205 research outputs found
Gas perturbations in cool cores of galaxy clusters: effective equation of state, velocity power spectra and turbulent heating
We present the statistical analysis of X-ray surface brightness and gas density fluctuations in cool cores of ten, nearby and bright galaxy clusters that have deep Chandra observations and show observational indications of radio-mechanical AGN feedback. Within the central parts of cool cores the total variance of fluctuations is dominated by isobaric and/or isothermal fluctuations on spatial scales ~ 10-60 kpc, which are likely associated with slow gas motions and bubbles of relativistic plasma. Adiabatic fluctuations associated with weak shocks constitute less than 10 per cent of the total variance in all clusters. The typical amplitude of density fluctuations is small, ~ 10 per cent or less on scales of ~ 10-15 kpc. Subdominant contribution of adiabatic fluctuations and small amplitude of density fluctuations support a model of gentle AGN feedback as opposed to periodically explosive scenarios which are implemented in some numerical simulations. Measured one-component velocities of gas motions are typically below 100-150 km/s on scales < 50 kpc, and can be up to ~ 300 km/s on ~ 100 kpc scales. The non-thermal energy is < 12 per cent of the thermal energy. Regardless of the source that drives these motions the dissipation of the energy in such motions provides heat that is sufficient to balance radiative cooling on average, albeit the uncertainties are large. Presented results here support previous conclusions based on the analysis of the Virgo and Perseus Clusters, and agree with the Hitomi measurements. With next generation observatories like Athena and Lynx, these techniques will be yet more powerful
Low-Temperature Mobility of Surface Electrons and Ripplon-Phonon Interaction in Liquid Helium
The low-temperature dc mobility of the two-dimensional electron system
localized above the surface of superfluid helium is determined by the slowest
stage of the longitudinal momentum transfer to the bulk liquid, namely, by the
interaction of surface and volume excitations of liquid helium, which rapidly
decreases with temperature. Thus, the temperature dependence of the
low-frequency mobility is \mu_{dc} = 8.4x10^{-11}n_e T^{-20/3} cm^4 K^{20/3}/(V
s), where n_e is the surface electron density. The relation
T^{20/3}E_\perp^{-3} << 2x10^{-7} between the pressing electric field (in
kV/cm) and temperature (in K) and the value \omega < 10^8 T^5 K^{-5}s^{-1} of
the driving-field frequency have been obtained, at which the above effect can
be observed. In particular, E_\perp = 1 kV/cm corresponds to T < 70 mK and
\omega/2\pi < 30 Hz.Comment: 4 pages, 1 figur
Type Ia supernova parameter estimation: a comparison of two approaches using current datasets
By using the Sloan Digital Sky Survey (SDSS) first year type Ia supernova (SN
Ia) compilation, we compare two different approaches (traditional \chi^2 and
complete likelihood) to determine parameter constraints when the magnitude
dispersion is to be estimated as well. We consider cosmological constant + Cold
Dark Matter (\Lambda CDM) and spatially flat, constant w Dark Energy + Cold
Dark Matter (FwCDM) cosmological models and show that, for current data, there
is a small difference in the best fit values and 30% difference in
confidence contour areas in case the MLCS2k2 light-curve fitter is adopted. For
the SALT2 light-curve fitter the differences are less significant (
13% difference in areas). In both cases the likelihood approach gives more
restrictive constraints. We argue for the importance of using the complete
likelihood instead of the \chi^2 approach when dealing with parameters in the
expression for the variance.Comment: 16 pages, 5 figures. More complete analysis by including peculiar
velocities and correlations among SALT2 parameters. Use of 2D contours
instead of 1D intervals for comparison. There can be now a significant
difference between the approaches, around 30% in contour area for MLCS2k2 and
up to 13% for SALT2. Generic streamlining of text and suppression of section
on model selectio
Surface Region of Superfluid Helium as an Inhomogeneous Bose-Condensed Gas
We present arguments that the low density surface region of self-bounded
superfluid He systems is an inhomogeneous dilute Bose gas, with almost all
of the atoms occupying the same single-particle state at . Numerical
evidence for this complete Bose-Einstein condensation was first given by the
many-body variational calculations of He droplets by Lewart, Pandharipande
and Pieper in 1988. We show that the low density surface region can be treated
rigorously using a generalized Gross-Pitaevskii equation for the Bose order
parameter.Comment: 4 pages, 1 Postscript figur
The XXL Survey:XIII. Baryon content of the bright cluster sample
Traditionally, galaxy clusters have been expected to retain all the material
accreted since their formation epoch. For this reason, their matter content
should be representative of the Universe as a whole, and thus their baryon
fraction should be close to the Universal baryon fraction. We make use of the
sample of the 100 brightest galaxy clusters discovered in the XXL Survey to
investigate the fraction of baryons in the form of hot gas and stars in the
cluster population. We measure the gas masses of the detected halos and use a
mass--temperature relation directly calibrated using weak-lensing measurements
for a subset of XXL clusters to estimate the halo mass. We find that the
weak-lensing calibrated gas fraction of XXL-100-GC clusters is substantially
lower than was found in previous studies using hydrostatic masses. Our best-fit
relation between gas fraction and mass reads . The baryon budget of galaxy
clusters therefore falls short of the Universal baryon fraction by about a
factor of two at . Our measurements require a hydrostatic bias
to match the gas fraction obtained
using lensing and hydrostatic equilibrium. Comparing our gas fraction
measurements with the expectations from numerical simulations, our results
favour an extreme feedback scheme in which a significant fraction of the
baryons are expelled from the cores of halos. This model is, however, in
contrast with the thermodynamical properties of observed halos, which might
suggest that weak-lensing masses are overestimated. We note that a mass bias
as required to reconcile Planck CMB and cluster counts should
translate into an even lower baryon fraction, which poses a major challenge to
our current understanding of galaxy clusters. [Abridged]Comment: 13th paper in the XXL series, A&A in pres
Constraints on the CMB Temperature Evolution using Multi-Band Measurements of the Sunyaev Zel'dovich Effect with the South Pole Telescope
The adiabatic evolution of the temperature of the cosmic microwave background
(CMB) is a key prediction of standard cosmology. We study deviations from the
expected adiabatic evolution of the CMB temperature of the form using measurements of the spectrum of the Sunyaev
Zel'dovich Effect with the South Pole Telescope (SPT). We present a method for
using the ratio of the Sunyaev Zel'dovich signal measured at 95 and 150 GHz in
the SPT data to constrain the temperature of the CMB. We demonstrate that this
approach provides unbiased results using mock observations of clusters from a
new set of hydrodynamical simulations. We apply this method to a sample of 158
SPT-selected clusters, spanning the redshift range , and
measure , consistent with the standard model
prediction of . In combination with other published results, we
constrain , an improvement of over
published constraints. This measurement also provides a strong constraint on
the effective equation of state in models of decaying dark energy
.Comment: Submitted to MNRAS Letter
Cosmological Constraints from Galaxy Clusters in the 2500 square-degree SPT-SZ Survey
(abridged) We present cosmological constraints obtained from galaxy clusters
identified by their Sunyaev-Zel'dovich effect signature in the 2500 square
degree South Pole Telescope Sunyaev Zel'dovich survey. We consider the 377
cluster candidates identified at z>0.25 with a detection significance greater
than five, corresponding to the 95% purity threshold for the survey. We compute
constraints on cosmological models using the measured cluster abundance as a
function of mass and redshift. We include additional constraints from
multi-wavelength observations, including Chandra X-ray data for 82 clusters and
a weak lensing-based prior on the normalization of the mass-observable scaling
relations. Assuming a LCDM cosmology, where the species-summed neutrino mass
has the minimum allowed value (mnu = 0.06 eV) from neutrino oscillation
experiments, we combine the cluster data with a prior on H0 and find sigma_8 =
0.797+-0.031 and Omega_m = 0.289+-0.042, with the parameter combination
sigma_8(Omega_m/0.27)^0.3 = 0.784+-0.039. These results are in good agreement
with constraints from the CMB from SPT, WMAP, and Planck, as well as with
constraints from other cluster datasets. Adding mnu as a free parameter, we
find mnu = 0.14+-0.08 eV when combining the SPT cluster data with Planck CMB
data and BAO data, consistent with the minimum allowed value. Finally, we
consider a cosmology where mnu and N_eff are fixed to the LCDM values, but the
dark energy equation of state parameter w is free. Using the SPT cluster data
in combination with an H0 prior, we measure w = -1.28+-0.31, a constraint
consistent with the LCDM cosmological model and derived from the combination of
growth of structure and geometry. When combined with primarily geometrical
constraints from Planck CMB, H0, BAO and SNe, adding the SPT cluster data
improves the w constraint from the geometrical data alone by 14%, to w =
-1.023+-0.042
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