3,516 research outputs found
Detecting dark matter-dark energy coupling with the halo mass function
We use high-resolution simulations of large-scale structure formation to
analyze the effects of interacting dark matter and dark energy on the evolution
of the halo mass function. Using a chi-square likelihood analysis, we find
significant differences in the mass function between models of coupled dark
matter-dark energy and standard concordance cosmology Lambda-CDM out to
redshift z=1.5. We also find a preliminary indication that the Dark Energy
Survey should be able to distinguish these models from Lambda-CDM within its
mass and redshift contraints. While we can distinguish the effects of these
models from Lambda-CDM cosmologies with different fundamental parameters, DES
will require independent measurements of sigma-8 to confirm these effects.Comment: 5 pages, 3 figures, responded to referee comments, accepted by Ap
Pre-Processing and Post-Processing in Group-Cluster Mergers
Galaxies in clusters are more likely to be of early type and to have lower
star formation rates than galaxies in the field. Recent observations and
simulations suggest that cluster galaxies may be `pre-processed' by group or
filament environments and that galaxies that fall into a cluster as part of a
larger group can stay coherent within the cluster for up to one orbital period
(`post-processing'). We investigate these ideas by means of a cosmological
-body simulation and idealized -body plus hydrodynamics simulations of a
group-cluster merger. We find that group environments can contribute
significantly to galaxy pre-processing by means of enhanced galaxy-galaxy
merger rates, removal of galaxies' hot halo gas by ram pressure stripping, and
tidal truncation of their galaxies. Tidal distortion of the group during infall
does not contribute to pre-processing. Post-processing is also shown to be
effective: galaxy-galaxy collisions are enhanced during a group's pericentric
passage within a cluster, the merger shock enhances the ram pressure on group
and cluster galaxies, and an increase in local density during the merger leads
to greater galactic tidal truncation.Comment: Accepted for publication in MNRAS. 25 pages, 21 figure
Elastic theory of icosahedral quasicrystals - application to straight dislocations
In quasicrystals, there are not only conventional, but also phason
displacement fields and associated Burgers vectors. We have calculated
approximate solutions for the elastic fields induced by two-, three- and
fivefold straight screw- and edge-dislocations in infinite icosahedral
quasicrystals by means of a generalized perturbation method. Starting from the
solution for elastic isotropy in phonon and phason spaces, corrections of
higher order reflect the two-, three- and fivefold symmetry of the elastic
fields surrounding screw dislocations. The fields of special edge dislocations
display characteristic symmetries also, which can be seen from the
contributions of all orders.Comment: 13 pages, 11 figure
Cluster magnetic fields from active galactic nuclei
Active galactic nuclei (AGN) found at the centers of clusters of galaxies are
a possible source for weak cluster-wide magnetic fields. To evaluate this
scenario, we present 3D adaptive mesh refinement MHD simulations of a cool-core
cluster that include injection of kinetic, thermal, and magnetic energy via an
AGN-powered jet. Using the MHD solver in FLASH 2, we compare several
sub-resolution approaches that link the estimated accretion rate as measured on
the simulation mesh to the accretion rate onto the central black hole and the
resulting feedback. We examine the effects of magnetized outflows on the
accretion history of the black hole and discuss the ability of these models to
magnetize the cluster medium.Comment: 4 pages, 2 figures, submitted to conference proceedings "The
Monster's Fiery Breath: Feedback in Groups, Galaxies, and Clusters
The Influence of AGN Feedback on Galaxy Cluster Observables
Galaxy clusters are valuable cosmological probes. However, cluster mass
estimates rely on observable quantities that are affected by complicated
baryonic physics in the intracluster medium (ICM), including feedback from
active galactic nuclei (AGN). Cosmological simulations have started to include
AGN feedback using subgrid models. In order to make robust predictions, the
systematics of different implementations and parametrizations need to be
understood. We have developed an AGN subgrid model in FLASH that supports a few
different black hole accretion models and feedback models. We use this model to
study the effect of AGN on X-ray cluster observables and its dependence on
model variations.Comment: minor error corrected, to appear in proceedings of the conference
"The Monster's Fiery Breath: Feedback in Galaxies, Groups, and Clusters",
June 2009, Madison, Wisconsi
Glassy behavior of molecular crystals: A comparison between results from MD-simulation and mode coupling theory
We have investigated the glassy behavior of a molecular crystal built up with
chloroadamantane molecules. For a simple model of this molecule and a rigid fcc
lattice a MD simulation was performed from which we obtained the dynamical
orientational correlators and the ``self''
correlators , with ,
. Our investigations are for the diagonal correlators
. Since the lattice constant decreases with decreasing
temperature which leads to an increase of the steric hindrance of the
molecules, we find a strong slowing down of the relaxation. It has a high
sensitivity on , . For most , there is a two-step
relaxation process, but practically not for , ,
and . Our results are consistent with the -relaxation
scaling laws predicted by mode coupling theory from which we deduce the glass
transition temperature . From a first principle solution
of the mode coupling equations we find . Furthermore mode
coupling theory reproduces the absence of a two-step relaxation process for
, , and , but underestimates the critical
nonergodicity parameters by about 50 per cent for all other . It is
suggested that this underestimation originates from the anisotropic crystal
field which is not accounted for by mode coupling theory. Our results also
imply that phonons have no essential influence on the long time relaxation
Simulations of the symbiotic recurrent nova V407 Cyg. I. Accretion and shock evolutions
The shock interaction and evolution of nova ejecta with a wind from a red
giant star in a symbiotic binary system are investigated via three-dimensional
hydrodynamics simulations. We specifically model the March 2010 outburst of the
symbiotic recurrent nova V407~Cygni from the quiescent phase to its eruption
phase. The circumstellar density enhancement due to wind-white dwarf
interaction is studied in detail. It is found that the density-enhancement
efficiency depends on the ratio of the orbital speed to the red giant wind
speed. Unlike another recurrent nova, RS~Ophiuchi, we do not observe a strong
disk-like density enhancement, but instead observe an aspherical density
distribution with higher density in the equatorial plane than at
the poles. To model the 2010 outburst, we consider several physical parameters,
including the red giant mass loss rate, nova eruption energy, and ejecta mass.
A detailed study of the shock interaction and evolution reveals that the
interaction of shocks with the red giant wind generates strong Rayleigh-Taylor
instabilities. In addition, the presence of the companion and circumstellar
density enhancement greatly alter the shock evolution during the nova phase.
The ejecta speed after sweeping out most of the circumstellar medium decreases
to km-s, depending on model, which is consistent with the
observed extended redward emission in [N~II] lines in April 2011.Comment: ApJ, In Press. Simulation Animation: https://youtu.be/g5Nu7vDfCO
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