779 research outputs found
2-Point Moments in Cosmological Large Scale Structure: I. Theory and Comparison with Simulations
We present new perturbation theory (PT) predictions in the Spherical Collapse
(SC) model for the 2-point moments of the large-scale distribution of dark
matter density in the universe. We assume that these fluctuations grow under
gravity from small Gaussian initial conditions. These predictions are compared
with numerical simulations and with previous PT results to assess their domain
of validity. We find that the SC model provides in practice a more accurate
description of 2-point moments than previous tree-level PT calculations. The
agreement with simulations is excellent for a wide range of scales (5-50 Mpc/h)
and fluctuations amplitudes (0.02-2 variance). When normalized to unit variance
these results are independent of the cosmological parameters and of the initial
amplitude of fluctuations. The 2-point moments provide a convenient tool to
study the statistical properties of gravitational clustering for fairly
non-linear scales and complicated survey geometries, such as those probing the
clustering of the Ly-alpha forest. In this context, the perturbative SC
predictions presented here, provide a simple and novel way to test the
gravitational instability paradigm.Comment: 10 LaTeX pages, 9 figs, submitted to MNRA
Comparison of the Large Scale Clustering in the APM and the EDSGC Galaxy Surveys
Clustering statistics are compared in the Automatic Plate Machine (APM) and
the Edinburgh/Durham Southern Galaxy Catalogue (EDSGC) angular galaxy surveys.
Both surveys were independently constructed from scans of the same adjacent UK
IIIa--J Schmidt photographic plates with the APM and COSMOS microdensitometers,
respectively. The comparison of these catalogs is a rare practical opportunity
to study systematic errors, which cannot be achieved via simulations or
theoretical methods. On intermediate scales, ,
we find good agreement for the cumulants or reduced moments of counts in cells
up to sixth order. On larger scales there is a small disagreement due to edge
effects in the EDSGC, which covers a smaller area. On smaller scales, we find a
significant disagreement that can only be attributed to differences in the
construction of the surveys, most likely the dissimilar deblending of crowded
fields. The overall agreement of the APM and EDSGC is encouraging, and shows
that the results for intermediate scales should be fairly robust. On the other
hand, the systematic deviations found at small scales are significant in a
regime, where comparison with theory and simulations is possible. This is an
important fact to bear in mind when planning the construction of future
digitized galaxy catalogs.Comment: 4 pages with 3 figures included. Submitted for MNRAS 'pink pages
The Real and Redshift Space Density Distribution Function for Large-Scale Structure in the Spherical Collapse Approximation
We use the spherical collapse (SC) approximation to derive expressions for
the smoothed redshift-space probability distribution function (PDF), as well as
the -order hierarchical amplitudes , in both real and redshift space.
We compare our results with numerical simulations, focusing on the
standard CDM model, where redshift distortions are strongest. We find good
agreement between the SC predictions and the numerical PDF in real space even
for \sigma_L \simgt 1, where is the linearly-evolved rms
fluctuation on the smoothing scale. In redshift space, reasonable agreement is
possible only for \sigma_L \simlt 0.4. Numerical simulations also yield a
simple empirical relation between the real-space PDF and redshift-space PDF: we
find that for \sigma \simlt 1, the redshift space PDF, P[\delta_z], is, to a
good approximation, a simple rescaling of the real space PDF, P[\delta], i.e.,
P[\delta/\sigma] d[\delta/\sigma] = P[\delta_z/\sigma_z] d[\delta_z/\sigma_z],
where and \sigma_z are the real-space and redshift-space rms
fluctuations, respectively. This result applies well beyond the validity of
linear perturbation theory, and it is a good fit for both the standard CDM
model and the Lambda-CDM model. It breaks down for SCDM at ,
but provides a good fit to the \Lambda-CDM models for as large as 0.8.Comment: 9 pages, latex, 12 figures added (26 total), minor changes to
conclusions, to appear in MNRA
Intrinsic leakage and adsorption currents associated with the electrocaloric effect in multilayer capacitors
During the last few years, the increasing demand of energy for refrigeration
applications has relived the interest of the scientific community in the study
of alternative methods to the traditional gas-based refrigeration. Within this
framework, the use of solid state refrigeration based on the electrocaloric
effect reveals itself as one of the most promising technologies. In this work,
we analyze how the temperature change associated with the electrocaloric effect
shows a correlation with the electrical properties of a commercial multilayer
capacitor. In that sense we established a clear relation between the adsorption
currents and the temperature change produced by the electrocaloric effect.
Additionally, intrinsic leakage currents are responsible for the sample heating
due to the Joule effect. These well distinguished contributions can be useful
during the design of solid state refrigeration devices based on the
electrocaloric effect.Comment: Acepted to be published in Applied Physics Letter
Modeling the angular correlation function and its full covariance in Photometric Galaxy Surveys
Near future cosmology will see the advent of wide area photometric galaxy
surveys, like the Dark Energy Survey (DES), that extent to high redshifts (z ~
1 - 2) but with poor radial distance resolution. In such cases splitting the
data into redshift bins and using the angular correlation function ,
or the power spectrum, will become the standard approach to extract
cosmological information or to study the nature of dark energy through the
Baryon Acoustic Oscillations (BAO) probe. In this work we present a detailed
model for at large scales as a function of redshift and bin width,
including all relevant effects, namely nonlinear gravitational clustering,
bias, redshift space distortions and photo-z uncertainties. We also present a
model for the full covariance matrix characterizing the angular correlation
measurements, that takes into account the same effects as for and
also the possibility of a shot-noise component and partial sky coverage.
Provided with a large volume N-body simulation from the MICE collaboration we
built several ensembles of mock redshift bins with a sky coverage and depth
typical of forthcoming photometric surveys. The model for the angular
correlation and the one for the covariance matrix agree remarkably well with
the mock measurements in all configurations. The prospects for a full shape
analysis of at BAO scales in forthcoming photometric surveys such
as DES are thus very encouraging.Comment: 23 pages, 21 figures Revised version accepted by MNRAS. Description
of mocks re-structured. Mocks including redshift distortions and Photo-z
publicly available at http://www.ice.cat/mic
Inverting the Angular Correlation Function
The two point angular correlation function is an excellent measure of
structure in the universe. To extract from it the three dimensional power
spectrum, one must invert Limber's Equation. Here we perform this inversion
using a Bayesian prior constraining the smoothness of the power spectrum. Among
other virtues, this technique allows for the possibility that the estimates of
the angular correlation function are correlated from bin to bin. The output of
this technique are estimators for the binned power spectrum and a full
covariance matrix. Angular correlations mix small and large scales but after
the inversion, small scale data can be trivially eliminated, thereby allowing
for realistic constraints on theories of large scale structure. We analyze the
APM catalogue as an example, comparing our results with previous results. As a
byproduct of these tests, we find -- in rough agreement with previous work --
that APM places stringent constraints on Cold Dark Matter inspired models, with
the shape parameter constrained to be (using data with
wavenumber ). This range of allowed values use the
full power spectrum covariance matrix, but assumes negligible covariance in the
off-diagonal angular correlation error matrix, which is estimated with a large
angular resolution of 0.5degrees (in the range 0.5 and 20 degrees).Comment: 7 pages, 11 figures, replace to match accepted version, MNRAS in
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Measurement of the gravitational potential evolution from the cross-correlation between WMAP and the APM Galaxy survey
Models with late time cosmic acceleration, such as the Lambda-dominated CDM
model, predict a freeze out for the growth of linear gravitational potential at
moderate redshift z<1, what can be observed as temperature anisotropies in the
CMB: the so called integrated Sachs-Wolfe (ISW) effect. We present a direct
measurement of the ISW effect based on the angular cross-correlation function,
w_{TG}, of CMB temperature anisotropies and dark-matter fluctuations traced by
galaxies. We cross-correlate the first-year WMAP data in combination with the
APM Galaxy survey. On the largest scales, theta = 4-10 deg, we detect a
non-vanishing cross-correlation at 98.8 % significance level, with a 1-sigma
error of w_{TG} = 0.35 +/- 0.14 microK, which favors large values of
Omega_Lambda \simeq 0.8 for flat FRW models. On smaller scales, theta < 1deg,
the correlations disappear. This is contrary to what would be expected from the
ISW effect, but the absence of correlations may be simply explained if the ISW
signal was being cancelled by anti-correlations arising from the thermal
Sunyaev-Zeldovich (SZ) effect.Comment: Matches version accepted for publication in MNRAS Letter
What determines large scale galaxy clustering: halo mass or local density?
Using dark matter simulations we show how halo bias is determined by local
density and not by halo mass. This is not totally surprising, as according to
the peak-background split model, local density is the property that constraints
bias at large scales. Massive haloes have a high clustering because they reside
in high density regions. Small haloes can be found in a wide range of
environments which determine their clustering amplitudes differently. This
contradicts the assumption of standard Halo Occupation Distribution (HOD)
models that the bias and occupation of haloes is determined solely by their
mass. We show that the bias of central galaxies from semi-analytic models of
galaxy formation as a function of luminosity and colour is not correctly
predicted by the standard HOD model. Using local density instead of halo mass
the HOD model correctly predicts galaxy bias. These results indicate the need
to include information about local density and not only mass in order to
correctly apply HOD analysis in these galaxy samples. This new model can be
readily applied to observations and has the advantage that the galaxy density
can be directly observed, in contrast with the dark matter halo mass.Comment: 11 pages, 9 figure
On the interpretation of clustering from the angular APM Galaxy Survey
We analyze the uncertainties in the amplitudes of the spatial correlation
functions estimated from angular correlations in a sample from the APM Galaxy
Survey, with . We model the uncertainties in the selection function
and in the evolution of clustering. In particular we estimate ,
the rms galaxy number fluctuations in spheres of radius at 8 \Mpc, from the
measured angular variance in the APM. The uncertainty in has
three main contributions: 8\% from sampling and selection function
uncertainties, 7\% from the uncertainty in the evolution of clustering and 3\%
from the uncertainty in the value of . Including all these
contributions, we find is in the range . If the
galaxy clustering in the APM evolves as expected from gravitational clustering
of matter fluctuations, then ()
for (), close to the values for nearby
optical samples. On the other hand, if we assume that clustering evolution is
fixed in comoving coordinates (),
closer to the results for nearby IRAS samples. The final uncertainty in the
range of values for the hierarchical amplitudes S_J\equiv
\xibar_J/\xibar_2^{J-1} is typically twice the estimated sampling errors, with
the highest values for the case of less clustering evolution. We compare our
estimates with other results and discuss the implications for models of
structure formation.Comment: 11 pages plus 12 figures, uuencoded compress postscrip
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