2,447 research outputs found
Measuring the hydrostatic mass bias in galaxy clusters by combining Sunyaev-Zel'dovich and CMB lensing data
The cosmological parameters prefered by the cosmic microwave background (CMB)
primary anisotropies predict many more galaxy clusters than those that have
been detected via the thermal Sunyaev-Zeldovich (tSZ) effect. This tension has
attracted considerable attention since it could be evidence of physics beyond
the simplest CDM model. However, an accurate and robust calibration of
the mass-observable relation for clusters is necessary for the comparison,
which has been proven difficult to obtain so far. Here, we present new
contraints on the mass-pressure relation by combining tSZ and CMB lensing
measurements about optically-selected clusters. Consequently, our galaxy
cluster sample is independent from the data employed to derive cosmological
constrains. We estimate an average hydrostatic mass bias of , with no significant mass nor redshift evolution. This value greatly
reduces the tension between the predictions of CDM and the observed
abundance of tSZ clusters while being in agreement with recent estimations from
tSZ clustering. On the other hand, our value for is higher than the
predictions from hydro-dynamical simulations. This suggests the existence of
mechanisms driving large departures from hydrostatic equilibrium and that are
not included in state-of-the-art simulations, and/or unaccounted systematic
errors such as biases in the cluster catalogue due to the optical selection.Comment: 4 pages, 3 figure
How BAO measurements can fail to detect quintessence
We model the nonlinear growth of cosmic structure in different dark energy
models, using large volume N-body simulations. We consider a range of
quintessence models which feature both rapidly and slowly varying dark energy
equations of state, and compare the growth of structure to that in a universe
with a cosmological constant. The adoption of a quintessence model changes the
expansion history of the universe, the form of the linear theory power spectrum
and can alter key observables, such as the horizon scale and the distance to
last scattering. The difference in structure formation can be explained to
first order by the difference in growth factor at a given epoch; this scaling
also accounts for the nonlinear growth at the 15% level. We find that
quintessence models which feature late , rapid transitions towards
in the equation of state, can have identical baryonic acoustic
oscillation (BAO) peak positions to those in CDM, despite being very
different from CDM both today and at high redshifts .
We find that a second class of models which feature non-negligible amounts of
dark energy at early times cannot be distinguished from CDM using
measurements of the mass function or the BAO. These results highlight the need
to accurately model quintessence dark energy in N-body simulations when testing
cosmological probes of dynamical dark energy.Comment: 10 pages, 7 figures, to appear in the Invisible Univers International
Conference AIP proceedings serie
Direct measurement of the 14N(p,g)15O S-factor
We have measured the 14N(p,g)15O excitation function for energies in the
range E_p = 155--524 keV. Fits of these data using R-matrix theory yield a
value for the S-factor at zero energy of 1.64(17) keV b, which is significantly
smaller than the result of a previous direct measurement. The corresponding
reduction in the stellar reaction rate for 14N(p,g)15O has a number of
interesting consequences, including an impact on estimates for the age of the
Galaxy derived from globular clusters.Comment: 5 pages, 3 figures, submitted to Phys. Rev. Let
Few-body decay and recombination in nuclear astrophysics
Three-body continuum problems are investigated for light nuclei of
astrophysical relevance. We focus on three-body decays of resonances or
recombination via resonances or the continuum background. The concepts of
widths, decay mechanisms and dynamic evolution are discussed. We also discuss
results for the triple decay in connection with resonances and
density and temperature dependence rates of recombination into light nuclei
from -particles and neutrons.Comment: 9 pages, 8 figures. Proceedings of the 21st European Few Body
Conference held in Salamanca (Spain) in August-September 201
Implementation of a 10.24 GS/s 12-bit Optoelectronics Analog-to-Digital Converter Based on a Polyphase Demultiplexing Architecture
AbstractIn this paper we present the practical implementation of a high-speed polyphase sampling and demultiplexing architecture for optoelectronics analog-to-digital converters (OADCs). The architecture consists of a one-stage divide-by-eight decimator circuit where optically-triggered samplers are cascaded to sample an analog input signal, and demultiplex different phases of the sampled signal to yield low data rate for electronic quantization. Electrical-in to electrical-out data format is maintained through the sampling, demultiplexing and quantization processes of the architecture thereby avoiding the need for electrical-to-optical and optical-to-electrical signal conversions. We experimentally demonstrate a 10.24 giga samples per second (GS/s), 12-bit resolution OADC system comprising the optically-triggered sampling circuits integrated with commercial electronic quantizers. Measurements performed on the OADC yielded an effective bit resolution (ENOB) of 10.3 bits, spurious free dynamic range (SFDR) of -32 dB and signal-to-noise and distortion ratio (SNDR) of 63.7 dB
The Birth and Growth of Neutralino Haloes
We use the Extended-Press-Schechter (EPS) formalism to study halo assembly
histories in a standard CDM cosmology. A large ensemble of Monte Carlo
random walks provides the {\it entire} halo membership histories of a
representative set of dark matter particles, which we assume to be neutralinos.
The first generation halos of most particles do not have a mass similar to the
free-streaming cut-off of the neutralino power spectrum, nor do they
form at high redshift. Median values are to and
to 8 depending on the form of the collapse barrier assumed in the
EPS model. For almost a third of all particles the first generation halo has
. At redshifts beyond 20, most neutralinos are not yet part
of any halo but are still diffuse. These numbers apply with little modification
to the neutralinos which are today part of halos similar to that of the Milky
Way. Up to 10% of the particles in such halos were never part of a smaller
object; the typical particle has undergone "accretion events' where
the halo it was part of falls into a more massive object. Available N-body
simulations agree well with the EPS predictions for an "ellipsoidal" collapse
barrier, so these may provide a reliable extension of simulation results to
smaller scales. The late formation times and large masses of the first
generation halos of most neutralinos imply that they will be disrupted with
high efficiency during halo assembly.Comment: 7 pages, 7 figure
Impact of Scale Dependent Bias and Nonlinear Structure Growth on the ISW Effect: Angular Power Spectra
We investigate the impact of nonlinear evolution of the gravitational
potentials in the LCDM model on the Integrated Sachs-Wolfe (ISW) contribution
to the CMB temperature power spectrum, and on the cross-power spectrum of the
CMB and a set of biased tracers of the mass. We use an ensemble of N-body
simulations to directly follow the potentials and compare results to
perturbation theory (PT). The predictions from PT match the results to high
precision for k<0.2 h/Mpc. We compute the nonlinear corrections to the angular
power spectrum and find them to be <10% of linear theory for l<100. These
corrections are swamped by cosmic variance. On scales l>100 the departures are
more significant, however the CMB signal is more than a factor 10^3 larger at
this scale. Nonlinear ISW effects therefore play no role in shaping the CMB
power spectrum for l<1500. We analyze the CMB--density tracer cross-spectrum
using simulations and renormalized bias PT, and find good agreement. The usual
assumption is that nonlinear evolution enhances the growth of structure and
counteracts linear ISW on small scales, leading to a change in sign of the
CMB-LSS cross-spectrum at small scales. However, PT analysis suggests that this
trend reverses at late times when the logarithmic growth rate
f(a)=dlnD/dlna<0.5 or om_m(a)<0.3. Numerical results confirm these expectations
and we find no sign change in ISW-LSS cross-power for low redshifts.
Corrections due to nonlinearity and scale dependence of the bias are found to
be <10% for l<100, therefore below the S/N of the current and future
measurements. Finally, we estimate the CMB--halo cross-correlation coefficient
and show that it can be made to match that for CMB--dark matter to within 5%
for thin redshift shells, mitigating the need to model bias evolution.Comment: 27 pages, 19 figure. Hi-res. version:
http://www.itp.uzh.ch/~res/NonlinearISW.HiRes.pd
Nucleosynthesis of light element isotopes in evolved stars experiencing extended mixing
We present computations of nucleosynthesis in red giants and asymptotic giant
branch stars of Population I experiencing extended mixing. The assumed physical
cause for mass transport is the buoyancy of magnetized structures, according to
recent suggestions. The peculiar property of such a mechanism is to allow for
both fast and slow mixing phenomena, as required for reproducing the spread in
Li abundances displayed by red giants and as discussed in an accompanying
paper. We explore here the effects of this kind of mass transport on CNO and
intermediatemass nuclei and compare the results with the available evidence
from evolved red giants and from the isotopic composition of presolar grains of
AGB origin. It is found that a good general accord exists between predictions
and measurements; in this framework we also show which type of observational
data best constrains the various parameters. We conclude that magnetic
buoyancy, allowing for mixing at rather different speeds, can be an interesting
scenario to explore for explaining together the abundances of CNO nuclei and of
Li.Comment: 8 pages, 7 figures, proceeding of 'The Origin of the Elements Heavier
than Fe' September 24-28, 2008, Torino, Italy. PASA (accepted for
publication
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