M. Brüggen

The role of black holes in galaxy formation and evolution

Virtually all massive galaxies, including our own, host central black holes ranging in mass from millions to billions of solar masses. The growth of these black holes releases vast amounts of energy that powers quasars and other weaker active galactic nuclei. A tiny fraction of this energy, if absorbed by the host galaxy, could halt star formation by heating and ejecting ambient gas. A central question in galaxy evolution is the degree to which this process has caused the decline of star formation in large elliptical galaxies, which typically have little cold gas and few young stars, unlike spiral galaxies.

Particle Acceleration on Megaparsec Scales in a Merging Galaxy Cluster

Shocking Radio Relic Radio relics are diffuse, elongated radio sources located on the outskirts of galaxy clusters thought to trace shocks generated by collisions between galaxy clusters. Particles may be accelerated within the shock waves by a diffusive shock acceleration mechanism, which also accelerates particles in shock waves produced by supernova explosions. Van Weeren et al. (p. 347, published online 23 September) report the detection of a megaparsec-scale radio relic showing all the properties of diffusive shock acceleration expected at radio wavelengths. The results suggest that this acceleration mechanism operates on scales larger than those of supernova remnants and imply that merging clusters of galaxies can accelerate particles to energies much higher than those achieved in supernova remnants Observations show that shocks within the intracluster medium are capable of producing extremely energetic cosmic rays. Galaxy clusters form through a sequence of mergers of smaller galaxy clusters and groups. Models of diffusive shock acceleration suggest that in shocks that occur during cluster mergers, particles are accelerated to relativistic energies, similar to conditions within supernova remnants. In the presence of magnetic fields, these particles emit synchrotron radiation and may form so-called radio relics. We detected a radio relic that displays highly aligned magnetic fields, a strong spectral index gradient, and a narrow relic width, giving a measure of the magnetic field in an unexplored site of the universe. Our observations show that diffusive shock acceleration also operates on scales much larger than in supernova remnants and that shocks in galaxy clusters are capable of producing extremely energetic cosmic rays.

CLUSTER HEATING BY VISCOUS DISSIPATION OF SOUND WAVES

We simulate the effects of viscous dissipation of waves that are generated by active galactic nucleus (AGN) activity in clusters of galaxies. We demonstrate that the amount of viscous heating associated with the dissipation of these waves can offset radiative cooling rates in cooling flow clusters of galaxies. This heating mechanism leads to spatially distributed and approximately symmetrical dissipation. The heating waves reach a given distance from the cluster center on a timescale shorter than the cooling time. This means that this heating mechanism has the potential to quench cooling flows in a quasi-stable fashion. Moreover, the heating is gentle, as no strong shocks are present in the simulations. We first investigated whether a single continuous episode of AGN activity can lead to adequate dissipation to balance cooling rates. These simulations demonstrated that whereas secondary waves generated by the interaction of the rising bubble with the intracluster medium are clearly present, viscous heating associated with the dissipation of these waves is insufficient to balance radiative cooling. It is only when the central source is intermittent that the viscous dissipation of waves associated with subsequent episodes of activity can offset cooling. This suggests that the ripples observed in the Perseus Cluster can be interpreted as being due to the AGN duty cycle; i.e., they trace AGN activity history. The simulations were performed using the adaptive mesh refinement code FLASH in two dimensions.

Massive and refined - II. The statistical properties of turbulent motions in massive galaxy clusters with high spatial resolution

We study the properties of chaotic motions in the intra clust er medium using a set of 20 galaxy clusters simulated with large dynamical range, using the Adaptive Mesh Refinement co de ENZO (e.g. Norman et al.2007). The adopted setup allows us to study the spectral and spatial properties of turbulent motions in galaxy clusters with unprecedented detail, achi eving an maximum available Reynolds number of the order of Re ∼ 500− 1000 for the largest eddies. The correlations between the energy of these motions in the Intra Cluster Medium and the dy namical state of the host systems are studied, and the statis tical properties of turbulent motions and their evolution with ti me support that major merger events are responsible for the i njection of the bulk of turbulent kinetic energy inside cluster. Turb lence is found to account for a ∼ 20− 30 per cent of the thermal energy in merging clusters, while it accounts for a ∼ 5 per cent in relaxed clusters. A comparison of the energies o f turbulence and motions in our simulated clusters with present upper-li mits in real nearby clusters, recently derived with XMM-New ton (Sanders et al.2010), is provided. When the same spatial sca les of turbulent motions are compared, the data from simulat ions result well within the range presently allowed by observati ons. Finally, we comment on the possibility that turbulence may accelerate relativistic particles leading to the formatio n of giant radio halos in turbulent (merging) clusters. Base d on our simulations we confirm previous semi-analytical studies th at suggest that the fraction of turbulent clusters is consis tent with that of clusters hosting radio halos.We study the properties of chaotic motions in the intra cluster medium using a set of 20 galaxy clusters simulated with large dynamical range, using the adaptive mesh refinement code ENZO. The adopted setup allows us to study the spectral and spatial properties of turbulent motions in galaxy clusters with unprecedented detail, achieving an maximum available Reynolds number of the order of Re ∼ 500−1000 for the largest eddies. We investigated the correlations between the energy of these motions in the intra cluster medium and the dynamical state of the host systems. We find that the statistical properties of turbulent motions and their evolution with time imply that major merger events are responsible for the injection of the bulk of turbulent kinetic energy into the cluster. Turbulence is found to account for ∼20−30 per cent of the thermal energy in merging clusters, and ∼5 per cent in relaxed clusters. We compare the energies of turbulence and motions in our simulated clusters with upper-limits for real nearby clusters derived from XMM-Newton data. When turbulent motions are compared on the same spatial scales, the data from simulations are well within the range presently allowed by observations. Finally, we comment on the possibility that turbulence may accelerate relativistic particles leading to the formation of giant radio halos in turbulent (merging) clusters. On the basis of our simulations, we confirm the conclusions of previous semi-analytical studies that the fraction of turbulent clusters appears to be consistent with that of clusters hosting radio halos.

Radio signature of cosmological structure formation shocks

In the course of the formation of cosmological structures, large shock waves are generated in the intracluster medium (ICM). In analogy to processes in supernova remnants, these shock waves may generate a significant population of relativistic electrons which, in turn, produce observable synchrotron emission. The extended radio relics found at the periphery of several clusters and possibly also a fraction of radio halo emission may have this origin. Here, we derive an analytic expression for (i) the total radio power in the downstream region of a cosmological shock wave, and (ii) the width of the radio-emitting region. These expressions predict a spectral slope close to -1 for strong shocks. Moderate shocks, such as those produced . in mergers between clusters of galaxies, lead to a somewhat steeper spectrum. Moreover, we predict an upper limit for the radio power of cosmological shocks. Comparing our results to the radio relics in Abell 115, 2256 and 3667, we conclude that the magnetic field in these relics is typically at a level of 0.1 μG. Magnetic fields in the ICM are presumably generated by the shocks themselves; this allows us to calculate the radio emission as a function of the cluster temperature. The resulting emissions agree very well with the radio power-temperature relation found for cluster haloes. Finally, we show that cosmic accretion shocks generate less radio emission than merger shock waves. The latter may, however, be detected with upcoming radio telescopes.

Ram pressure stripping of disc galaxies orbiting in clusters – I. Mass and radius of the remaining gas disc

We present the first 3D hydrodynamical simulations of ram pressure stripping of a disc galaxy orbiting in a galaxy cluster. Along the orbit, the ram pressure that this galaxy experiences varies with time. In this paper, we focus on the evolution of the radius and mass of the remaining gas disc, and compare it with the classical analytical estimate proposed by Gunn & Gott. We find that this simple estimate works well in predicting the evolution of the radius of the remaining gas disc. Only if the ram pressure increases faster than the stripping time-scale, the disc radius remains larger than predicted. However, orbits with such short ram pressure peaks are unlikely to occur in other than compact clusters. Unlike the radius evolution, the mass-loss history for the galaxy is not accurately described by the analytical estimate. Generally, in the simulations the galaxy loses its gas more slowly than predicted.

Impact of tangled magnetic fields on fossil radio bubbles

There is growing consensus that feedback from active galactic nuclei (AGN) is the main mechanism responsible for stopping cooling flows in clusters of galaxies. AGN are known to inflate buoyant bubbles that supply mechanical power to the intracluster gas (ICM). High Reynolds number hydrodynamical simulations show that such bubbles get entirely disrupted within 100 Myr, as they rise in cluster atmospheres, which is contrary to observations. This artificial mixing has consequences for models trying to quantify the amount of heating and star formation in cool core clusters of galaxies. It has been suggested that magnetic fields can stabilize bubbles against disruption. We perform magnetohydrodynamical (MHD) simulations of fossil bubbles in the presence of tangled magnetic fields using the high order PENCIL code. We focus on the physically-motivated case where thermal pressure dominates over magnetic pressure and consider randomly oriented fields with and without maximum helicity and a case where large scale external fields drape the bubble. We find that helicity has some stabilizing effect. However, unless the coherence length of magnetic fields exceeds the bubble size, the bubbles are quickly shredded. As observations of Hydra A suggest that lengthscale of magnetic fields may be smaller then typical bubble size, this may suggest that other mechanisms, such as viscosity, may be responsible for stabilizing the bubbles. However, since Faraday rotation observations of radio lobes do not constrain large scale ICM fields well if they are aligned with the bubble surface, the draping case may be a viable alternative solution to the problem. A generic feature found in our simulations is the formation of magnetic wakes where fields are ordered and amplified. We suggest that this effect could prevent evaporation by thermal conduction of cold Hα filaments observed in the Perseus cluster.

Ram pressure stripping of disc galaxies: the role of the inclination angle

We present three-dimensional (3D) hydrodynamical simulations of ram pressure stripping of massive disc galaxies in clusters. Studies of galaxies that move face-on have predicted that in such a geometry the galaxy can lose a substantial amount of its interstellar medium. But only a small fraction of galaxies is moving face-on. In this work we focus on a systematic study of the effect of the inclination angle between the direction of motion and the galaxys rotation axis. In agreement with some previous works, we find that the inclination angle does not play a major role for the mass loss as long as the galaxy is not moving close to edge-on (inclination angle ≲60°). We explain this behaviour by extending Gunn & Gotts estimate of the stripping radius, which is valid for face-on geometries, to moderate inclinations. The inclination plays a role as long as the ram pressure is comparable to pressures in the galactic plane, which can span two orders of magnitude. For very strong ram pressures, the disc will be stripped completely, and for very weak ram pressures, mass loss is negligible independent of inclination. We show that in non-edge-on geometries the stripping proceeds remarkably similar. A major difference between different inclinations is the degree of asymmetry introduced in the remaining gas disc. We demonstrate that the tail of gas stripped from the galaxy does not necessarily point in a direction opposite to the galaxys direction of motion. Therefore, the observation of a galaxys gas tail may be misleading about the galaxys direction of motion.

Three-Dimensional Simulations of Viscous Dissipation in the Intracluster Medium

We present three-dimensional simulations of viscous dissipation of active galactic nuclei (AGN)-induced gas motions and waves in clusters of galaxies. These simulations are motivated by recent detections of ripples in the Perseus and Virgo Clusters. Although the sound waves generated by buoyant bubbles decay with distance from the cluster center, we show that these waves can contribute substantially to offsetting the radiative cooling at distances significantly exceeding the bubble size. The energy flux of the waves declines more steeply with radius than the inverse-square law predicted by energy conservation, implying that dissipation plays an important role in tapping the wave energy. We show that such dispersing sound waves/weak shocks are detectable as ripples on unsharp-masked X-ray cluster maps and point out that the interfaces between the intracluster medium and old bubbles are also clearly detectable in unsharp-masked X-ray maps. This opens up the possibility of detecting fossil bubbles that are difficult to detect in radio emission. This mode of heating is consistent with other observational constraints, such as the presence of cool rims around the bubbles and the absence of strong shocks. Thus, the mechanism offers a way of heating clusters in a spatially distributed and gentle fashion. We also discuss the energy transfer between the central AGN and the surrounding medium. In our numerical experiments, we find that roughly 65% of the energy injected by the AGN is transferred to the intracluster medium, and approximately 25% of the injected energy is dissipated by viscous effects and contributes to heating of the gas. The overall transfer of heat from the AGN to the gas is comparable to the radiative cooling losses. The simulations were performed with the FLASH adaptive mesh refinement code.

Chemical enrichment in the cluster of galaxies Hydra A

We analyzed global properties, radial profiles, and 2D maps of the metal abundances and temperature in the cool core cluster of galaxies Hydra A using a deep ∼120 ks XMM-Newton exposure. The best fit among the available spectral models is provided by a Gaussian distribution of the emission measure (gdem). We can accurately determine abundances for 7 elements in the cluster core with EPIC (O, Si, S, Ar, Ca, Fe, Ni) and 3 elements (O, Ne, Fe) with RGS. The gdem model gives lower Fe abundances than a singletemperature model. Based on this, we explain why simulations show that the best-fit Fe abundance in clusters with intermediate temperatures is overestimated. The abundance profiles for Fe, Si, S, but also O are centrally peaked. Combining the Hydra A results with 5 other clusters for which detailed chemical abundance studies are available, we find a significant decrease in O with radius, while the increase in the O/Fe ratio with radius is small within 0.1 r200, where the O abundances can be accurately determined, with d(O/Fe)/d(log10r/r200) = 0.25 ± 0.09. We compare the observed abundance ratios with the mixing of various supernova type Ia and core-collapse yield models in different relative amounts. Producing the estimated O, Si, and S peaks in Hydra A requires either the amount of metals ejected by stellar winds to be 3–8 times higher than predicted by available models or the initial enrichment by core-collapse supernovae in the protocluster phase not to be as well mixed on large scales as previously thought. The temperature map shows cooler gas extending in arm-like structures towards the north and south. These structures, and especially the northern one, appear to be richer in metals than the ambient medium and spatially correlated with the large-scale radio lobes. With different sets of assumptions, we estimate the mass of cool gas, which was probably uplifted by buoyant bubbles of relativistic plasma produced by the AGN, to 1.6−6.1 × 10 9 M� , and the energy associated with this uplift to 3.3−12.5 × 10 58 erg. The best estimate of the mass of Fe uplifted together with the cool gas is 1.7 × 10 7 M� , 15% of the total mass of Fe in the central 0.5 � region.