Mechanical Feedback from Active Galactic Nuclei in Galaxies, Groups, and Clusters

The radiative cooling timescales at the centers of hot atmospheres surrounding elliptical galaxies, groups, and clusters are much shorter than their ages. Therefore, hot atmospheres are expected to cool and to form stars. Cold gas and star formation are observed in central cluster galaxies but at levels below those expected from an unimpeded cooling flow. X-ray observations have shown that wholesale cooling is being offset by mechanical heating from radio active galactic nuclei. Feedback is widely considered to be an important and perhaps unavoidable consequence of the evolution of galaxies and supermassive black holes. We show that cooling X-ray atmospheres and the ensuing star formation and nuclear activity are probably coupled to a self-regulated feedback loop. While the energetics are now reasonably well understood, other aspects of feedback are not. We highlight the problems of atmospheric heating and transport processes, accretion, and nuclear activity, and we discuss the potential role of black hole spin. We discuss X-ray imagery showing that the chemical elements produced by central galaxies are being dispersed on large scales by outflows launched from the vicinity of supermassive black holes. Finally, we comment on the growing evidence for mechanical heating of distant cluster atmospheres by radio jets and its potential consequences for the excess entropy in hot halos and a possible decline in the number of distant cooling flows.Comment: Accepted for publication in New Journal of Physics Focus Issue on Clusters of Galaxie

The role of cooling flows in galaxy formation

The present structure of galaxies is governed by the radiative dissipation of the gravitational and supernova energy injected during formation. A crucial aspect of this process is whether the gas cools as fast as it falls into the gravitational potential well. If it does then rapid normal star formation is assumed to ensue. If not, and the gas can still cool by the present time, then the situation resembles that of a cooling flow, such as commonly found in clusters of galaxies. The cooled matter is assumed to accumulate as very cold clouds and/or low mass stars, i.e. as baryonic dark matter. In this paper we investigate the likelihood of a cooling flow phase during the hierarchical formation of galaxies. We concentrate on the behaviour of the gas, using a highly simplified treatment of the evolution of the dark matter potential within which the gas evolves. We assume that normal star formation is limited by how much gas the associated supernovae can unbind and allow the gas profile to flatten as a consequence of supernova energy injection. We find that cooling flows are an important phase in the formation of most galaxies with total (dark plus luminous) masses approxgt 10^12 Msun , creating about 20 per cent of the total dark halo in a galaxy such as our own and a smaller but comparable fraction of an elliptical galaxy of similar mass. The onset of a cooling flow determines the upper mass limit for the formation of a visible spheroid from gas, setting a characteristic mass scale for normal galaxies. We argue that disk formation requires a cooling flow phase and that dissipation in the cooling flow phase is the most important factor in the survival of normal galaxies during subsequent hierarchical mergers.Comment: uuencoded compressed postscript. The preprint is also available at http://www.ast.cam.ac.uk/preprint/PrePrint.htm

Fuelling quasars with hot gas

We consider a model for quasar formation in which massive black holes are formed and fuelled largely by the accretion of hot gas during the process of galaxy formation. In standard hierarchical collapse models, objects about the size of normal galaxies and larger form a dense hot atmosphere when they collapse. We show that if such an atmosphere forms a nearly "maximal" cooling flow, then a central black hole can accrete at close to its Eddington limit. This leads to exponential growth of a seed black hole, resulting in a quasar in some cases. In this model, the first quasars form soon after the first collapses to produce hot gas. The hot gas is depleted as time progresses, mostly by cooling, so that the accretion rate eventually falls below the threshold for advection-dominated accretion, at which stage radiative efficiency plummets and any quasar turns off. A simple implementation of this model, incorporated into a semi-analytical model for galaxy formation, over-produces quasars when compared with observed luminosity functions, but is consistent with models of the X-ray Background which indicate that most accretion is obscured. It produces few quasars at high redshift due to the lack of time needed to grow massive black holes. Quasar fuelling by hot gas provides a minimum level, sufficient to power most quasars at redshifts between one and two, to which other sources of fuel can be added. The results are sensitive to feedback effects, such as might be due to radio jets and other outflows.Comment: 12 pages, 6 figures, MN Latex style, accepted for publication in MNRA

Stripped elliptical galaxies as probes of ICM physics: II. Stirred, but mixed? Viscous and inviscid gas stripping of the Virgo elliptical M89

Elliptical galaxies moving through the intra-cluster medium (ICM) are progressively stripped of their gaseous atmospheres. X-ray observations reveal the structure of galactic tails, wakes, and the interface between the galactic gas and the ICM. This fine-structure depends on dynamic conditions (galaxy potential, initial gas contents, orbit in the host cluster), orbital stage (early infall, pre-/post-pericenter passage), as well as on the still ill-constrained ICM plasma properties (thermal conductivity, viscosity, magnetic field structure). Paper I describes flow patterns and stages of inviscid gas stripping. Here we study the effect of a Spitzer-like temperature dependent viscosity corresponding to Reynolds numbers, Re, of 50 to 5000 with respect to the ICM flow around the remnant atmosphere. Global flow patterns are independent of viscosity in this Reynolds number range. Viscosity influences two aspects: In inviscid stripping, Kelvin-Helmholtz instabilities (KHIs) at the sides of the remnant atmosphere lead to observable horns or wings. Increasing viscosity suppresses KHIs of increasing length scale, and thus observable horns and wings. Furthermore, in inviscid stripping, stripped galactic gas can mix with the ambient ICM in the galaxy's wake. This mixing is suppressed increasingly with increasing viscosity, such that viscously stripped galaxies have long X-ray bright, cool wakes. We provide mock X-ray images for different stripping stages and conditions. While these qualitative results are generic, we tailor our simulations to the Virgo galaxy M89 (NGC 4552), where Re~ 50 corresponds to a viscosity of 10% of the Spitzer level. Paper III compares new deep Chandra and archival XMM-Newton data to our simulations.Comment: ApJ in press. 16 pages, 16 figures. Text clarified, conclusions unchange

Stripped elliptical galaxies as probes of ICM physics: I. Tails, wakes, and flow patterns in and around stripped ellipticals

Elliptical cluster galaxies are progressively stripped of their atmospheres due to their motion through the intra-cluster medium (ICM). Deep X-ray observations reveal the fine-structure of the galaxy's remnant atmosphere and its gas tail and wake. This fine-structure depends on dynamic conditions (galaxy potential, initial gas contents, orbit through the host cluster), orbital stage (early infall, pre-/post-pericenter passage), and ICM plasma properties (thermal conductivity, viscosity, magnetic field structure). We aim to disentangle dynamic and plasma effects in order to use stripped ellipticals as probes of ICM plasma properties. This first paper of a series investigates the hydrodynamics of progressive gas stripping by means of inviscid hydrodynamical simulations. We distinguish a long-lasting initial relaxation phase and a quasi-steady stripping phase. During quasi-steady stripping, the ICM flow around the remnant atmosphere resembles the flow around solid bodies, including a `deadwater' region in the near wake. Gas is stripped from the remnant atmosphere predominantly at its sides via Kelvin-Helmholtz instabilities. The downstream atmosphere is largely shielded from the ICM wind and thus shaped into a tail. Observationally, both, this `remnant tail' and the stripped gas in the wake can appear as a `tail', but only in the wake can galactic gas mix with the ambient ICM. While the qualitative results are generic, the simulations presented here are tailored to the Virgo elliptical galaxy M89 (NGC 4552) for the most direct comparison to observations. Papers II and III of this series describe the effect of viscosity and compare to Chandra and XMM-Newton observations, respectively.Comment: ApJ, in press. 19 pages, 13 figures. Clarifications added, text restructured. Conclusions unchange

The effect of supernova heating on cluster properties and constraints on galaxy formation models

Models of galaxy formation should be able to predict the properties of clusters of galaxies, in particular their gas fractions, metallicities, X-ray luminosity-temperature relation, temperature function and mass-deposition-rate function. Fitting these properties places important constaints on galaxy formation on all scales. By following gas processes in detail, our semi-analytic model (based on that of Nulsen & Fabian 1997) is the only such model able to predict all of the above cluster properties. We use realistic gas fractions and gas density profiles, and as required by observations we break the self-similarity of cluster structure by including supernova heating of intracluster gas, the amount of which is indicated by the observed metallicities. We also highlight the importance of the mass-deposition-rate function as an independent and very sensitive probe of cluster structure.Comment: 5 pages, 4 figures, accepted for publication in MNRAS as a lette

Kelvin-Helmholtz instabilities at the sloshing cold fronts in the Virgo cluster as a measure for the effective ICM viscosity

Sloshing cold fronts (CFs) arise from minor merger triggered gas sloshing. Their detailed structure depends on the properties of the intra-cluster medium (ICM): hydrodynamical simulations predict the CFs to be distorted by Kelvin-Helmholtz instabilities (KHIs), but aligned magnetic fields, viscosity, or thermal conduction can suppress the KHIs. Thus, observing the detailed structure of sloshing CFs can be used to constrain these ICM properties. Both smooth and distorted sloshing CFs have been observed, indicating that the KHI is suppressed in some clusters, but not in all. Consequently, we need to address at least some sloshing clusters individually before drawing general conclusions about the ICM properties. We present the first detailed attempt to constrain the ICM properties in a specific cluster from the structure of its sloshing CF. Proximity and brightness make the Virgo cluster an ideal target. We combine observations and Virgo-specific hydrodynamical sloshing simulations. Here we focus on a Spitzer-like temperature dependent viscosity as a mechanism to suppress the KHI, but discuss the alternative mechanisms in detail. We identify the CF at 90 kpc north and north-east of the Virgo center as the best location in the cluster to observe a possible KHI suppression. For viscosities $\gtrsim$ 10% of the Spitzer value KHIs at this CF are suppressed. We describe in detail the observable signatures at low and high viscosities, i.e. in the presence or absence of KHIs. We find indications for a low ICM viscosity in archival XMM-Newton data and demonstrate the detectability of the predicted features in deep Chandra observations.Comment: Accepted for ApJ; 15 pages, 11 figures. A movie can be found here: http://www.hs.uni-hamburg.de/DE/Ins/Per/Roediger/research.html#Virgo-viscou