11,987 research outputs found

### Constraining our Universe with X-ray & Optical Cluster Data

We have used recent X-ray and optical data in order to impose some
constraints on the cosmology and cluster scaling relations. Generically two
kind of hypotheses define our model. First we consider that the cluster
population is well described by the standard Press-Schechter (PS) formalism,
and second, these clusters are supposed to follow scaling relations with mass:
Temperature-Mass (T-M) and X-ray Luminosity-Mass (L_x - M). As a difference
with many other authors we do not assume specific scaling relations to model
cluster properties such as the usual $T-M$ virial relation or one observational
determination of the $L_x-T$ relation. Instead we consider general free
parameter scaling relations. With the previous model (PS plus scalings) we fit
our free parameters to several X-ray and optical data with the advantage over
many other works that we consider all the data sets at the same time. This
prevents us from being inconsistent with some of the available observations.
Among other interesting conclusions, we find that only low-density universes
are compatible with all the data considered and that the degeneracy between
$\Omega_m$ and $\sigma_8$ is broken. Also we obtain interesting limits on the
parameters characterizing the scaling relations.Comment: 11 pages, 7 figures. MNRAS accepted versio

### Genus and spot density in the COBE DMR first year anisotropy maps

A statistical analysis of texture on the {\it COBE}-DMR first year sky maps
based on the genus and spot number is presented. A generalized $\chi^2$
statistic is defined in terms of ``observable'' quantities: the genus and spot
density that would be measured by different cosmic observers. This strategy
together with the use of Monte Carlo simulations of the temperature
fluctuations, including all the relevant experimental parameters, represent the
main difference with previous analyses. Based on the genus analysis we find a
strong anticorrelation between the quadrupole amplitude $Q_{rms-PS}$ and the
spectral index $n$ of the density fluctuation power spectrum at recombination
of the form $Q_{rms-PS}= 22.2 \pm 1.7 - (4.7 \pm 1.3) \times n\ \mu$K for fixed
$n$, consistent with previous works. The result obtained based on the spot
density is consistent with this $Q_{rms-PS} (n)$ relation. In addition to the
previous results we have determined, using Monte Carlo simulations, the minimum
uncertainty due to cosmic variance for the determination of the spectral index
with the genus analysis. This uncertainty is $\delta n\approx 0.2$.Comment: 5 pages, uuencode file containing text and 1 figure. MNRAS in press

### Geometrical estimators as a test of Gaussianity in the CMB

We investigate the power of geometrical estimators on detecting
non-Gaussianity in the cosmic microwave background. In particular the number,
eccentricity and Gaussian curvature of excursion sets above (and below) a
threshold are studied. We compare their different performance when applied to
non-Gaussian simulated maps of small patches of the sky, which take into
account the angular resolution and instrumental noise of the Planck satellite.
These non-Gaussian simulations are obtained as perturbations of a Gaussian
field in two different ways which introduce a small level of skewness or
kurtosis in the distribution. A comparison with a classical estimator, the
genus, is also shown. We find that the Gaussian curvature is the best of our
estimators in all the considered cases. Therefore we propose the use of this
quantity as a particularly useful test to look for non-Gaussianity in the CMB.Comment: 9 pages, 6 postscript figures, submitted to MNRA

### Quasar-galaxy associations revisited

Gravitational lensing predicts an enhancement of the density of bright,
distant QSOs around foreground galaxies. We measure this QSO-galaxy correlation
w_qg for two complete samples of radio-loud quasars, the southern 1Jy and
Half-Jansky samples. The existence of a positive correlation between z~1
quasars and z~0.15 galaxies is confirmed at a p=99.0% significance level
(>99.9%) if previous measurements on the northern hemisphere are included). A
comparison with the results obtained for incomplete quasar catalogs (e.g. the
Veron-Cetty and Veron compilation) suggests the existence of an `identification
bias', which spuriously increases the estimated amplitude of the quasar-galaxy
correlation for incomplete samples. This effect may explain many of the
surprisingly strong quasar-galaxy associations found in the literature.
Nevertheless, the value of w_qg that we measure in our complete catalogs is
still considerably higher than the predictions from weak lensing. We consider
two effects which could help to explain this discrepancy: galactic dust
extinction and strong lensing.Comment: 9 pages, 6 figures, MNRAS accepte

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