91 research outputs found
Reversing cooling flows with AGN jets: shock waves, rarefaction waves, and trailing outflows
The cooling flow problem is one of the central problems in galaxy clusters,
and active galactic nucleus (AGN) feedback is considered to play a key role in
offsetting cooling. However, how AGN jets heat and suppress cooling flows
remains highly debated. Using an idealized simulation of a cool-core cluster,
we study the development of central cooling catastrophe and how a subsequent
powerful AGN jet event averts cooling flows, with a focus on complex
gasdynamical processes involved. We find that the jet drives a bow shock, which
reverses cooling inflows and overheats inner cool core regions. The shocked gas
moves outward in a rarefaction wave, which rarefies the dense core and
adiabatically transports a significant fraction of heated energy to outer
regions. As the rarefaction wave propagates away, inflows resume in the cluster
core, but a trailing outflow is uplifted by the AGN bubble, preventing gas
accumulation and catastrophic cooling in central regions. Inflows and trailing
outflows constitute meridional circulations in the cluster core. At later
times, trailing outflows fall back to the cluster centre, triggering central
cooling catastrophe and potentially a new generation of AGN feedback. We thus
envisage a picture of cool cluster cores going through cycles of
cooling-induced contraction and AGN-induced expansion. This picture naturally
predicts an anti-correlation between the gas fraction (or X-ray luminosity) of
cool cores and the central gas entropy, which may be tested by X-ray
observations.Comment: Slightly revised version, accepted for publication in MNRAS. 14
pages, 10 figure
AGN Feedback and Bimodality in Cluster Core Entropy
We investigate a series of steady-state models of galaxy clusters, in which
the hot intracluster gas is efficiently heated by active galactic nucleus (AGN)
feedback and thermal conduction, and in which the mass accretion rates are
highly reduced compared to those predicted by the standard cooling flow models.
We perform a global Lagrangian stability analysis. We show for the first time
that the global radial instability in cool core clusters can be suppressed by
the AGN feedback mechanism, provided that the feedback efficiency exceeds a
critical lower limit. Furthermore, our analysis naturally shows that the
clusters can exist in two distinct forms. Globally stable clusters are expected
to have either: 1) cool cores stabilized by both AGN feedback and conduction,
or 2) non-cool cores stabilized primarily by conduction. Intermediate central
temperatures typically lead to globally unstable solutions. This bimodality is
consistent with the recently observed anticorrelation between the flatness of
the temperature profiles and the AGN activity (Dunn & Fabian 2008) and the
observation by Rafferty et al. (2008) that the shorter central cooling times
tend to correspond to significantly younger AGN X-ray cavities.Comment: 4 pages, to appear in the proceedings of "The Monster's Fiery Breath:
Feedback in Galaxies, Groups, and Clusters", Eds. Sebastian Heinz, Eric
Wilcots (AIP conference series
On the Efficiency of Thermal Conduction in Galaxy Clusters
Galaxy clusters host a large reservoir of diffuse plasma with
radially-varying temperature profiles. The efficiency of thermal conduction in
the intracluster medium (ICM) is complicated by the existence of turbulence and
magnetic fields, and has received a lot of attention in the literature.
Previous studies suggest that the magnetothermal instability developed in outer
regions of galaxy clusters would drive magnetic field lines preferentially
radial, resulting in efficient conduction along the radial direction. Using a
series of spherically-symmetric simulations, here we investigate the impact of
thermal conduction on the observed temperature distributions in outer regions
of three massive clusters, and find that thermal conduction substantially
modifies the ICM temperature profile. Within 3 Gyr, the gas temperature at a
representative radius of typically decreases by ~10 - 20% and the
average temperature slope between and drops by ~ 30 -
40%, indicating that the observed ICM would not stay in a long-term equilibrium
state in the presence of thermal conduction. However, X-ray observations show
that the outer regions of massive clusters have remarkably similar
radially-declining temperature profiles, suggesting that they should be quite
stable. Our study thus suggests that the effective conductivity along the
radial direction must be suppressed below the Spitzer value by a factor of 10
or more, unless additional heating sources offset conductive cooling and
maintain the observed temperature distributions. Our study provides a
smoking-gun evidence for the suppression of parallel conduction along magnetic
field lines in low-collisionality plasmas by kinetic mirror or whistler
instabilities.Comment: Slightly revised version, accepted for publication in ApJ. 11 pages,
7 figure
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