1,885 research outputs found
Cluster Alignments and Ellipticities in LCDM Cosmology
The ellipticities and alignments of clusters of galaxies, and their evolution
with redshift, are examined in the context of a Lambda-dominated cold dark
matter cosmology. We use a large-scale, high-resolution N-body simulation to
model the matter distribution in a light cone containing ~10^6 clusters out to
redshifts of z=3. Cluster ellipticities are determined as a function of mass,
radius, and redshift, both in 3D and in projection. We find strong cluster
ellipticities: the mean ellipticity increases with redshift from 0.3 at z=0 to
0.5 at z=3, for both 3D and 2D ellipticities; the evolution is well-fit by
e=0.33+0.05z. The ellipticities increase with cluster mass and with cluster
radius; the main cluster body is more elliptical than the cluster cores, but
the increase of ellipticities with redshift is preserved. Using the fitted
cluster ellipsoids, we determine the alignment of clusters as a function of
their separation. We find strong alignment of clusters for separations <100
Mpc/h; the alignment increases with decreasing separation and with increasing
redshift. The evolution of clusters from highly aligned and elongated systems
at early times to lower alignment and elongation at present reflects the
hierarchical and filamentary nature of structure formation. These measures of
cluster ellipticity and alignment will provide a new test of the current
cosmological model when compared with upcoming cluster surveys.Comment: 29 pages including 13 figures, to appear in ApJ Jan. 2005 (corrected
typos, added reference
A perturbative determination of O(a) boundary improvement coefficients for the Schr\"odinger Functional coupling at 1-loop with improved gauge actions
We determine O() boundary improvement coefficients up to 1-loop level for
the Schr\"odinger Functional coupling with improved gauge actions including
plaquette and rectangle loops. These coefficients are required to implement
1-loop O() improvement in full QCD simulations for the coupling with the
improved gauge actions. To this order, lattice artifacts of step scaling
function for each improved gauge action are also investigated. In addition,
passing through the SF scheme, we estimate the ratio of -parameters
between the improved gauge actions and the plaquette action more accurately.Comment: 17 pages, 2 figures, 6 table
Cluster Ellipticities as a Cosmological Probe
We investigate the dependence of ellipticities of clusters of galaxies on
cosmological parameters using large-scale cosmological simulations. We
determine cluster ellipticities out to redshift unity for LCDM models with
different mean densities and amplitudes of mass fluctuation
. The mean ellipticity increases monotonically with redshift for
all models. Larger values of , i.e., earlier cluster formation
time, produce lower ellipticities. The dependence of ellipticity on
is relatively weak in the range for high mass
clusters. The mean ellipticity decreases linearly with the
amplitude of fluctuations at the cluster redshift , nearly independent of
; on average, older clusters are more relaxed and are thus less
elliptical. The distribution of ellipticities about the mean is approximated by
a Gaussian, allowing a simple characterization of the evolution of ellipticity
with redshift as a function of cosmological parameters. At , the mean
ellipticity of high mass clusters is approximated by . This relation opens up the
possibility that, when compared with future observations of large cluster
samples, the mean cluster ellipticity and its evolution could be used as a new,
independent tool to constrain cosmological parameters, especially the amplitude
of mass fluctuations, .Comment: 16 pages, 4 figure
Statistical kinetic treatment of relativistic binary collisions
In particle-based algorithms, the effect of binary collisions is commonly
described in a statistical way, using Monte Carlo techniques. It is shown that,
in the relativistic regime, stringent constraints should be considered on the
sampling of particle pairs for collision, which are critical to ensure
physically meaningful results, and that nonrelativistic sampling criteria
(e.g., uniform random pairing) yield qualitatively wrong results, including
equilibrium distributions that differ from the theoretical J\"uttner
distribution. A general procedure for relativistically consistent algorithms is
provided, and verified with three-dimensional Monte Carlo simulations, thus
opening the way to the numerical exploration of the statistical properties of
collisional relativistic systems.Comment: Accepted for publication as a Rapid Communication in Phys. Rev.
Dissipational versus Dissipationless Galaxy Formation and the Dark Matter Content of Galaxies
We examine two extreme models for the build-up of the stellar component of
luminous elliptical galaxies. In one case, we assume the build-up of stars is
dissipational, with centrally accreted gas radiating away its orbital and
thermal energy; the dark matter halo will undergo adiabatic contraction and the
central dark matter density profile will steepen. For the second model, we
assume the central galaxy is assembled by a series of dissipationless mergers
of stellar clumps that have formed far from the nascent galaxy. In order to be
accreted, these clumps lose their orbital energy to the dark matter halo via
dynamical friction, thereby heating the central dark matter and smoothing the
dark matter density cusp. The central dark matter density profiles differ
drastically between these models. For the isolated elliptical galaxy, NGC 4494,
the central dark matter densities follow the power-laws r^(-0.2) and r^(-1.7)
for the dissipational and dissipationless models, respectively. By matching the
dissipational and dissipationless models to observations of the stellar
component of elliptical galaxies, we examine the relative contributions of
dissipational and dissipationless mergers to the formation of elliptical
galaxies and look for observational tests that will distinguish between these
models. Comparisons to strong lensing brightest cluster galaxies yield median
M*/L_B ratios of 2.1+/-0.8 and 5.2+/-1.7 at z=0.39 for the dissipational and
dissipationless models, respectively. For NGC 4494, the best-fit dissipational
and dissipationless models have M*/L_B=2.97 and 3.96. Comparisons to expected
stellar mass-to-light ratios from passive evolution and population syntheses
appear to rule out a purely dissipational formation mechanism for the central
stellar regions of giant elliptical galaxies.Comment: 15 pages, 9 figures, accepted to Ap
Dissecting accretion and outflows in accreting white dwarf binaries
This is a White Paper in support of the mission concept of the Large
Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We
discuss the potential of LOFT for the study of accreting white dwarfs. For a
summary, we refer to the paper.Comment: White Paper in Support of the Mission Concept of the Large
Observatory for X-ray Timin
On initial conditions for the Hot Big Bang
We analyse the process of reheating the Universe in the electroweak theory
where the Higgs field plays a role of the inflaton. We estimate the maximal
temperature of the Universe and fix the initial conditions for
radiation-dominated phase of the Universe expansion in the framework of the
Standard Model (SM) and of the nuMSM -- the minimal extension of the SM by
three right-handed singlet fermions. We show that the inflationary epoch is
followed by a matter dominated stage related to the Higgs field oscillations.
We investigate the energy transfer from Higgs-inflaton to the SM particles and
show that the radiation dominated phase of the Universe expansion starts at
temperature T_r~(3-15)*10^{13} GeV, where the upper bound depends on the Higgs
boson mass. We estimate the production rate of singlet fermions at preheating
and find that their concentrations at T_r are negligibly small. This suggests
that the sterile neutrino Dark Matter (DM) production and baryogenesis in the
nuMSM with Higgs-driven inflation are low energy phenomena, having nothing to
do with inflation. We study then a modification of the nuMSM, adding to its
Lagrangian higher dimensional operators suppressed by the Planck scale. The
role of these operators in Higgs-driven inflation is clarified. We find that
these operators do not contribute to the production of Warm Dark Matter (WDM)
and to baryogenesis. We also demonstrate that the sterile neutrino with mass
exceeding 100 keV (a Cold Dark Matter (CDM) candidate) can be created during
the reheating stage of the Universe in necessary amounts. We argue that the
mass of DM sterile neutrino should not exceed few MeV in order not to overclose
the Universe.Comment: 41 pages, 5 figures. Journal version accepted in JCA
Classical novae from the POINT-AGAPE microlensing survey of M31 -- I. The nova catalogue
The POINT-AGAPE survey is an optical search for gravitational microlensing
events towards the Andromeda Galaxy (M31). As well as microlensing, the survey
is sensitive to many different classes of variable stars and transients. Here
we describe the automated detection and selection pipeline used to identify M31
classical novae (CNe) and we present the resulting catalogue of 20 CN
candidates observed over three seasons. CNe are observed both in the bulge
region as well as over a wide area of the M31 disk. Nine of the CNe are caught
during the final rise phase and all are well sampled in at least two colours.
The excellent light-curve coverage has allowed us to detect and classify CNe
over a wide range of speed class, from very fast to very slow. Among the
light-curves is a moderately fast CN exhibiting entry into a deep transition
minimum, followed by its final decline. We have also observed in detail a very
slow CN which faded by only 0.01 mag day over a 150 day period. We
detect other interesting variable objects, including one of the longest period
and most luminous Mira variables. The CN catalogue constitutes a uniquely
well-sampled and objectively-selected data set with which to study the
statistical properties of classical novae in M31, such as the global nova rate,
the reliability of novae as standard-candle distance indicators and the
dependence of the nova population on stellar environment. The findings of this
statistical study will be reported in a follow-up paper.Comment: 21 pages, 13 figures, re-submitted for publication in MNRAS, typos
corrected, references updated, figures 5-9 made cleare
Effect of interface bonding on spin-dependent tunneling from the oxidized Co surface
We demonstrate that the factorization of the tunneling transmission into the
product of two surface transmission functions and a vacuum decay factor allows
one to generalize Julliere's formula and explain the meaning of the ``tunneling
density of states'' in some limiting cases. Using this factorization we
calculate spin-dependent tunneling from clean and oxidized fcc Co surfaces
through vacuum into Al using the principal-layer Green's function approach. We
demonstrate that a monolayer of oxygen on the Co (111) surface creates a
spin-filter effect due to the Co-O bonding which produces an additional
tunneling barrier in the minority-spin channel. This changes the minority-spin
dominated conductance for the clean Co surface into a majority spin dominated
conductance for the oxidized Co surface.Comment: 7 pages, revtex4, 4 embedded eps figure
Non-minimally coupled dark matter: effective pressure and structure formation
We propose a phenomenological model in which a non-minimal coupling between
gravity and dark matter is present in order to address some of the apparent
small scales issues of \lcdm model. When described in a frame in which gravity
dynamics is given by the standard Einstein-Hilbert action, the non-minimal
coupling translates into an effective pressure for the dark matter component.
We consider some phenomenological examples and describe both background and
linear perturbations. We show that the presence of an effective pressure may
lead these scenarios to differ from \lcdm at the scales where the non-minimal
coupling (and therefore the pressure) is active. In particular two effects are
present: a pressure term for the dark matter component that is able to reduce
the growth of structures at galactic scales, possibly reconciling simulations
and observations; an effective interaction term between dark matter and baryons
that could explain observed correlations between the two components of the
cosmic fluid within Tully-Fisher analysis.Comment: 18 pages, 6 figures, references added. Published in JCA
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