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Qualitative picture of the current sheets formation by AGN-inflated buoyant bubbles of relativistic plasma, rising in the cluster atmosphere. Nearby elliptical galaxy M87/Virgo is used in this example. Left: Morphology of soft X-ray filaments in M87 (Forman et al. 2007) and overall morphology of the radio emitting plasma (Owen, Eilek, & Kassim 2000), superposed as contours. Optical filaments are largely co-spatial with X-ray filaments. Buoyant bubbles rise in the atmosphere, entraining the low-entropy gas from the core (Churazov et al. 2000, 2001) and stretching/squeezing the fluid elements in the wake. Radio emission traces the distribution of the relativistic plasma produced by the AGN. Right: Schematic evolution of the magnetic field in the wake. As the bubbles rise, they stretch the magnetic field lines in the entrained fluid elements, thus increasing the strength of the field. The field lines, anchored to the gas in the cluster core, have opposite directions in the wake. They are forced together as the bubble rises. This setup bears strong similarity to the coronal loops on the Sun or to the Earth's magneto-tail, where reconnection is believed (and, in some cases, observed) to take place.
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Cool-core clusters (e.g. Perseus or M87) often possess a network of bright gaseous filaments, observed in radio, infrared,
optical and X-ray bands. We propose that these filaments are powered by the reconnection of the magnetic field in the wakes
of buoyant bubbles. Active galactic nucleus (AGN)-inflated bubbles of relativistic plasma rise buoyantl...
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... filaments. Here we consider a differ- ent scenario, in which buoyant bubbles of relativistic plasma stretch the magnetic field lines and drive the fields of op- posite direction together. In this model the active galac- tic nucleus (AGN)-inflated bubbles provide the energy that powers the filaments. A schematic picture of this process is shown in Fig. ...
Citations
... At even later times, these inflated bubbles rise further and form vortex rings, as has been discussed extensively in the literature on radio sources in cool core clusters (see e.g. Churazov et al. 2001;Gardini 2007;Werner et al. 2010;Churazov et al. 2013). If viewed from an inclined line-of-sight (Fig. 11), a synchrotron emitting torus is visible at 100 Myr in each simulation. ...
The propagation of active galactic nucleus jets depends both on the environment into which they propagate and on their internal structure. To test the impact that different magnetic topologies have on the observable properties of radio galaxies on kpc scales, we conducted a series of magneto-hydrodynamic simulations of jets injected with different magnetic field configurations propagating into a gaseous atmosphere modeled on the Perseus cluster. The simulations show that the structure of the field affects the collimation and propagation of the jets on cluster scales and thus the morphology of the radio lobes inflated by the jets, due both to magnetic collimation and the development of dynamical instabilities in jets with different magnetic topologies. In all cases, the simulations show a distinct reversal of the sychrotron spectral age gradient in the radio lobes about a dynamical time after the jets turn off due to large scale circulation inside the radio lobe, driven primarily by buoyancy, which could provide a way to constrain the age of radio sources in cluster environments without the need for detailed spectral modeling and thus constrain the radio mode feedback efficiency. We suggest a robust diagnostic to search for such age gradients in multi-frequency radio data.
... Despite a subdominant average magnetic field in the galactic haloes, some of these magnetic processes can be interesting to explain the local energetics near condensed gas where a strong magnetic field can develop. In fact, in the context of AGN uplifted cold filamentary gas and dragged magnetic fields, magnetic reconnection has already been evoked to explain how emission lines (e.g., Hα) are powered in the dense, cold filaments (Churazov et al., 2013). ...
Thermal instability (TI) potentially explains the origin of cold gas in the intracluster medium (ICM), which is heated sufficiently by AGN feedback. The H α filaments seen in cluster cores provide strong motivation for TI. The hot ( ∼ 1 0 7 K) ICM coronae allow the growth of isobaric TI. The multiphase medium (cold-dense—hot-diffuse) forms once TI saturates. However, gravitational stratification can spatially constrain TI, and thermal conduction is known to stabilize all scales below the field length ( λ F ). In addition, the transport of energy is anisotropic along magnetic fields. Thermal conduction may further trigger gyroscale instabilities and effective reduction of λ F . However, cold gas at small scales ( < λ F ) needs to be verified in observations. The virial temperature in galactic haloes is lower ( ∼ 1 0 6 K) and opens the regime of isochoric TI. In this regime, the cooling time is typically shorter than the sound-crossing time, and large-scale isochoric clouds are rendered unstable. The linear and non-linear isochoric clouds have interesting differences which potentially lead to either fragmentation of the cloud or not. On saturation, TI produces a turbulent medium that helps mix phases and thermalize kinetic energy and thus completes a cycle of condensation and heating. Various aspects of condensation, stratified turbulence, and magnetized transport are physically identical in solar coronae but scaled down to lower luminosity (similar temperatures). We will discuss the recent progress in TI, its connection to observations, and the analogy to solar prominences.
... Over the years, many processes have been suggested as the source of this heating, from photoionization by the central AGN (e.g. Heckman et al. 1989;McNamara & Nulsen 2012) or massive stars (Canning et al. 2014), via thermal conduction (Voit et al. 2008) and magnetic reconnection (Hanasz & Lesch 1998;Tanuma et al. 2003;Churazov et al. 2013) to cosmic ray streaming heating (Ruszkowski et al. 2018), to name a few. Stars can form in "knots" along filaments (Vantyghem et al. 2018), and these sites may also provide some heating and feedback to the cold gas once fragmented. ...
In galaxy clusters, the hot intracluster medium (ICM) can develop a striking multi-phase structure around the brightest cluster galaxy. Much work has been done on understanding the origin of this central nebula, but less work has studied its eventual fate after the originally filamentary structure is broken into individual cold clumps. In this paper we perform a suite of 30 (magneto-)hydrodynamical simulations of kpc-scale cold clouds with typical parameters as found by galaxy cluster simulations, to understand whether clouds are mixed back into the hot ICM or can persist. We investigate the effects of radiative cooling, small-scale heating, magnetic fields, and (anisotropic) thermal conduction on the long-term evolution of clouds. We find that filament fragments cool on timescales shorter than the crushing timescale, fall out of pressure equilibrium with the hot medium, and shatter, forming smaller clumplets. These act as nucleation sites for further condensation, and mixing via Kelvin-Helmholtz instability, causing cold gas mass to double within 75 Myr. Cloud growth depends on density, as well as on local heating processes, which determine whether clouds undergo ablation- or shattering-driven evolution. Magnetic fields slow down but don’t prevent cloud growth, with the evolution of both cold and warm phase sensitive to the field topology. Counter-intuitively, anisotropic thermal conduction increases the cold gas growth rate compared to non-conductive clouds, leading to larger amounts of warm phase as well. We conclude that dense clumps on scales of 500 pc or more cannot be ignored when studying the long-term cooling flow evolution of galaxy clusters.
... Over the years, many processes have been suggested as the source of this heating, from photoionization by the central AGN (e.g. Heckman et al. 1989;McNamara & Nulsen 2012) or massive stars (Canning et al. 2014), via thermal conduction (Voit et al. 2008) and magnetic reconnection (Hanasz & Lesch 1998;Tanuma et al. 2003;Churazov et al. 2013) to cosmic ray streaming heating (Ruszkowski et al. 2018), to name a few. Stars can form in "knots" along filaments (Vantyghem et al. 2018), and these sites may also provide some heating and feedback to the cold gas once fragmented. ...
In galaxy clusters, the hot intracluster medium (ICM) can develop a striking multi-phase structure around the brightest cluster galaxy. Much work has been done on understanding the origin of this central nebula, but less work has studied its eventual fate after the originally filamentary structure is broken into individual cold clumps. In this paper we perform a suite of 30 (magneto-)hydrodynamical simulations of kpc-scale cold clouds with typical parameters as found by galaxy cluster simulations, to understand whether clouds are mixed back into the hot ICM or can persist. We investigate the effects of radiative cooling, small-scale heating, magnetic fields, and (anisotropic) thermal conduction on the long-term evolution of clouds. We find that filament fragments cool on timescales shorter than the crushing timescale, fall out of pressure equilibrium with the hot medium, and shatter, forming smaller clumplets. These act as nucleation sites for further condensation, and mixing via Kelvin-Helmholtz instability, causing cold gas mass to double within 75 Myr. Cloud growth depends on density, as well as on local heating processes, which determine whether clouds undergo ablation- or shattering-driven evolution. Magnetic fields slow down but don't prevent cloud growth, with the evolution of both cold and warm phase sensitive to the field topology. Counter-intuitively, anisotropic thermal conduction increases the cold gas growth rate compared to non-conductive clouds, leading to larger amounts of warm phase as well. We conclude that dense clumps on scales of $500$ pc or more cannot be ignored when studying the long-term cooling flow evolution of galaxy clusters.
... These buoyant radio bubbles interact with the ICM, inducing subsonic turbulence and offsetting the overall cooling of the gas. So far, no evidence of strong shock has been found near these bubbles (Churazov et al., 2013). Simulations have shown that the rise of the bubbles in the ICM undergoes Rayleigh-Taylor (RT), and Kelvin-Helmholtz instabilities, and their expansion is closely comparable to the mushroom clouds formed by massive explosions on earth (Saxton et al., 2001;Reynolds et al., 2005;Gardini, 2007). ...
Diffuse radio emission has been detected in a considerable number of galaxy clusters and groups, revealing the presence of pervasive cosmic magnetic fields, and of relativistic particles in the large-scale structure (LSS) of the Universe. Since cluster radio emission is faint and steep spectrum, its observations are largely limited by the instrument sensitivity and frequency of observation, leading to a dearth of information, more so for lower-mass systems. The unprecedented sensitivity of recently commissioned low-frequency radio telescope arrays, aided by the development of advanced calibration and imaging techniques, have helped in achieving unparalleled image quality. At the same time, the development of sophisticated numerical simulations and the availability of supercomputing facilities have paved the way for high-resolution numerical modeling of radio emission, and the structure of the cosmic magnetic fields in LSS, leading to predictions matching the capabilities of observational facilities. In view of these rapidly-evolving scenerio in modeling and observations, in this review, we summarise the role of the new telescope arrays and the development of advanced imaging techniques and discuss the detections of various kinds of cluster radio sources. In particular, we discuss observations of the cosmic web in the form of supercluster filaments, studies of emission in poor clusters and groups of galaxies, and of ultra-steep spectrum sources. We also review the current theoretical understanding of various diffuse cluster radio sources and the associated magnetic field and polarization. As the statistics of detections improve along with our theoretical understanding, we update the source classification schemes based on their intrinsic properties. We conclude by summarising the role of the upgraded GMRT and our expectations from the upcoming Square Kilometre Array (SKA) observatories.
... The magnetic field may further reduce fragmentation. Mixing with hot surrounding gas and magnetic reconnection may reheat the cold filaments before star formation can ensue Churazov, Ruszkowski & Schekochihin 2013 ). The gas in the nuclear regions is also dynamically disturbed in many BCGs. ...
Molecular gas flows are analyzed in 14 cluster galaxies (BCGs) centered 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 two to three 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 percent 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 disks that are more impervious to feedback and are better able to promote star formation.
... The magnetic field may further reduce fragmentation. Mixing with hot surrounding gas and magnetic reconnection may reheat the cold filaments before star formation can ensue Churazov et al. 2013). The gas in the nuclear regions is also dynamically disturbed in many BCGs. ...
Molecular gas flows are analyzed in 14 cluster galaxies (BCGs) centered 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 $\sim$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 $\sim$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 two to three 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 percent 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 disks that are more impervious to feedback and are better able to promote star formation.
... Along similar lines, observations of Hα emission in some clusters that reveal the presence of colder gas structured into long, coherent filaments have been taken as evidence for dynamically important intracluster fields, because of the filaments' apparent stability against tidal shear and dissipation into the surrounding hot gas (e.g., [41]). If these filaments were produced in the wakes of buoyantly rising bubbles, Ref. [32] argues that the advected magnetic-field lines would be anti-parallel and gradually forced together, ultimately leading to their reconnection and to resistive heating capable of powering the filaments' luminosity. ...
... The most recent LOFAR observations of the Toothbrush relic measured α inj ≈ −0.8 at the northern edge of the relic and steepening towards the south to α inj ≈ −2 (see figure 5, left panel). Applying equation (32) gives a Mach number ≈2.8 with the lower limit M ≈ 2.5 -values that are inconsistent with the Mach number obtained from X-ray observations. This discrepancy between the X-ray-and radiomeasured Mach numbers is not unique to the Toothbrush relic and requires additional theoretical work on the DSA scenario for shocks as relic sources in clusters. ...
This Chapter provides a brief tutorial on some aspects of plasma physics that are fundamental to understanding the dynamics and energetics of the intracluster medium (ICM). The tutorial is split into two parts: one that focuses on the thermal plasma component -- its stability, viscosity, conductivity, and ability to amplify magnetic fields to dynamical strengths via turbulence and other plasma processes; and one that focuses on the non-thermal population of charged particles known as cosmic rays -- their acceleration, re-acceleration, and transport throughout the cluster volume. Observational context is woven throughout the narrative, from constraints on the strength and geometry of intracluster magnetic fields and the effective viscosity of the ICM, to examples of radio halos, radio relics, and cluster shocks that can test theories of particle acceleration. The promise of future X-ray missions to probe intracluster turbulence and discover the impact of small-scale plasma physics, coupled with sensitive, high-resolution radio observations of synchrotron-emitting plasma that reveal the properties of intracluster magnetic fields and particle-acceleration mechanisms, are likely to establish galaxy clusters as the premier cosmic laboratories for deciphering the fundamental physics of hot, dilute plasmas.
... In Chapter 3, we have shown that the thermal energy of the radiative cooling gas is sufficient as the power source for the optical/UV nebula in clusters with a cooling rate below ∼10 M ⊙ yr −1 , but the most luminous clusters are likely powered by hotter gas or otherwise ( [113]). Churazov et al. [30] argued that buoyant bubbles stretch fluid elements to form gaseous filaments with amplified magnetic field. The release of magnetic energy allows dissipation into filaments. ...
In this thesis, I present the results of my PhD research on the cooling flow problem in galaxy clusters. The centre of relaxed galaxy clusters has a short radiative cooling time suggesting the presence of a massive cooling flow. However, early studies using high resolution X-ray spectroscopy indicated much lower levels of cooling rate. AGN feedback is thought to be the most likely energy source to balance radiative cooling, though the energy transport and dissipation mechanisms are still under debate. In this work, I study whether AGN feedback can actually balance radiative cooling in a large number of galaxy clusters using high resolution X-ray spectroscopy. The first chapter contains the necessary background of the cooling flow problem and AGN feedback. This is followed by a chapter describing data reduction of XMM-Newton observations. In the third chapter, I present a study of 45 nearby cool core galaxy clusters and groups, where I measure the radiative cooling rate in the softest X-ray band. Then I select a small sub sample of bright clusters to understand the mass temperature profile of the gas in Chapter 4. In Chapter 5, I present a deep study of recent XMM-Newton observations of two luminous clusters at intermediate redshift. Finally, I extend my research on 40 more clusters within a much larger range of distances corresponding to redshift up to 0.6.
... The role of magnetic fields in the stretched tail could also be important (e.g. Churazov et al. 2013). ...
Galaxy clusters grow primarily through the continuous accretion of group-scale haloes. Group galaxies experience preprocessing during their journey into clusters. A star-bursting compact group, the Blue Infalling Group (BIG), is plunging into the nearby cluster A1367. Previous optical observations reveal rich tidal features in the BIG members, and a long H$\alpha$ trail behind. Here we report the discovery of a projected $\sim 250$ kpc X-ray tail behind the BIG using Chandra and XMM-Newton observations. The total hot gas mass in the tail is $\sim 7\times 10^{10}\ {\rm M}_\odot$ with an X-ray bolometric luminosity of $\sim 3.8\times 10^{41}$ erg s$^{-1}$. The temperature along the tail is $\sim 1$ keV, but the apparent metallicity is very low, an indication of the multi-$T$ nature of the gas. The X-ray and H$\alpha$ surface brightnesses in the front part of the BIG tail follow the tight correlation established from a sample of stripped tails in nearby clusters, which suggests the multiphase gas originates from the mixing of the stripped interstellar medium (ISM) with the hot intracluster medium (ICM). Because thermal conduction and hydrodynamic instabilities are significantly suppressed, the stripped ISM can be long lived and produce ICM clumps. The BIG provides us a rare laboratory to study galaxy transformation and preprocessing.
