15 research outputs found
Cool core cycles: Cold gas and AGN jet feedback in cluster cores
Using high-resolution 3-D and 2-D (axisymmetric) hydrodynamic simulations in
spherical geometry, we study the evolution of cool cluster cores heated by
feedback-driven bipolar active galactic nuclei (AGN) jets. Condensation of cold
gas, and the consequent enhanced accretion, is required for AGN feedback to
balance radiative cooling with reasonable efficiencies, and to match the
observed cool core properties. A feedback efficiency (mechanical luminosity
; where is the mass
accretion rate at 1 kpc) as small as is sufficient to reduce
the cooling/accretion rate by compared to a pure cooling flow. This
value is smaller compared to the ones considered earlier, and is consistent
with the jet efficiency and the fact that only a small fraction of gas at 1 kpc
is accreted on to the supermassive black hole (SMBH). We find hysteresis cycles
in all our simulations with cold mode feedback: {\em condensation} of cold gas
when the ratio of the cooling-time to the free-fall time () is leads to a sudden enhancement in the accretion rate; a
large accretion rate causes strong jets and {\em overheating} of the hot ICM
such that ; further condensation of cold gas is
suppressed and the accretion rate falls, leading to slow cooling of the core
and condensation of cold gas, restarting the cycle. Therefore, there is a
spread in core properties, such as the jet power, accretion rate, for the same
value of core entropy or . A fewer number of cycles
are observed for higher efficiencies and for lower mass halos because the core
is overheated to a longer cooling time. The 3-D simulations show the formation
of a few-kpc scale, rotationally-supported, massive ()
cold gas torus. (abstract abridged)Comment: 22 pages, 15 figures; ApJ accepted version (figures downgraded to
smaller size, as required for arxiv submission
Scalable explicit implementation of anisotropic diffusion with Runge-Kutta-Legendre super-time-stepping
An important ingredient in numerical modelling of high temperature magnetised
astrophysical plasmas is the anisotropic transport of heat along magnetic field
lines from higher to lower temperatures.Magnetohydrodynamics (MHD) typically
involves solving the hyperbolic set of conservation equations along with the
induction equation. Incorporating anisotropic thermal conduction requires to
also treat parabolic terms arising from the diffusion operator. An explicit
treatment of parabolic terms will considerably reduce the simulation time step
due to its dependence on the square of the grid resolution () for
stability. Although an implicit scheme relaxes the constraint on stability, it
is difficult to distribute efficiently on a parallel architecture. Treating
parabolic terms with accelerated super-time stepping (STS) methods has been
discussed in literature but these methods suffer from poor accuracy (first
order in time) and also have difficult-to-choose tuneable stability parameters.
In this work we highlight a second order (in time) Runge Kutta Legendre (RKL)
scheme (first described by Meyer et. al. 2012) that is robust, fast and
accurate in treating parabolic terms alongside the hyperbolic conversation
laws. We demonstrate its superiority over the first order super time stepping
schemes with standard tests and astrophysical applications. We also show that
explicit conduction is particularly robust in handling saturated thermal
conduction. Parallel scaling of explicit conduction using RKL scheme is
demonstrated up to more than processors.Comment: 15 pages, 9 figures, incorporated comments from the referee. This
version is now accepted for publication in MNRA
Cool-Core Clusters : Role of BCG, Star Formation & AGN-Driven Turbulence
Recent analysis shows that it is important to explicitly include the
gravitational potential of the central brightest central galaxy (BCG) to infer
the acceleration due to gravity () and the free-fall time () in cool cluster cores. Accurately measuring
is crucial because according to numerical simulations cold gas condensation and
strong feedback occur in cluster cores with min()
below a threshold value close to 10. Recent observations which include the BCG
gravity show that the observed threshold in min() lies
at a somewhat higher value, close to 10-30; there are only a few clusters in
which this ratio falls much below 10. In this paper we compare numerical
simulations of feedback AGN (Active Galactic Nuclei) jets interacting with the
intracluster medium (ICM), with and without a BCG potential. We find that, for
a fixed feedback efficiency, the presence of a BCG does not significantly
affect the temperature but increases (decreases) the core density (entropy) on
average. Most importantly, min() is only affected
slightly by the inclusion of the BCG gravity. Also notable is that the lowest
value of min() in the NFW+BCG runs are about twice
larger than in the NFW runs. We also look at the role of depletion of cold gas
due to star formation and show that it only affects the rotationally dominant
component (torus), while the radially dominant component (which regulates the
feedback cycle) remains largely unaffected. The distribution of metals due to
AGN jets in our simulations is predominantly along the jet direction and the
radial spread of metals is less. We also show that the turbulence in cool core
clusters is weak, consistent with recent Hitomi results on Perseus cluster.Comment: Submitted to ApJ; Suggestions and comments are welcome; Substantially
updated from the previous version; Abstract abridged; 12 pages, 11 figure
The Case for Hot-Mode Accretion in Abell 2029
Radiative cooling and AGN heating are thought to form a feedback loop that
regulates the evolution of low redshift cool-core galaxy clusters. Numerical
simulations suggest that formation of multiphase gas in the cluster core
imposes a floor on the ratio of cooling time () to free-fall time
() at . Observations
of galaxy clusters show evidence for such a floor, and usually the cluster
cores with contain abundant
multiphase gas. However, there are important outliers. One of them is Abell
2029, a massive galaxy cluster ( M) with
, but little apparent multiphase gas.
In this paper, we present high resolution 3D hydrodynamic AMR simulations of a
cluster similar to A2029 and study how it evolves over a period of 1-2 Gyr.
Those simulations suggest that Abell 2029 self-regulates without producing
multiphase gas because the mass of its central black hole () is great enough for Bondi accretion of hot ambient gas to
produce enough feedback energy to compensate for radiative cooling.Comment: 8 pages, 5 figures, submitted to MNRA
Figuring Out Gas & Galaxies In Enzo (FOGGIE). IV. The Stochasticity of Ram Pressure Stripping in Galactic Halos
We study ram pressure stripping in simulated Milky Way-like halos at z>=2
from the Figuring Out Gas & Galaxies In Enzo (FOGGIE) project. These
simulations reach exquisite resolution in their circumgalactic medium (CGM) gas
owing to FOGGIE's novel refinement scheme. The CGM of each halo spans a wide
dynamic range in density and velocity over its volume---roughly 6 dex and 1000
km/s, respectively---translating into a 5 dex range in ram pressure imparted to
interacting satellites. The ram pressure profiles of the simulated CGM are
highly stochastic, owing to kpc-scale variations of the density and velocity
fields of the CGM gas. As a result, the efficacy of ram pressure stripping
depends strongly on the specific path a satellite takes through the CGM. The
ram-pressure history of a single satellite is generally unpredictable and not
well correlated with its approach vector with respect to the host galaxy. The
cumulative impact of ram pressure on the simulated satellites is dominated by
only a few short strong impulses---on average, 90% of the total surface
momentum gained through ram pressure is imparted in 20% or less of the total
orbital time. These results reveal an erratic mode of ram pressure stripping in
Milky-Way like halos at high redshift---one that is not captured by a smooth
spherically-averaged model of the circumgalactic medium.Comment: 18 pages, 10 figures. Submitted to Ap
AGN jet-driven stochastic cold accretion in cluster cores
International audienceSeveral arguments suggest that stochastic condensation of cold gas and its accretion on to the central supermassive black hole (SMBH) is essential for active galactic nuclei (AGNs) feedback to work in the most massive galaxies that lie at the centres of galaxy clusters. Our 3-D hydrodynamic AGN jet-ICM (intracluster medium) simulations, looking at the detailed angular momentum distribution of cold gas and its time variability for the first time, show that the angular momentum of the cold gas crossing ≲1 kpc is essentially isotropic. With almost equal mass in clockwise and counterclockwise orientations, we expect a cancellation of the angular momentum on roughly the dynamical time. This means that a compact accretion flow with a short viscous time ought to form, through which enough accretion power can be channeled into jet mechanical energy sufficiently quickly to prevent a cooling flow. The inherent stochasticity, expected in feedback cycles driven by cold gas condensation, gives rise to a large variation in the cold gas mass at the centres of galaxy clusters, for similar cluster and SMBH masses, in agreement with the observations. Such correlations are expected to be much tighter for the smoother hot/Bondi accretion. The weak correlation between cavity power and Bondi power obtained from our simulations also matches observations
Cool-core Clusters: The Role of BCG, Star Formation, and AGN-driven Turbulence
Recent observations of cool cluster cores that include the BCG gravity claim that the observed threshold in min(t(cool)/t(ff)) (cooling time to free-fall time ratio) lies at a somewhat higher value, close to 10-30, compared with the threshold seen in numerical simulations. There are only a few clusters in which this ratio falls much below 10. In this paper, we compare 3D hydrodynamic simulations of feedback active galactic nuclei (AGNs) jets interacting with the intracluster medium, with and without a BCG potential. We find that, for a fixed feedback efficiency, the presence of a BCG does not significantly affect the temperature, but increases (decreases) the core density (entropy) on average. Most importantly, min(t(cool)/t(ff)) is only affected slightly by the inclusion of the BCG gravity. Also notable is that the lowest value of min(t(cool)/t(ff)) in the NFW+BCG runs is about twice as large as in the NFW runs. We also look at the role of depletion of cold gas due to star formation, and show that it only affects the rotationally dominant component, while the radially dominant component remains largely unaffected. Stellar gas depletion also increases the repetition rate of AGN jets. The distribution of metals due to AGN jets in our simulations is predominantly along the jet direction, and the equatorial spread of metals is less compared with the observations. We also show that the turbulence in cool-core clusters is weak, which is consistent with recent Hitomi results on the Perseus cluster