4,941 research outputs found
Operations of and Future Plans for the Pierre Auger Observatory
Technical reports on operations and features of the Pierre Auger Observatory,
including ongoing and planned enhancements and the status of the future
northern hemisphere portion of the Observatory. Contributions to the 31st
International Cosmic Ray Conference, Lodz, Poland, July 2009.Comment: Contributions to the 31st ICRC, Lodz, Poland, July 200
Investigating the influence of magnetic fields upon structure formation with AMIGA - a C code for cosmological magnetohydrodynamics
Despite greatly improved observational methods, the presence of magnetic
fields at cosmological scales and their role in the process of large-scale
structure formation still remains unclear. In this paper we want to address the
question how the presence of a hypothetical primordial magnetic field on large
scales influences the cosmic structure formation in numerical simulations. As a
tool for carrying out such simulations, we present our new numerical code
AMIGA. It combines an N-body code with an Eulerian grid-based solver for the
full set of MHD equations in order to conduct simulations of dark matter,
baryons and magnetic fields in a self-consistent way in a fully cosmological
setting. Our numerical scheme includes effective methodes to ensure proper
capturing of shocks and highly supersonic flows and a divergence-free magnetic
field. The high accuracy of the code is demonstrated by a number of numerical
tests. We then present a series of cosmological MHD simulations and confirm
that, in order to have a significant effect on the distribution of matter on
large scales, the primordial magnetic field strength would have to be
significantly higher than the current observational and theoretical
constraints.Comment: accepted by MNRAS, 24 pages, 14 figure
Spartan Daily, October 29, 1990
Volume 95, Issue 42https://scholarworks.sjsu.edu/spartandaily/8041/thumbnail.jp
Constrained simulations of the local universe: I. Mass and motion in the Local Volume
It has been recently claimed that there is no correlation between the
distribution of galaxies and their peculiar velocities within the Local Volume
(LV), namely a sphere of R=7/h Mpc around the Local Group (LG). It has been
then stated that this implies that either locally dark matter is not
distributed in the same way as luminous matter, or peculiar velocities are not
due to fluctuations in mass. To test that statement a set of constrained N-body
cosmological simulations, designed to reproduce the main observed large scale
structure, have been analyzed. The simulations were performed within the
flat-Lambda, open and flat matter only CDM cosmogonies. Two unconstrained
simulations of the flat-Lambda and open CDM models were performed for
comparison. LG-like objects have been selected so as to mimic the real LG
environment. The local gravitational field due to all halos found within each
LV is compared with the exact gravitational field induced by all matter in the
simulation. We conclude that there is no correlation between the exact and the
local gravitational field obtained by pairwise newtonian forces between halos.
Moreover, the local gravitational field is uncorrelated with the peculiar
velocities of halos. The exact gravitational field has a linear correlation
with peculiar velocities but the proportionality constant relating the velocity
with gravitational field falls below the prediction of the linear theory. Upon
considering all matter inside the LVs, the exact and local gravitational
accelerations show a much better correlation, but with a considerable scatter
independent on the cosmological models. The main conclusion is that the lack of
correlation between the local gravitation and the peculiar velocity fields
around LG-like objects is naturally expected in the CDM cosmologies.Comment: 10 pages, 19 figures. Accepted for publication in MNRA
The AMIGA sample of isolated galaxies. IV. A catalogue of neighbours around isolated galaxies
Studies of the effects of environment on galaxy properties and evolution
require well defined control samples. Such isolated galaxy samples have up to
now been small or poorly defined. The AMIGA project (Analysis of the
interstellar Medium of Isolated GAlaxies) represents an attempt to define a
statistically useful sample of the most isolated galaxies in the local (z <
0.05) Universe. A suitable large sample for the AMIGA project already exists,
the Catalogue of Isolated Galaxies (CIG, Karachentseva 1973; 1050 galaxies),
and we use this sample as a starting point to refine and perform a better
quantification of its isolation properties. Digitised POSS-I E images were
analysed out to a minimum projected radius R > 0.5 Mpc around 950 CIG galaxies
(those within Vr = 1500 km s-1 were excluded). We identified all galaxy
candidates in each field brighter than B = 17.5 with a high degree of
confidence using the LMORPHO software. We generated a catalogue of
approximately 54 000 potential neighbours (redshifts exist for 30% of this
sample). Six hundred sixty-six galaxies pass and two hundred eighty-four fail
the original CIG isolation criterion. The available redshift data confirm that
our catalogue involves a largely background population rather than physically
associated neighbours. We find that the exclusion of neighbours within a factor
of four in size around each CIG galaxy, employed in the original isolation
criterion, corresponds to Delta Vr ~ 18000 km s-1 indicating that it was a
conservative limit. Galaxies in the CIG have been found to show different
degrees of isolation. We conclude that a quantitative measure of this is
mandatory. It will be the subject of future work based on the catalogue of
neighbours obtained here.Comment: Accepted by A&A, 10 pages, 8 figures, 4 table
Halo abundances and shear in void models
We study the non-linear gravitational collapse of dark matter into halos
through numerical N-body simulations of Lemaitre-Tolman-Bondi void models. We
extend the halo mass function formalism to these models in a consistent way.
This extension not only compares well with the simulated data at all times and
radii, but it also gives interesting clues about the impact of the background
shear on the growth of perturbations. Our results give hints about the
possibility of constraining the background shear via cluster number counts,
which could then give rise to strong constraints on general inhomogeneous
models, of any scale.Comment: 5 pages, 3 figures, Accepted in Physics of the Dark Universe,
preprint IFT-UAM/CSIC-12-3
- …