy formation model we supplement previous treatments of the growth and activity of central black holes with a new model for `radio' feedback from those active galactic nuclei that lie at the centre of a quasistatic X-ray emitting atmosphere in a galaxy group or cluster. With this we can simultaneously explain (i) the low observed mass drop-out rate in cooling flows, (ii) the exponential cut-off at the bright end of the galaxy luminosity function, and (iii) the fact that the most massive galaxies tend to be bulge-dominated systems in clusters and contain systematically older stars than lower mass galaxies. This success occurs because static hot atmospheres form only in the most massive structures, and radio feedback (in contrast, for example, to supernova or starburst feedback) can suppress further cooling and thus star formation without itself requiring star formation. Matching galaxy formation models with such observations has previously proved quite challenging.

The many lives of active galactic nuclei: cooling flows, black holes and the luminosities and colours of galaxies

We simulate the growth of galaxies and their central supermassive black holes by implementing a suite of semi-analytic models on the output of the Millennium Run, a very large simulation of the concordance LCDM cosmogony. Our procedures follow the detailed assembly history of each object and are able to track the evolution of all galaxies more massive than the Small Magellanic Cloud throughout a volume comparable to that of large modern redshift surveys. In this first paper we supplement previous treatments of the growth and activity of central black holes with a new model for `radio' feedback from those AGN that lie at the centre of a quasistatic X-ray emitting atmosphere in a galaxy group or cluster. We show that for energetically and observationally plausible parameters such a model can simultaneously explain: (i) the low observed mass drop-out rate in cooling flows; (ii) the exponential cut-off at the bright end of the galaxy luminosity function; and (iii) the fact that the most massive galaxies tend to be bulge-dominated systems in clusters and to contain systematically older stars than lower mass galaxies. This success occurs because static hot atmospheres form only in the most massive structures, and radio feedback (in contrast, for example, to supernova or starburst feedback) can suppress further cooling and star formation without itself requiring star formation. We discuss possible physical models which might explain the accretion rate scalings required for our phenomenological `radio mode' model to be successful.

Qos enhancement for adaptive streaming services over wcdma

This paper presents a new method to adapt the quality-of-service (QoS) in a wideband code-division multiple-access (WCDMA) system, called radio network feedback (RNF). The concept of RNF is in general applicable to all the services requiring a minimum quality (i.e., non best effort) and in this paper, the focus is on streaming. In this context, RNF makes it possible for a streaming server to adapt its source bit rate to a WCDMA radio link, whose bandwidth may vary in time due, for example, to decongestion/congestion situations over the air interface or to handover. This is very beneficial for the end user as it allows to increase the bandwidth (i.e., the quality) when possible and to decrease it (instead of just dropping the service) when needed. RNF was compared with Client-based adaptation solutions. Simulation results show that RNF is fast and accurate and performs better than Client-based adaptation. For example, rebuffering occurs only with Client-based method. Moreover, with RNF the up-switch is performed only when needed, whereas the up-switch performed by means of the Client-based method is often erroneous and may annoy the end user.