2,455 research outputs found
Variance and Skewness in the FIRST survey
We investigate the large-scale clustering of radio sources in the FIRST
1.4-GHz survey by analysing the distribution function (counts in cells). We
select a reliable sample from the the FIRST catalogue, paying particular
attention to the problem of how to define single radio sources from the
multiple components listed. We also consider the incompleteness of the
catalogue. We estimate the angular two-point correlation function ,
the variance , and skewness of the distribution for the
various sub-samples chosen on different criteria. Both and
show power-law behaviour with an amplitude corresponding a spatial correlation
length of Mpc. We detect significant skewness in the
distribution, the first such detection in radio surveys. This skewness is found
to be related to the variance through , with
, consistent with the non-linear gravitational growth of
perturbations from primordial Gaussian initial conditions. We show that the
amplitude of variance and skewness are consistent with realistic models of
galaxy clustering.Comment: 13 pages, 21 inline figures, to appear in MNRA
The 2dF gravitational lens survey
The 2 degree Field (2dF) galaxy redshift survey will involve obtaining
approximately 2.5 x 10^5 spectra of objects previously identified as galaxy
candidates on morphological grounds. Included in these spectra should be about
ten gravitationally-lensed quasars, all with low-redshift galaxies as
deflectors (as the more common lenses with high-redshift deflectors will be
rejected from the survey as multiple point-sources). The lenses will appear as
superpositions of galaxy and quasar spectra, and both cross-correlation
techniques and principal components analysis should be able to identify
candidates systematically. With the 2dF survey approximately half-completed it
is now viable to begin a systematic search for these spectroscopic lenses, and
the first steps of this project are described here.Comment: PASA (OzLens edition), in press; 4 pages, 0 figure
Initial Conditions for Large Cosmological Simulations
This technical paper describes a software package that was designed to
produce initial conditions for large cosmological simulations in the context of
the Horizon collaboration. These tools generalize E. Bertschinger's Grafic1
software to distributed parallel architectures and offer a flexible alternative
to the Grafic2 software for ``zoom'' initial conditions, at the price of large
cumulated cpu and memory usage. The codes have been validated up to resolutions
of 4096^3 and were used to generate the initial conditions of large
hydrodynamical and dark matter simulations. They also provide means to generate
constrained realisations for the purpose of generating initial conditions
compatible with, e.g. the local group, or the SDSS catalog.Comment: 12 pages, 11 figures, submitted to ApJ
Using Wii technology to explore real spaces via virtual environments for people who are blind
Purpose - Virtual environments (VEs) that represent real spaces (RSs) give people who are blind the opportunity to build a cognitive map in advance that they will be able to use when arriving at the RS. Design - In this research study Nintendo Wii based technology was used for exploring VEs via the Wiici application. The Wiimote allows the user to interact with VEs by simulating walking and scanning the space. Finding - By getting haptic and auditory feedback the user learned to explore new spaces. We examined the participants' abilities to explore new simple and complex places, construct a cognitive map, and perform orientation tasks in the RS. Originality â To our knowledge, this finding presents the first virtual environment for people who are blind that allow the participants to scan the environment and by this to construct map model spatial representations
Acoustic peaks and dips in the CMB power spectrum: observational data and cosmological constraints
The locations and amplitudes of three acoustic peaks and two dips in the last
releases of the Boomerang, MAXIMA and DASI measurements of the cosmic microwave
background (CMB) anisotropy power spectra as well as their statistical
confidence levels are determined in a model-independent way. It is shown that
the Boomerang-2001 data (Netterfield et al. 2001) fixes the location and
amplitude of the first acoustic peak at more than 3\sigma confidence level. The
next two peaks and dips are determined at a confidence level above 1\sigma but
below 2\sigma. The locations and amplitudes of the first three peaks and two
dips are 212+/-17, 5426+/-1218\mu K^2, 544+/-56, 2266+/-607\mu K^2, 843+/-35,
2077+/-876\mu K^2, 413+/-50, 1960+/-503\mu K^2, 746+/-89, 1605+/-650\mu K^2
respectively (1\sigma errors include statistical and systematic errors). The
MAXIMA and DASI experiments give similar values for the extrema which they
determine. The determined cosmological parameters from the CMB acoustic extrema
data show good agreement with other determinations, especially with the baryon
content as deduced from standard nucleosynthesis constraints. These data
supplemented by the constraints from direct measurements of some cosmological
parameters and data on large scale structure lead to a best-fit model which
agrees with practically all the used experimental data within 1\sigma. The
best-fit parameters are: \Omega_{\Lambda}=0.64^{+0.14}_{-0.27}, \Omega_{m}=
0.36^{+0.21}_{-0.11}, \Omega_b=0.047^{+0.093}_{-0.024},
n_s=1.0^{+0.59}_{-0.17}, h=0.65^{+0.35}_{-0.27} and \tau_c=0.15^{+0.95}_{-0.15}
(plus/minus values show 1\sigma upper/lower limits obtained by marginalization
over all other model parameters). The best-fit values of \Omega_{\nu} and T/S
are close to zero, their 1\sigma upper limits are 0.17 and 1.7 respectively.Comment: 34 pages, 10 figures; accepted by ApJ; some corrections in the text
are made and a few references are adde
Intracluster Comptonization of the CMB: Mean Spectral Distrortion and Cluster Number Counts
The mean sky-averaged Comptonization parameter, y, describing the scattering
of the CMB by hot gas in clusters of galaxies is calculated in an array of flat
and open cosmological and dark matter models. The models are globally
normalized to fit cluster X-ray data, and intracluster gas is assumed to have
evolved in a manner consistent with current observations. We predict values of
y lower than the COBE/FIRAS upper limit. The corresponding values of the
overall optical thickness to Compton scattering are < 10^{-4} for relevant
parameter values. Of more practical importance are number counts of clusters
across which a net flux (with respect to the CMB) higher than some limiting
value can be detected. Such number counts are specifically predicted for the
COBRAS/SAMBA and BOOMERANG missions.Comment: 23 pages, Latex, 11 PostScript figures, 5 PostScript tables, to
appear in Ap
Cosmological redshift distortion: deceleration, bias and density parameters from future redshift surveys of galaxies
The observed two-point correlation functions of galaxies in redshift space
become anisotropic due to the geometry of the universe as well as due to the
presence of the peculiar velocity field. On the basis of linear perturbation
theory, we expand the induced anisotropies of the correlation functions with
respect to the redshift , and obtain analytic formulae to infer the
deceleration parameter , the density parameter and the
derivative of the bias parameter at in terms of the
observable statistical quantities. The present method does not require any
assumption of the shape and amplitude of the underlying fluctuation spectrum,
and thus can be applied to future redshift surveys of galaxies including the
Sloan Digital Sky Survey. We also evaluate quantitatively the systematic error
in estimating the value of from a galaxy
redshift survey on the basis of a conventional estimator for which
neglects both the geometrical distortion effect and the time evolution of the
parameter . If the magnitude limit of the survey is as faint as 18.5
(in B-band) as in the case of the Sloan Digital Sky Survey, the systematic
error ranges between -20% and 10% depending on the cosmological parameters.
Although such systematic errors are smaller than the statistical errors in the
current surveys, they will dominate the expected statistical error for future
surveys.Comment: 9 pages, 5 figs, aastex, ApJ in press, replaced version includes
minor correction
Combining cosmological datasets: hyperparameters and Bayesian evidence
A method is presented for performing joint analyses of cosmological datasets,
in which the weight assigned to each dataset is determined directly by it own
statistical properties. The weights are considered in a Bayesian context as a
set of hyperparameters, which are then marginalised over in order to recover
the posterior distribution as a function only of the cosmological parameters of
interest. In the case of a Gaussian likelihood function, this marginalisation
may be performed analytically. Calculation of the Bayesian evidence for the
data, with and without the introduction of hyperparameters, enables a direct
determination of whether the data warrant the introduction of weights into the
analysis; this generalises the standard likelihood ratio approach to model
comparison. The method is illustrated by application to the classic toy problem
of fitting a straight line to a set of data. A cosmological illustration of the
technique is also presented, in which the latest measurements of the cosmic
microwave background power spectrum are used to infer constraints on
cosmological parameters.Comment: 12 pages, 6 figures, submitted to MNRA
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