30 research outputs found
An MHD Gadget for cosmological simulations
Various radio observations have showed that the hot atmospheres of galaxy
clusters are magnetized. However, our understanding of the origin of these
magnetic fields, their implications on structure formation and their interplay
with the dynamics of the cluster atmosphere, especially in the centers of
galaxy clusters is still very limited. In preparation to the upcoming new
generation of radio telescopes (like EVLA, LWA, LOFAR and SKA), a huge effort
is being made to learn more about cosmological magnetic fields from the
observational perspective. Here we present the implementation of magneto
hydrodynamics in the cosmological SPH code GADGET. We discuss the details of
the implementation and various schemes to suppress numerical instabilities as
well as regularization schemes, in the context of cosmological simulations. The
performance of the SPH MHD code is demonstrated in various one and two
dimensional test problems, which we performed if a fully, three dimensional
setup to test the code under realistic circumstances. Comparing with solutions
obtained with ATHENA, we find excellent agreement with our SPH MHD
implementation. Finally we apply our SPH MHD implementation to forming galaxy
clusters within a large, cosmological box. Performing a resolution study we
demonstrate the robustness of the predicted shape of the magnetic field
profiles in galaxy clusters, which is in good agreement with previous studies.Comment: Updated version, 21 pages, 21 figures, submitted to MNRA
Measuring cosmic magnetic fields by rotation measure-galaxy cross-correlations in cosmological simulations
Using cosmological MHD simulations of the magnetic field in galaxy clusters
and filaments we evaluate the possibility to infer the magnetic field strength
in filaments by measuring cross-correlation functions between Faraday Rotation
Measures (RM) and the galaxy density field. We also test the reliability of
recent estimates considering the problem of data quality and Galactic
foreground (GF) removal in current datasets. Besides the two self-consistent
simulations of cosmological magnetic fields based on primordial seed fields and
galactic outflows analyzed here, we also explore a larger range of models
scaling up the resulting magnetic fields of one of the simulations. We find
that, if an unnormalized estimator for the cross-correlation functions and a GF
removal procedure is used, the detectability of the cosmological signal is only
possible for future instruments (e.g. SKA and ASKAP). However, mapping of the
observed RM signal to the underlying magnetization of the Universe (both in
space and time) is an extremely challenging task which is limited by the
ambiguities of our model parameters, as well as to the weak response of the RM
signal in low density environments. Therefore, we conclude that current data
cannot constrain the amplitude and distribution of magnetic fields within the
large scale structure and a detailed theoretical understanding of the build up
and distribution of magnetic fields within the Universe will be needed for the
interpretation of future observations.Comment: 11 pages, 11 figures, comparation between RM data and simulations in
fig. 8, submited to MNRAS
Infalling clouds onto super-massive black hole binaries - I. Formation of discs, accretion and gas dynamics
There is compelling evidence that most -if not all- galaxies harbour a
super-massive black hole (SMBH) at their nucleus, hence binaries of these
massive objects are an inevitable product of the hierarchical evolution of
structures in the universe, and represent an important but thus-far elusive
phase of galaxy evolution. Gas accretion via a circumbinary disc is thought to
be important for the dynamical evolution of SMBH binaries, as well as in
producing luminous emission that can be used to infer their properties. One
plausible source of the gaseous fuel is clumps of gas formed due to turbulence
and gravitational instabilities in the interstellar medium, that later fall
toward and interact with the binary. In this context, we model numerically the
evolution of turbulent clouds in near-radial infall onto equal-mass SMBH
binaries, using a modified version of the SPH code GADGET-3. We present a total
of 12 simulations that explore different possible pericentre distances and
relative inclinations, and show that the formation of circumbinary discs and
discs around each SMBH ('mini-discs') depend on those parameters. We also study
the dynamics of the formed discs, and the variability of the feeding rate onto
the SMBHs in the different configurations.Comment: Accepted for publication in MNRAS, 17 pages, 13 figures. Animations
available at http://www.astro.puc.cl/~fgarrido/animation
Magnetic field structure due to the global velocity field in spiral galaxies
We present a set of global, self-consistent N-body/SPH simulations of the
dynamic evolution of galactic discs with gas and including magnetic fields. We
have implemented a description to follow the evolution of magnetic fields with
the ideal induction equation in the SPH part of the Vine code. Results from a
direct implementation of the field equations are compared to a representation
by Euler potentials, which pose a div(B)-free description, an constraint not
fulfilled for the direct implementation. All simulations are compared to an
implementation of magnetic fields in the Gadget code which includes also
cleaning methods for div(B).
Starting with a homogeneous seed field we find that by differential rotation
and spiral structure formation of the disc the field is amplified by one order
of magnitude within five rotation periods of the disc. The amplification is
stronger for higher numerical resolution. Moreover, we find a tight connection
of the magnetic field structure to the density pattern of the galaxy in our
simulations, with the magnetic field lines being aligned with the developing
spiral pattern of the gas. Our simulations clearly show the importance of
non-axisymmetry for the evolution of the magnetic field.Comment: 17 pages, 18 figure
Simulating magnetic fields in the Antennae galaxies
We present self-consistent high-resolution simulations of NGC4038/4039 (the
"Antennae galaxies") including star formation, supernova feedback and magnetic
fields performed with the N-body/SPH code Gadget, in which magnetohydrodynamics
are followed with the SPH method. We vary the initial magnetic field in the
progenitor disks from 1 nG to 100 muG. At the time of the best match with the
central region of the Antennae system the magnetic field has been amplified by
compression and shear flows to an equilibrium field of approximately 10 muG,
independent of the initial seed field. These simulations are a proof of the
principle that galaxy mergers are efficient drivers for the cosmic evolution of
magnetic fields. We present a detailed analysis of the magnetic field structure
in the central overlap region. Simulated radio and polarization maps are in
good morphological and quantitative agreement with the observations. In
particular, the two cores with the highest synchrotron intensity and ridges of
regular magnetic fields between the cores and at the root of the southern tidal
arm develop naturally in our simulations. This indicates that the simulations
are capable of realistically following the evolution of the magnetic fields in
a highly non-linear environment. We also discuss the relevance of the
amplification effect for present day magnetic fields in the context of
hierarchical structure formation.Comment: 18 pages, 14 figures, accepte
A divergence-cleaning scheme for cosmological SPMHD simulations
In magnetohydrodynamics (MHD), the magnetic field is evolved by the induction
equation and coupled to the gas dynamics by the Lorentz force. We perform
numerical smoothed particle magnetohydrodynamics (Spmhd) simulations and study
the influence of a numerical magnetic divergence. For instabilities arising
from divergence B related errors, we find the hyperbolic/parabolic cleaning
scheme suggested by Dedner et al. 2002 to give good results and prevent
numerical artifacts from growing. Additionally, we demonstrate that certain
current Spmhd implementations of magnetic field regularizations give rise to
unphysical instabilities in long-time simulations. We also find this effect
when employing Euler potentials (divergenceless by definition), which are not
able to follow the winding-up process of magnetic field lines properly.
Furthermore, we present cosmological simulations of galaxy cluster formation at
extremely high resolution including the evolution of magnetic fields. We show
synthetic Faraday rotation maps and derive structure functions to compare them
with observations. Comparing all the simulations with and without divergence
cleaning, we are able to confirm the results of previous simulations performed
with the standard implementation of MHD in Spmhd at normal resolution. However,
at extremely high resolution, a cleaning scheme is needed to prevent the growth
of numerical errors at small scales.Comment: 15 pages, 19 figures, submitted to MNRA
Magnetic Field Amplification in Galaxy Clusters and its Simulation
We review the present theoretical and numerical understanding of magnetic
field amplification in cosmic large-scale structure, on length scales of galaxy
clusters and beyond. Structure formation drives compression and turbulence,
which amplify tiny magnetic seed fields to the microGauss values that are
observed in the intracluster medium. This process is intimately connected to
the properties of turbulence and the microphysics of the intra-cluster medium.
Additional roles are played by merger induced shocks that sweep through the
intra-cluster medium and motions induced by sloshing cool cores. The accurate
simulation of magnetic field amplification in clusters still poses a serious
challenge for simulations of cosmological structure formation. We review the
current literature on cosmological simulations that include magnetic fields and
outline theoretical as well as numerical challenges.Comment: 60 pages, 19 Figure
A non-ideal MHD Gadget: Simulating massive galaxy clusters
Magnetic fields in the intra-cluster medium of galaxy clusters have been
studied in the past years through different methods. In the next years the
up-coming generation of radio telescopes is going to provide new data that have
the potential of setting constraints on the properties of magnetic fields in
galaxy clusters. Here we present zoomed-in simulations for a set of massive
galaxy clusters (M_v > 10^15 M_sun/h). This is an ideal sample to study the
evolution of magnetic field during the process of structure formation in
detail. Turbulent motions of the gas within the ICM will manifest themselves in
a macroscopic magnetic resistivity eta_m, which has to be taken explicitly into
account, especially at scales below the resolution limit. We have adapted the
MHD GADGET code by Dolag & Stasyszyn (2009) to include the treatment of the
magnetic resistivity and for the first time we have included non-ideal MHD
equations to better follow the evolution of the magnetic field within galaxy
clusters. We investigate which value of the magnetic resistivity eta_m is
required to match the magnetic field profile derived from radio observations.
We find that a value of eta_m ~ 6 10^27 cm^2/s is necessary to recover the
shape of the magnetic field profile inferred from radio observations of the
Coma cluster. This value agrees well with the expected level of turbulent
motions within the ICM at our resolution limit. The magnetic field profiles can
be fitted by a beta-model like profile (Cavaliere & Fusco-Femiano 1976), with
small dispersion of the parameters. We find also that that the temperature,
density and entropy profiles of the clusters depend on the magnetic resistivity
constant, having flatter profiles in the inner regions when the magnetic
resistivity increases.Comment: 19 pages, 11 figures. Resubmitted to MNRAS after referee repor