608 research outputs found
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
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
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