1,721,066 research outputs found
Radio mini-halos and AGN heating in cool core clusters of galaxies
Full text linkThe brightest cluster galaxy (BCG) in the majority of relaxed, cool core galaxy clusters is radio loud, showing non-thermal radio jets and lobes ejected by the central active galactic nucleus (AGN). Such relativistic plasma has been unambiguously shown to interact with the surrounding thermal intra-cluster medium (ICM) thanks to spectacular images where the lobe radio emission is observed to fill the cavities in the X-ray-emitting gas. This `radio-mode AGN feedback' phenomenon, which is thought to quench cooling flows, is widespread and is critical to understand the physics of the inner regions of galaxy clusters and the properties of the central BCG. At the same time, mechanically-powerful AGN are likely to drive turbulence in the central ICM which may contribute to gas heating and also play a role for the origin of non-thermal emission on cluster-scales. Diffuse non-thermal emission has been observed in a number of cool core clusters in the form of a radio mini-halo surrounding the radio-loud BCG on scales comparable to that of the cooling region. This contribution outlines the main points covered by the talk on these topics. In particular, after summarizing the cooling flow regulation by AGN heating and the non-thermal emission from cool core clusters, we present a recent study of the largest collection of known mini-halo clusters (~ 20 objects) which investigated the scenario of a common origin of radio mini-halos and gas heating. We further discuss the prospects offered by future radio surveys with the Square Kilometre Array (SKA) for building large (>> 100 objects), unbiased mini-halo samples while probing at the same time the presence of radio-AGN feedback in the host clusters
Discovery of a fourth arc in Abell 2626 at 610 MHz with the GMRT: Spectral properties and possibilities for the origin
No full textWe report the discovery of a fourth eastern arc (Arc E) towards the cool-core cluster Abell 2626 using 610 MHz Giant Metrewave Radio Telescope observations. Three arcs towards north, west and south were known from earlier works at 1400 MHz and proposed to have originated in precessing radio jets of the central active galactic nucleus. The 610 - 1400 MHz integrated spectral indices of the arcs are in the range 3.2 - 3.6 and the spectral index map shows uniform distribution along the lengths of the arcs. If associated with A2626, the arcs have linear extents in the range 79 - 152 kpc. The detection of Arc E favours the scenario in which a pair of bipolar precessing jets were active and halted to produce the arc system. Based on the morphological symmetry and spectral similarity, we indicate a possible role of gravitational lensing. Further high resolution low frequency observations and measurements of the mass of the system are needed to disentangle the mystery of this source
Evidence for AGN Feedback in Galaxy Clusters and Groups
Full text linkThe current generation of flagship X-ray missions, Chandra and XMM-Newton, has changed our understanding of the so-called “cool-core” galaxy clusters and groups. Instead of the initial idea that the thermal gas is cooling and flowing toward the center, the new picture envisages a complex dynamical evolution of the intracluster medium (ICM) regulated by the radiative cooling and the nongravitational heating from the active galactic nucleus (AGN). Understanding the physics of the hot gas and its interplay with the relativistic plasma ejected by the AGN is key for understanding the growth and evolution of galaxies and their central black holes, the history of star formation, and the formation of large-scale structures. It has thus become clear that the feedback from the central black hole must be taken into account in any model of galaxy evolution. In this paper, we draw a qualitative picture of the current knowledge of the effects of the AGN feedback on the ICM by summarizing the recent results in this field
The Complete Local-volume Groups Sample - II. A study of the central radio galaxies in the high-richness sub-sample
Full text linkWe present a study of the radio properties of the dominant early-type galaxies in 26 galaxy groups, the high-richness sub-sample of theComplete Local-volumeGroups Sample (CLoGS). Combining new 610 and 235 MHz observations of 21 groups from the Giant Metrewave Radio Telescope (GMRT) with archival GMRT and Very Large Array (VLA) survey data, we find a high detection rate, with 92 per cent of the dominant galaxies hosting radio sources. The sources have a wide range of luminosities, 1020-1024 WHz-1 in the 235 and 610 MHz bands. Themajority (54 per cent) are point-like, but 23 per cent have radio jets, and another 15 per cent are diffuse radio sources with no clear jet/lobe morphology. Star formation may dominate the radio emission in 2 of the point-like systems and may make a significant contribution to a further 1-3, but is unlikely to be important in the remaining 21 galaxies. The spectral index of the detected radio sources ranges from very flat values of ~0.2 to typical radio synchrotron spectra of ~0.9 with only two presenting steep radio spectra with α235610 > 1. We find that jet sources are more common in X-ray bright groups, with radio non-detections found only in X-ray faint systems. Radio point sources appear in all group environments irrespective of their X-ray properties or spiral fraction.We estimate the mechanical power (Pcav) of the jet sources in the X-ray bright groups to be 1041-1043 erg s-1, with the two large-scale jet systems (NGC 193 and NGC 4261) showing jet powers two orders of magnitude greater than the radiative losses from the cool cores of their groups. This suggests that central AGN are not always in balance with cooling, but may instead produce powerful periodical bursts of feedback heating
A comprehensive study of the AGN feedback cycle in galaxy clusters from high resolution X-ray and radio observations
Full text linkAt the center of galaxy clusters, a dramatic interplay known as feedback cycle occurs between the hot intracluster medium (ICM) and the active galactic nucleus (AGN) of the central galaxy. The footprints of this interplay are evident from X-ray observations of the ICM, where X-ray cavities and shock fronts are associated with radio lobe emission tracing energetic AGN outbursts. While such jet activity reduces the efficiency of the hot gas to cool to lower temperatures, residual cooling can generate warm and cold gas clouds around the central galaxy. The condensed gas parcels can ultimately reach the core of the galaxy and be accreted by the AGN. This picture is the result of tremendous advances over the last three decades. Yet, a deeper understanding of the details of how the heating–cooling regulation is achieved and maintained is still missing. In this Thesis, we delve into key aspects of the feedback cycle. To this end, we leverage high-resolution (sub-arcsecond), multifrequency observations (mainly X-ray and radio) of several top-level facilities (e.g., Chandra, JVLA, VLBA, LOFAR). First, we investigate which conditions trigger a feedback response to gas cooling, by studying the properties of clusters where feedback is just about to start. Then, we focus on the details of how the AGN–ICM interaction progresses by examining cavity and shock heating in the cluster RBS797, an exemplary case of the jet feedback paradigm. Furthermore, we explore the importance of shock heating and the coupling of distinct jet power regimes (i.e., FRII, FRI and FR0 radio galaxies) to the environment. Ultimately, as heating models rely on the connection between the direct evidence (the jets) and the smoking gun (the X-ray cavities) of feedback, we examine the cases in which these two are dramatically misaligned
Connection between x-ray and radio properties of cool-core galaxy clusters hosting radio mini-halos
No full textGli ammassi di galassie sono gli oggetti gravitazionalmente legati più grandi dell’Universo. Questi emettono principalmente in banda X tramite bremsstrahlung. Una frazione mostra anche emissione radio diffusa da parte di elettroni relativistici che spiraleggiano nel campo magnetico. Si possono classificare tre tipi di emissione: alon, relitti e mini-aloni radio (MH). I MH sono sorgenti radio su scale di ≥ 200 – 500 kpc, osservate al centro di ammassi caratterizzati dalla presenze di cool-core (CC).
L’origine dei MH non è ancora chiara. Gli elettroni relativistici che emettono in banda radio hanno tempi di vita radiativi di molto inferiori a quelli necessari per diffondere sulle scale dell’emissione diffusa. Quindi non sono semplicemente iniettati dalle galassie presenti negli ammassi ed è necessario un meccanismo di accelerazione “in-situ” nell’ICM. I MH testimoniano la presenza di meccanismi che canalizzano parte del budget energetico disponibile nei CC nell’ICM.Quindi lo studio è importante per comprendere la fisica dell’ICM e l’interazione fra le componenti non termiche e termiche. I MH si formano attraverso la riaccelerazione delle particelle relativistiche ad opera della turbolenza del gas. L’origine di questa turbolenza tuttavia non è ancora ben compresa.
Gli ammassi CC sono caratterizzati da un picco della brillanza X nelle regioni centrali e da un drop della temperatura verso il centro accompagnata da aumento della densità del gas. Si ritiene che questo sia dovuto al raffreddamento del gas che quindi fluisce nelle zone centrali. Recenti osservazioni in X risultan inconsistenti con il modello classico di CF, suggerendo la presenza di una sorgente di riscaldamento del gas su scale del core degli ammassi. Recentemente Zhuravleva (2014) hanno mostrato che il riscaldamento dovuto alla turbolenza prodotta dall'AGN centrale è in grado di bilanciare il processo di raffreddamento.
Abbiamo assunto che la turbolenza responsabile del riscaldamento del gas è anche responsabile dell’accelerazione delle particelle nei MH.
Nell’ambito di questo scenario ci si aspetta una correlazione tra la potenza del cooling flow, PCF, che è una misura del tasso di energia emessa dal gas che raffredda nei CC, e la luminosità radio, che è una frazione dell’energia della turbolenza che è canalizzata nell’accelerazione delle particelle.
In questo lavoro di tesi abbiamo utilizzato il più grande campione disponibile di MH, allo scopo di studiare la connessione fra le proprietà dei MH e quelle del gas termico nei core degli ammassi che li ospitano. Abbiamo analizzato i dati di 21 ammassi e ricavato i parametri fisici all’interno del raggio di cooling e del MH. Abbiamo ricavato la correlazione fra luminosità radio, e PCF. Abbiamo trovato che le due quantità correlano in modo quasi-lineare confermando i risultati precedenti. Tale correlazione suggerisce uno stretto legame fra le proprietà del gas nei CC e l’origine dei MH
Interplay between relativistic and thermal plasma in relaxed galaxy clusters
Full text linkGalaxy clusters are complex ecosystems, where galaxies with powerful active galactic nuclei, thermal plasma, magnetic fields, and cosmic rays can interact and flourish in spectacular astrophysical phenomena. Decades of studies have investigated these systems, and as the boundaries of our knowledge were pushed forward, new questions emerged awaiting to be answered. The main focus of this Thesis is the interplay between the relativistic and thermal plasma of the intra-cluster medium. The study of the intra-cluster medium (ICM) allows to understand the way energy is injected from large scale dynamics and dissipated in different channels. These channels include viscous heating and
the generation of non-thermal components that, eventually, generate diffuse radio emission. Furthermore, studying the ICM provides complementary insights into the physics of cluster galaxy evolution. We mainly focus on the case of relaxed galaxy clusters, where the connection between the central, massive, radio-loud galaxies and the diffuse radio emission is still unclear. In this context, we investigate also the interactions between the ICM and the cluster galaxies. The cornerstone of our works is a multi-wavelength analysis based on radio and X-ray observations, which features brand-new Low-Frequency Array (LOFAR) observations and new approaches to the study of the spatial correlation between these emissions
L’emissione di sincrotrone e applicazione allo studio di una radiogalassia nell'ammasso RBS 797
Full text linkL'analisi dei processi di emissione della radiazione è alla base dello studio di molti fenomeni fisici che osserviamo nell'universo. Questa tesi tratta l'emissione di radiazione per sincrotrone, un fenomeno di particolare rilevanza in astrofisica che riguarda l'emissione di radiazione da parte di particelle ultra-relativistiche cariche in moto in un campo magnetico. Questo fenomeno è di fondamentale importanza per quanto riguarda lo studio delle radiogalassie, una classe di galassie in cui si osserva una forte emissione in banda radio, e degli ammassi di galassie. Una prima parte di questo elaborato è stata dedicata alla descrizione della fisica alla base dell'emissione di radiazione per sincrotrone. Si è studiata l'emissione di radiazione da parte di un singolo elettrone partendo dalla cinematica di base di una particella carica che si muove in un campo magnetico. Particolare attenzione è stata dedicata successivamente allo studio dell'emissione da parte di una popolazione di elettroni, da cui si è ricavata la legge di potenza tipica del sincrotrone. Una seconda parte è stata dedicata ad una breve descrizione e classificazione delle radiogalassie con alcuni esempi e allo studio di una radiogalassia facente parte dell'ammasso RBS 797. Si è misurato il flusso proveniente dalla sorgente a due diverse frequenze, 5.5 GHz e 9.0 GHz. L'analisi delle misure del flusso ha inoltre permesso di stimare l'indice spettrale caratteristico della legge di potenza del sincrotrone da cui si sono fatte considerazioni sul tipo di popolazione elettronica presente nella radiosorgente
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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