794 research outputs found
The Correlation Function in Redshift Space: General Formula with Wide-angle Effects and Cosmological Distortions
A general formula for the correlation function in redshift space is derived
in linear theory. The formula simultaneously includes wide-angle effects and
cosmological distortions. The formula is applicable to any pair with arbitrary
angle between lines of sight, and arbitrary redshifts, , ,
which are not necessarily small. The effects of the spatial curvature both on
geometry and on fluctuation spectrum are properly taken into account, and thus
our formula holds in a Friedman-Lema\^{\i}tre universe with arbitrary
cosmological parameters and . We illustrate the pattern
of the resulting correlation function with several models, and also show that
validity region of the conventional distant observer approximation is .Comment: 45 pages including 9 figures, To Appear in Astrophys. J. 535 (2000
The observed infall of galaxies towards the Virgo cluster
We examine the velocity field of galaxies around the Virgo cluster induced by
its overdensity. A sample of 1792 galaxies with distances from the Tip of the
Red Giant Branch, the Cepheid luminosity, the SNIa luminosity, the surface
brightness fluctuation method, and the Tully-Fisher relation has been used to
study the velocity-distance relation in the Virgocentric coordinates. Attention
was paid to some observational biases affected the Hubble flow around Virgo.
We estimate the radius of the zero-velocity surface for the Virgo cluster to
be within (5.0 - 7.5) Mpc corresponding to (17 - 26)^\circ at the mean cluster
distance of 17.0 Mpc. In the case of spherical symmetry with cosmological
parameter \Omega_m=0.24 and the age of the Universe T_0= 13.7 Gyr, it yields
the total mass of the Virgo cluster to be within M_T=(2.7 - 8.9) * 10^{14}
M_\sun in reasonable agreement with the existing virial mass estimates for the
cluster.Comment: 22 pages, 11 figures, 2 tables. Accepted for publication in MNRA
Statistical Determination of Bulk Flow Motions
We present here a new parameterization for the bulk motions of galaxies and
clusters (in the linear regime) that can be measured statistically from the
shape and amplitude of the two-dimensional two-point correlation function. We
further propose the one-dimensional velocity dispersion (v_p) of the bulk flow
as a complementary measure of redshift-space distortions, which is
model-independent and not dependent on the normalisation method. As a
demonstration, we have applied our new methodology to the C4 cluster catalogue
constructed from Data Release Three (DR3) of the Sloan Digital Sky Survey. We
find v_p=270^{+433}km/s (also consistent with v_p=0) for this cluster sample
(at z=0.1), which is in agreement with that predicted for a WMAP5-normalised
LCDM model (i.e., v_p(LCDM=203km/s). This measurement does not lend support to
recent claims of excessive bulk motions (\simeq1000 km/s) which appear in
conflict with LCDM, although our large statistical error cannot rule them out.
From the measured coherent evolution of v_p, we develop a technique to
re-construct the perturbed potential, as well as estimating the unbiased matter
density fluctuations and scale--independent bias.Comment: 8 pages, 5 figure
Redshift space correlations and scale-dependent stochastic biasing of density peaks
We calculate the redshift space correlation function and the power spectrum
of density peaks of a Gaussian random field. In the linear regime k < 0.1
h/Mpc, the redshift space power spectrum is
P^s_{pk}(k,u) = exp(-f^2 s_{vel}^2 k^2 u^2) * [b_{pk}(k) + b_{vel}(k) f
u^2]^2 * P_m(k), where u is the angle with respect to the line of sight,
s_{vel} is the one-dimensional velocity dispersion, f is the growth rate, and
b_{pk}(k) and b_{vel}(k) are k-dependent linear spatial and velocity bias
factors. For peaks, the value of s_{vel} depends upon the functional form of
b_{vel}. The peaks model is remarkable because it has unbiased velocities --
peak motions are driven by dark matter flows -- but, in order to achieve this,
b_{vel} is k-dependent. We speculate that this is true in general: k-dependence
of the spatial bias will lead to k-dependence of b_{vel} even if the biased
tracers flow with the dark matter. Because of the k-dependence of the linear
bias parameters, standard manipulations applied to the peak model will lead to
k-dependent estimates of the growth factor that could erroneously be
interpreted as a signature of modified dark energy or gravity. We use the
Fisher formalism to show that the constraint on the growth rate f is degraded
by a factor of two if one allows for a k-dependent velocity bias of the peak
type. We discuss a simple estimate of nonlinear evolution and illustrate the
effect of the peak bias on the redshift space multipoles. For k < 0.1 h/Mpc,
the peak bias is deterministic but k-dependent, so the configuration space bias
is stochastic and scale dependent, both in real and redshift space. We provide
expressions for this stochasticity and its evolution (abridged).Comment: 24 pages, 7 figures (v3): references added (v4): added
figure+appendix. In press in PR
Model for initiation of quality factor degradation at high accelerating fields in superconducting radio-frequency cavities
A model for the onset of the reduction in SRF cavity quality factor, the
so-called Q-drop, at high accelerating electric fields is presented. Breakdown
of the surface barrier against magnetic flux penetration at the cavity equator
is considered to be the critical event that determines the onset of Q-drop. The
worst case of triangular grooves with low field of first flux penetration Hp,
as analyzed previously by Buzdin and Daumens, [1998 Physica C 294: 257], was
adapted. This approach incorporates both the geometry of the groove and local
contamination via the Ginzburg-Landau parameter kappa, so the proposed model
allows new comparisons of one effect in relation to the other. The model
predicts equivalent reduction of Hp when either roughness or contamination were
varied alone, so smooth but dirty surfaces limit cavity performance about as
much as rough but clean surfaces do. When in combination, contamination
exacerbates the negative effects of roughness and vice-versa. To test the model
with actual data, coupons were prepared by buffered chemical polishing and
electropolishing, and stylus profilometry was used to obtain distributions of
angles. From these data, curves for surface resistance generated by simple flux
flow as a function of magnetic field were generated by integrating over the
distribution of angles for reasonable values of kappa. This showed that
combined effects of roughness and contamination indeed reduce the Q-drop onset
field by ~30%, and that that contamination contributes to Q-drop as much as
roughness. The latter point may be overlooked by SRF cavity research, since
access to the cavity interior by spectroscopy tools is very difficult, whereas
optical images have become commonplace. The model was extended to fit cavity
test data, which indicated that reduction of the superconducting gap by
contaminants may also play a role in Q-drop.Comment: 15 pages with 7 figure
On the cosmological mass function theory
This paper provides, from one side, a review of the theory of the
cosmological mass function from a theoretical point of view, starting from the
seminal paper of Press & Shechter (1974) to the last developments (Del Popolo &
Gambera (1998, 1999), Sheth & Tormen 1999 (ST), Sheth, Mo & Tormen 2001 (ST1),
Jenkins et al. 2001 (J01), Shet & Tormen 2002 (ST2), Del Popolo 2002a, Yagi et
al. 2004 (YNY)), and from another side some improvements on the multiplicity
function models in literature. ...Comment: Astronomy Reports, in prin
Planck pre-launch status: Low Frequency Instrument calibration and expected scientific performance
We give the calibration and scientific performance parameters of the Planck
Low Frequency Instrument (LFI) measured during the ground cryogenic test
campaign. These parameters characterise the instrument response and constitute
our best pre-launch knowledge of the LFI scientific performance. The LFI shows
excellent stability and rejection of instrumental systematic effects;
measured noise performance shows that LFI is the most sensitive instrument of
its kind. The set of measured calibration parameters will be updated during
flight operations through the end of the mission.Comment: Accepted for publications in Astronomy and Astrophysics. Astronomy &
Astrophysics, 2010 (acceptance date: 12 Jan 2010
Planck intermediate results. VIII. Filaments between interacting clusters
About half of the baryons of the Universe are expected to be in the form of
filaments of hot and low density intergalactic medium. Most of these baryons
remain undetected even by the most advanced X-ray observatories which are
limited in sensitivity to the diffuse low density medium. The Planck satellite
has provided hundreds of detections of the hot gas in clusters of galaxies via
the thermal Sunyaev-Zel'dovich (tSZ) effect and is an ideal instrument for
studying extended low density media through the tSZ effect. In this paper we
use the Planck data to search for signatures of a fraction of these missing
baryons between pairs of galaxy clusters. Cluster pairs are good candidates for
searching for the hotter and denser phase of the intergalactic medium (which is
more easily observed through the SZ effect). Using an X-ray catalogue of
clusters and the Planck data, we select physical pairs of clusters as
candidates. Using the Planck data we construct a local map of the tSZ effect
centered on each pair of galaxy clusters. ROSAT data is used to construct X-ray
maps of these pairs. After having modelled and subtracted the tSZ effect and
X-ray emission for each cluster in the pair we study the residuals on both the
SZ and X-ray maps. For the merging cluster pair A399-A401 we observe a
significant tSZ effect signal in the intercluster region beyond the virial
radii of the clusters. A joint X-ray SZ analysis allows us to constrain the
temperature and density of this intercluster medium. We obtain a temperature of
kT = 7.1 +- 0.9, keV (consistent with previous estimates) and a baryon density
of (3.7 +- 0.2)x10^-4, cm^-3. The Planck satellite mission has provided the
first SZ detection of the hot and diffuse intercluster gas.Comment: Accepted by A&
Planck intermediate results. XLI. A map of lensing-induced B-modes
The secondary cosmic microwave background (CMB) -modes stem from the
post-decoupling distortion of the polarization -modes due to the
gravitational lensing effect of large-scale structures. These lensing-induced
-modes constitute both a valuable probe of the dark matter distribution and
an important contaminant for the extraction of the primary CMB -modes from
inflation. Planck provides accurate nearly all-sky measurements of both the
polarization -modes and the integrated mass distribution via the
reconstruction of the CMB lensing potential. By combining these two data
products, we have produced an all-sky template map of the lensing-induced
-modes using a real-space algorithm that minimizes the impact of sky masks.
The cross-correlation of this template with an observed (primordial and
secondary) -mode map can be used to measure the lensing -mode power
spectrum at multipoles up to . In particular, when cross-correlating with
the -mode contribution directly derived from the Planck polarization maps,
we obtain lensing-induced -mode power spectrum measurement at a significance
level of , which agrees with the theoretical expectation derived
from the Planck best-fit CDM model. This unique nearly all-sky
secondary -mode template, which includes the lensing-induced information
from intermediate to small () angular scales, is
delivered as part of the Planck 2015 public data release. It will be
particularly useful for experiments searching for primordial -modes, such as
BICEP2/Keck Array or LiteBIRD, since it will enable an estimate to be made of
the lensing-induced contribution to the measured total CMB -modes.Comment: 20 pages, 12 figures; Accepted for publication in A&A; The B-mode map
is part of the PR2-2015 Cosmology Products; available as Lensing Products in
the Planck Legacy Archive http://pla.esac.esa.int/pla/#cosmology; and
described in the 'Explanatory Supplement'
https://wiki.cosmos.esa.int/planckpla2015/index.php/Specially_processed_maps#2015_Lensing-induced_B-mode_ma
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