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

A Chandra study of the large-scale shock and cool filaments in Hydra A: Evidence for substantial gas dredge-up by the central outburst

We present the results of a Chandra study of the Hydra A galaxy cluster, where a powerful AGN outburst created a large-scale cocoon shock. We investigated possible azimuthal variations in shock strength and shape, finding indications for a weak shock with a Mach number in the range ~1.2-1.3. We measured the temperature change across the shock front. However, the detection of a temperature rise in the regions immediately inside of the front is complicated by the underlying temperature profile of the cluster atmosphere. We measured the global temperature profile of the cluster up to 700 kpc, which represents the farthest measurement obtained with Chandra for this cluster. A "plateau" in the temperature profile in the range ~70-150 kpc indicates the presence of cool gas, which is likely the result of uplift of material by the AGN outburst. After masking the cool filaments visible in the hardness ratio map, the plateau disappears and the temperature profile recovers a typical shape with a peak around 190 kpc, just inside the shock front. However, it is unlikely that such a temperature feature is produced by the shock as it is consistent with the general shape of the temperature profiles observed for relaxed galaxy clusters. We studied the spectral properties of the cool filaments finding evidence that ~10^11 M_sun of low-entropy material has been dredged up by the rising lobes from the central 30 kpc to the observed current position of 75-150 kpc. The energy required to lift the cool gas is >~2.2 x 10^60 erg, which is comparable to the work required to inflate the cavities and is ~25% of the total energy of the large-scale shock. Our results show that the AGN feedback in Hydra A is acting not only by directly heating the gas, but also by removing a substantial amount of potential fuel for the SMBH.Comment: 11 pages, 9 figures, accepted for publication in ApJ (version with full resolution figures available at http://www.bo.astro.it/~myriam/files/papers/gitti-hydra.pdf

Model-independent X-ray mass determinations

A new method is introduced for making X-ray mass determinations of spherical clusters of galaxies. Treating the distribution of gravitating matter as piecewise constant and the cluster atmosphere as piecewise isothermal, X-ray spectra of a hydrostatic atmosphere are determined up to a single overall normalizing factor. In contrast to more conventional approaches, this method relies on the minimum of assumptions, apart from the conditions of hydrostatic equilibrium and spherical symmetry. The method has been implemented as an XSPEC mixing model called CLMASS, which was used to determine masses for a sample of nine relaxed X-ray clusters. Compared to conventional mass determinations, CLMASS provides weak constraints on values of M_500, reflecting the quality of current X-ray data for cluster regions beyond r_500. At smaller radii, where there are high quality X-ray spectra inside and outside the radius of interest to constrain the mass, CLMASS gives confidence ranges for M_2500 that are only moderately less restrictive than those from more familiar mass determination methods. The CLMASS model provides some advantages over other methods and should prove useful for mass determinations in regions where there are high quality X-ray data.Comment: 12 pages, 8 figures, accepted for publication in Ap

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