15 research outputs found
Shape, spin and baryon fraction of clusters in the MareNostrum Universe
The MareNostrum Universe is one of the largest cosmological
SPH simulation done so far. It consists of dark and
gas particles in a box of 500 Mpc on a side. Here we study
the shapes and spins of the dark matter and gas components of the 10,000 most
massive objects extracted from the simulation as well as the gas fraction in
those objects. We find that the shapes of objects tend to be prolate both in
the dark matter and gas. There is a clear dependence of shape on halo mass, the
more massive ones being less spherical than the less massive objects. The gas
distribution is nevertheless much more spherical than the dark matter, although
the triaxiality parameters of gas and dark matter differ only by a few percent
and it increases with cluster mass. The spin parameters of gas and dark matter
can be well fitted by a lognormal distribution function. On average, the spin
of gas is 1.4 larger than the spin of dark matter. We find a similar behavior
for the spins at higher redshifts, with a slightly decrease of the spin ratios
to 1.16 at The cosmic normalized baryon fraction in the entire cluster
sample ranges from , at to at . At both
redshifts we find a slightly, but statistically significant decrease of
with cluster mass.Comment: 7 pages, 6 figures. Accepted for publication in The Astrophysical
Journa
Dwarf Dark Matter Halos
We study properties of dark matter halos at high redshifts z=2-10 for a vast
range of masses with the emphasis on dwarf halos with masses 10^7-10^9 Msun/h.
We find that the density profiles of relaxed dwarf halos are well fitted by the
NFW profile and do not have cores. We compute the halo mass function and the
halo spin parameter distribution and find that the former is very well
reproduced by the Sheth & Tormen model while the latter is well fitted by a
lognormal distribution with lambda_0 = 0.042 and sigma_lambda = 0.63. We
estimate the distribution of concentrations for halos in mass range that covers
six orders of magnitude from 10^7 Msun/h to 10^13} Msun/h, and find that the
data are well reproduced by the model of Bullock et al. The extrapolation of
our results to z = 0 predicts that present-day isolated dwarf halos should have
a very large median concentration of ~ 35. We measure the subhalo circular
velocity functions for halos with masses that range from 4.6 x 10^9 Msun/h to
10^13 Msun/h and find that they are similar when normalized to the circular
velocity of the parent halo. Dwarf halos studied in this paper are many orders
of magnitude smaller than well-studied cluster- and Milky Way-sized halos. Yet,
in all respects the dwarfs are just down-scaled versions of the large halos.
They are cuspy and, as expected, more concentrated. They have the same spin
parameter distribution and follow the same mass function that was measured for
large halos.Comment: Accepted to be pusblished by ApJ, 12 pages, 8 figures, LaTeX
(documentclass preprint2). Differences with respect to the previous
submission are: (i) abstract was modified slightly to make it more
transparent to the reader, (ii) an extra figure has been added, and (3) some
minor modifications to the main text were also don
Simulations of ultra-high energy cosmic rays in the local Universe and the origin of cosmic magnetic fields
We simulate the propagation of cosmic rays at ultra-high energies, ≳1018 eV, in models of extragalactic magnetic fields in constrained simulations of the local Universe. We use constrained initial conditions with the cosmological magnetohydrodynamics code ENZO. The resulting models of the distribution of magnetic fields in the local Universe are used in the CRPROPA code to simulate the propagation of ultra-high energy cosmic rays. We investigate the impact of six different magneto-genesis scenarios, both primordial and astrophysical, on the propagation of cosmic rays over cosmological distances. Moreover, we study the influence of different source distributions around the Milky Way. Our study shows that different scenarios of magneto-genesis do not have a large impact on the anisotropy measurements of ultra-high energy cosmic rays. However, at high energies above the Greisen–Zatsepin–Kuzmin (GZK)-limit, there is anisotropy caused by the distribution of nearby sources, independent of the magnetic field model. This provides a chance to identify cosmic ray sources with future full-sky measurements and high number statistics at the highest energies. Finally, we compare our results to the dipole signal measured by the Pierre Auger Observatory. All our source models and magnetic field models could reproduce the observed dipole amplitude with a pure iron injection composition. Our results indicate that the dipole is observed due to clustering of secondary nuclei in direction of nearby sources of heavy nuclei. A light injection composition is disfavoured, since the increase in dipole angular power from 4 to 8 EeV is too slow compared to observation by the Pierre Auger Observatory
How far do they go? The outer structure of dark matter halos
We study the density profiles of collapsed galaxy-size dark matter halos with
masses 1e11-5e12 Msun focusing mostly on the halo outer regions from the formal
virial radius Rvir up to 5-7Rvir. We find that isolated halos in this mass
range extend well beyond Rvir exhibiting all properties of virialized objects
up to 2-3Rvir: relatively smooth density profiles and no systematic infall
velocities. The dark matter halos in this mass range do not grow as one naively
may expect through a steady accretion of satellites, i.e., on average there is
no mass infall. This is strikingly different from more massive halos, which
have large infall velocities outside of the virial radius. We provide accurate
fit for the density profile of these galaxy-size halos. For a wide range
(0.01-2)Rvir of radii the halo density profiles are fit with the approximation
rho=rho_s exp(-2n[x^{1/n}-1])+rho_m, where x=r/r_s, rho_m is the mean matter
density of the Universe, and the index n is in the range n=6-7.5. These
profiles do not show a sudden change of behavior beyond the virial radius. For
larger radii we combine the statistics of the initial fluctuations with the
spherical collapse model to obtain predictions for the mean and most probable
density profiles for halos of several masses. The model give excellent results
beyond 2-3 formal virial radii.Comment: 15 pages, 10 figures, submitted to Ap
The dependence on environment of Cold Dark Matter Halo properties
High-resolution LCDM cosmological N-body simulations are used to study the
properties of galaxy-size dark halos in different environments (cluster, void,
and "field"). Halos in clusters and their surroundings have a median spin
parameter ~1.3 times lower, and tend to be more spherical and to have less
aligned internal angular momentum than halos in voids and the field. For halos
in clusters the concentration parameters decrease on average with mass with a
slope of ~0.1; for halos in voids these concentrations do not change with mass.
For masses <5 10^11 M_sh^-1, halos in clusters are on average ~30-40% more
concentrated and have ~2 times higher central densities than halos in voids.
When comparing only parent halos, the differences are less pronounced but they
are still significant. The Vmax-and Vrms-mass relations are shallower and more
scattered for halos in clusters than in voids, and for a given Vmax or Vrms,
the mass is smaller at z=1 than at z=0 in all the environments. At z=1, the
differences in the halo properties with environment almost dissapear,
suggesting this that the differences were stablished mainly after z~1. The
halos in clusters undergo more dramatic changes than those in the field or the
voids. The differences with environment are owing to (i) the dependence of halo
formation time on environment, and (ii) local effects as tidal stripping and
the tumultuos histories that halos suffer in high-density regions. We calculate
seminumerical models of disk galaxy evolution in halos with the properties
found for the different environments. For a given disk mass, the galaxy disks
have higher surface density, larger Vd,max and secular bulge-to-disk ratio,
lower gas fraction, and are redder as one goes from cluster to void
environments, in rough agreement with observations. (abridged)Comment: 28 pages, 13 figures included. To appear in The Astrophysical Journa