48 research outputs found
Statistics of gravitational potential perturbations: A novel approach to deriving the X-ray temperature function
Context. While the halo mass function is theoretically a very sensitive
measure of cosmological models, masses of dark-matter halos are poorly defined,
global, and unobservable quantities.
Aims. We argue that local, observable quantities such as the X-ray
temperatures of galaxy clusters can be directly compared to theoretical
predictions without invoking masses. We derive the X-ray temperature function
directly from the statistics of Gaussian random fluctuations in the
gravitational potential.
Methods. We derive the abundance of potential minima constrained by the
requirement that they belong to linearly collapsed structures. We then use the
spherical-collapse model to relate linear to non-linear perturbations, and the
virial theorem to convert potential depths to temperatures. No reference is
made to mass or other global quantities in the derivation.
Results. Applying a proper high-pass filter that removes large enough modes
from the gravitational potential, we derive an X-ray temperature function that
agrees very well with the classical Press-Schechter approach on relevant
temperature scales, but avoids the necessity of measuring masses.
Conclusions. TThis first study shows that and how an X-ray temperature
function of galaxy clusters can be analytically derived, avoiding the
introduction of poorly defined global quantities such as halo masses. This
approach will be useful for reducing scatter in observed cluster distributions
and thus in cosmological conclusions drawn from them.Comment: 10 pages, 5 figures, accepted for publication in A&A. Revision to
match the published version. Equation 8 corrected. Notable changes in section
4 including new figure
An analytic approach to number counts of weak-lensing peak detections
We develop and apply an analytic method to predict peak counts in
weak-lensing surveys. It is based on the theory of Gaussian random fields and
suitable to quantify the level of spurious detections caused by chance
projections of large-scale structures as well as the shape and shot noise
contributed by the background galaxies. We compare our method to peak counts
obtained from numerical ray-tracing simulations and find good agreement at the
expected level. The number of peak detections depends substantially on the
shape and size of the filter applied to the gravitational shear field. Our main
results are that weak-lensing peak counts are dominated by spurious detections
up to signal-to-noise ratios of 3--5 and that most filters yield only a few
detections per square degree above this level, while a filter optimised for
suppressing large-scale structure noise returns up to an order of magnitude
more.Comment: 9 pages, 5 figures, submitted to A&
The influence of mergers on the cluster temperature function and cosmological parameters derived from it
We develop a parameter-free analytic model to include the effects of mergers into the theoretical modelling of the X-ray temperature function of galaxy clusters. We include this description into our model for the cluster population based on fluctuations of the gravitational potential, which avoids any reference to mass. Comparisons with a numerical simulation reveal that the theoretical model is in good agreement with the simulation results. We show that building the model on the dynamics of spherical rather than ellipsoidal collapse yields better results if emission-weighted temperatures are used, while ellipsoidal collapse yields good agreement between model and simulation for mass-weighted temperatures. Analysing two different samples of X-ray clusters, we quantify the influence of mergers and a conversion between different temperature definitions on the joint determination of Ωm0 and Ï8. If effects of mergers are included, temperature functions based on cluster masses and on the gravitational potential built on spherical collapse are in good agreement with other cosmological probes without any conversion of temperatures