220 research outputs found

### Relative velocity of dark matter and barions in clusters of galaxies and measurements of their peculiar velocities

The increasing sensitivity of current experiments, which nowadays routinely
measure the thermal SZ effect within galaxy clusters, provide the hope that
peculiar velocities of individual clusters of galaxies will be measured rather
soon using the kinematic SZ effect. Also next generation of X-ray telescopes
with microcalorimeters, promise first detections of the motion of the intra
cluster medium (ICM) within clusters. We used a large set of cosmological,
hydrodynamical simulations, which cover very large cosmological volume, hosting
a large number of rich clusters of galaxies, as well as moderate volumes where
the internal structures of individual galaxy clusters can be resolved with very
high resolution to investigate, how the presence of baryons and their
associated physical processes like cooling and star-formation are affecting the
systematic difference between mass averaged velocities of dark matter and the
ICM inside a cluster. We, for the first time, quantify the peculiar motion of
galaxy clusters as function of the large scale environment. We also demonstrate
that especially in very massive systems, the relative velocity of the ICM
compared to the cluster peculiar velocity add significant scatter onto the
inferred peculiar velocity, especially when measurements are limited to the
central regions of the cluster. Depending on the aperture used, this scatter
varies between 50% and 20%, when going from the core (e.g. ten percent of the
virial radius) to the full cluster (e.g. the virial radius).Comment: 17 pages, 18 figures, submitted to MNRA

### Adaptive gravitational softening in GADGET

Cosmological simulations of structure formation follow the collisionless
evolution of dark matter starting from a nearly homogeneous field at early
times down to the highly clustered configuration at redshift zero. The density
field is sampled by a number of particles in number infinitely smaller than
those believed to be its actual components and this limits the mass and spatial
scales over which we can trust the results of a simulation. Softening of the
gravitational force is introduced in collisionless simulations to limit the
importance of close encounters between these particles. The scale of softening
is generally fixed and chosen as a compromise between the need for high spatial
resolution and the need to limit the particle noise. In the scenario of
cosmological simulations, where the density field evolves to a highly
inhomogeneous state, this compromise results in an appropriate choice only for
a certain class of objects, the others being subject to either a biased or a
noisy dynamical description. We have implemented adaptive gravitational
softening lengths in the cosmological simulation code GADGET; the formalism
allows the softening scale to vary in space and time according to the density
of the environment, at the price of modifying the equation of motion for the
particles in order to be consistent with the new dependencies introduced in the
system's Lagrangian. We have applied the technique to a number of test cases
and to a set of cosmological simulations of structure formation. We conclude
that the use of adaptive softening enhances the clustering of particles at
small scales, a result visible in the amplitude of the correlation function and
in the inner profile of massive objects, thereby anticipating the results
expected from much higher resolution simulations.Comment: 15 pages, 21 figures, 1 table. Accepted for publication in MNRA

### QSO-galaxy correlations due to weak lensing in arbitrary Friedmann-Lemaitre cosmologies

We calculate the angular cross-correlation function between background QSOs
and foreground galaxies induced by the weak lensing effect of large-scale
structures. Results are given for arbitrary Friedmann-Lemaitre cosmologies. The
non-linear growth of density perturbations is included. Compared to the linear
growth, the non-linear growth increases the correlation amplitude by about an
order of magnitude in an Einstein-de Sitter universe, and by even more for
lower Omega_0. The dependence of the correlation amplitude on the cosmological
parameters strongly depends on the normalization of the power spectrum. The
QSO-galaxy cross-correlation function is most sensitive to density structures
on scales in the range (1-10) Mpc/h, where the normalization of the power
spectrum to the observed cluster abundance appears most appropriate. In that
case, the correlation strength changes by less than a factor of <~ 2 when
Omega_0 varies between 0.3 and 1, quite independent of the value of
Omega_Lambda. For Omega_0 <~ 0.3, the correlation strength increases with
decreasing Omega_0, and it scales approximately linearly with the Hubble
constant h.Comment: revised version, accepted by MNRA

### SZ effects in the Magneticum Pathfinder Simulation: Comparison with the Planck, SPT, and ACT results

We calculate the one-point probability density distribution functions (PDF)
and the power spectra of the thermal and kinetic Sunyaev-Zeldovich (tSZ and
kSZ) effects and the mean Compton Y parameter using the Magneticum Pathfinder
simulations, state-of-the-art cosmological hydrodynamical simulations of a
large cosmological volume of (896 Mpc/h)^3. These simulations follow in detail
the thermal and chemical evolution of the intracluster medium as well as the
evolution of super-massive black holes and their associated feedback processes.
We construct full-sky maps of tSZ and kSZ from the light-cones out to z=0.17,
and one realization of 8.8x8.8 degree wide, deep light-cone out to z=5.2. The
local universe at z<0.027 is simulated by a constrained realisation. The tail
of the one-point PDF of tSZ from the deep light-cone follows a power-law shape
with an index of -3.2. Once convolved with the effective beam of Planck, it
agrees with the PDF measured by Planck. The predicted tSZ power spectrum agrees
with that of the Planck data at all multipoles up to l~1000, once the
calculations are scaled to the Planck 2015 cosmological parameters with
\Omega_m=0.308 and \sigma_8=0.8149. Consistent with the results in the
literature, however, we continue to find the tSZ power spectrum at l=3000 that
is significantly larger than that estimated from the high-resolution
ground-based data. The simulation predicts the mean fluctuating Compton Y value
of =1.18x10^{-6} for \Omega_m=0.272 and \sigma_8=0.809. Nearly half (~
5x10^{-7}) of the signal comes from halos below a virial mass of
10^{13}M_\odot/h. Scaling this to the Planck 2015 parameters, we find
=1.57x10^{-6}. The PDF and the power spectrum of kSZ from our simulation
agree broadly with the previous work.Comment: 16 pages, 10 figures, MNRAS in press, http://www.mageticum.or

### The temperature-mass relation in magnetized galaxy clusters

We use cosmological, magneto-hydrodynamic simulations of galaxy clusters to
quantify the dynamical importance of magnetic fields in these clusters. The
set-up of initial magnetic field strengths at high redshifts is chosen such
that observed Faraday-rotation measurements in low-redshift clusters are well
reproduced in the simulations. We compute the radial profiles of the
intracluster gas temperature and of the thermal and magnetic pressure in a set
of clusters simulated in the framework of an Einstein-de Sitter and a
low-density, spatially-flat CDM cosmological model. We find that, for a
realistic range of initial magnetic field strengths, the temperature of the
intracluster gas changes by less than $\approx5%$.Comment: Accepted for publication in A&

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