5,272 research outputs found
An analytic model for the epoch of halo creation
In this paper we describe the Bayesian link between the cosmological mass
function and the distribution of times at which isolated halos of a given mass
exist. By assuming that clumps of dark matter undergo monotonic growth on the
time-scales of interest, this distribution of times is also the distribution of
`creation' times of the halos. This monotonic growth is an inevitable aspect of
gravitational instability. The spherical top-hat collapse model is used to
estimate the rate at which clumps of dark matter collapse. This gives the prior
for the creation time given no information about halo mass. Applying Bayes'
theorem then allows any mass function to be converted into a distribution of
times at which halos of a given mass are created. This general result covers
both Gaussian and non-Gaussian models. We also demonstrate how the mass
function and the creation time distribution can be combined to give a joint
density function, and discuss the relation between the time distribution of
major merger events and the formula calculated. Finally, we determine the
creation time of halos within three N-body simulations, and compare the link
between the mass function and creation rate with the analytic theory.Comment: 7 pages, 2 figures, submitted to MNRA
Attraction of Acorn-Infesting \u3ci\u3eCydia Latiferreana\u3c/i\u3e (Lepidoptera: Tortricidae) to Pheromone-Baited Traps
Males of acorn-infesting Cydia latiferreana are attracted to an equilibrium mixture of the four isomers of 8, 10-dodecadien-l-ol acetate, the virgin female-produced pheromone. Trap height relative to the height of trees in which traps are placed seems to be a significant factor influencing moth catches at attractant-baited traps. In an oak woodlot and in an oak nursery, catches of male moths were greater in traps placed near the upper periphery of the canopy than at traps deployed at lower levels in the tree. Practical application of pheromone-baited traps in a forest situation will require further study on lure formulation and on trap deployment under forest conditions
Reionization Revisited: Secondary CMB Anisotropies and Polarization
Secondary CMB anisotropies and polarization provide a laboratory to study
structure formation in the reionized epoch. We consider the kinetic
Sunyaev-Zel'dovich effect from mildly nonlinear large-scale structure and show
that it is a natural extension of the perturbative Vishniac effect. If the gas
traces the dark matter to overdensities of order 10, as expected from
simulations, this effect is at least comparable to the Vishniac effect at
arcminute scales. On smaller scales, it may be used to study the thermal
history-dependent clustering of the gas. Polarization is generated through
Thomson scattering of primordial quadrupole anisotropies, kinetic (second order
Doppler) quadrupole anisotropies and intrinsic scattering quadrupole
anisotropies. Small scale polarization results from the density and ionization
modulation of these sources. These effects generically produce comparable E and
B-parity polarization, but of negligible amplitude (0.001-0.01 uK) in adiabatic
CDM models. However, the primordial and kinetic quadrupoles are observationally
comparable today so that a null detection of B-polarization would set
constraints on the evolution and coherence of the velocity field. Conversely, a
detection of a cosmological B-polarization even at large angles does not
necessarily imply the presence of gravity waves or vorticity. For these
calculations, we develop an all-sky generalization of the Limber equation that
allows for an arbitrary local angular dependence of the source for both scalar
and symmetric trace-free tensor fields on the sky.Comment: 14 pages, 12 figures, minor changes and typo fixes reflect published
versio
A study of human performance in a rotating environment
Consideration is given to the lack of sufficient data relative to the response of man to the attendant oculovestibular stimulations induced by multi-directional movement of an individual within the rotating environment to provide the required design criteria. This was done to determine the overall impact of artificial gravity simulations on potential design configurations and crew operational procedures. Gross locomotion and fine motor performance were evaluated. Results indicate that crew orientation, rotational rates, vehicle design configurations, and operational procedures may be used to reduce the severity of the adverse effects of the Coriolis and cross-coupled angular accelerations acting on masses moving within a rotating environment. Results further indicate that crew selection, motivation, and short-term exposures to the rotating environment may be important considerations for future crew indoctrination and training programs
How well do structured abstracts reflect the articles they summerize?
Background: evidence-based medicine requires
critical appraisal of published research. This is often
done by reading the abstracts alone of published
papers. This study examined how well structured
abstracts reflect the articles they summarize in
medical journals.Methods: a total of 20 papers reporting original
randomized trials were obtained from four general
medical journals. Key study details, results, and
conclusions were extracted from the full articles.
Abstracts were examined to see what information
from the article was included, and they were
scrutinized for inaccuracies, data not presented in
the main body, and ambiguous statements.Results: nineteen abstracts (95%; 95% CI 75 to 100%)
correctly stated the primary outcome. Eight abstracts
(40%; 19% to 64%) were deficient in some way. Three
(15%; 3% to 38%) contained incorrect or inconsistent
figures or data. Six abstracts (30%; 12% to 54%)
contained data not present in the full article.Discussion: almost half of the abstracts studied
contained some data inconsistent with the full article,
or missing altogether. Authors and editors need
to ensure that abstracts are of a high quality and
accurately reflect the papers they are summarizing.
CONSORT guidelines provide helpful indications as
to what should be included in abstracts reporting
clinical trial
An imaging K-band survey - I: The catalogue, star and galaxy counts
We present results from a large area (552\,\sqamin) imaging -band survey
to a 5 limit of . We have optical-infrared colours of
almost all the objects in the sample. Star-galaxy discrimination is performed
and the results used to derive the infrared star and galaxy counts. -band
``no-evolution'' galaxy-count models are constructed and compared with the
observed data. In the infrared, there is no counterpart for the large excess of
faint galaxies over the no-evolution model seen in optical counts. However, we
show that the counts can be remarkably insensitive to evolution under
certain reasonable assumptions. Finally, model predictions for -selected
redshift surveys are derived.Comment: MNRAS in press. 21 pages plain TeX; figs plus table 4 available via
anonymous ftp from /pub/kgb/paper1/sissa.uu at ftp.ast.cam.ac.u
Power Spectrum Correlations Induced by Non-Linear Clustering
Gravitational clustering is an intrinsically non-linear process that
generates significant non-Gaussian signatures in the density field. We consider
how these affect power spectrum determinations from galaxy and weak-lensing
surveys. Non-Gaussian effects not only increase the individual error bars
compared to the Gaussian case but, most importantly, lead to non-trivial
cross-correlations between different band-powers. We calculate the
power-spectrum covariance matrix in non-linear perturbation theory (weakly
non-linear regime), in the hierarchical model (strongly non-linear regime), and
from numerical simulations in real and redshift space. We discuss the impact of
these results on parameter estimation from power spectrum measurements and
their dependence on the size of the survey and the choice of band-powers. We
show that the non-Gaussian terms in the covariance matrix become dominant for
scales smaller than the non-linear scale, depending somewhat on power
normalization. Furthermore, we find that cross-correlations mostly deteriorate
the determination of the amplitude of a rescaled power spectrum, whereas its
shape is less affected. In weak lensing surveys the projection tends to reduce
the importance of non-Gaussian effects. Even so, for background galaxies at
redshift z=1, the non-Gaussian contribution rises significantly around l=1000,
and could become comparable to the Gaussian terms depending upon the power
spectrum normalization and cosmology. The projection has another interesting
effect: the ratio between non-Gaussian and Gaussian contributions saturates and
can even decrease at small enough angular scales if the power spectrum of the
3D field falls faster than 1/k^2.Comment: 34 pages, 15 figures. Revised version, includes a clearer explanation
of why the hierarchical ansatz does not provide a good model of the
covariance matrix in the non-linear regime, and new constraints on the
amplitudes Ra and Rb for general 4-pt function configurations in the
non-linear regim
Correlation between the Mean Matter Density and the Width of the Saturated Lyman Alpha Absorption
We report a scaling of the mean matter density with the width of the
saturated Lyman alpha absorptions. This property is established using the
``pseudo-hydro'' technique (Croft et al. 1998). It provides a constraint for
the inversion of the Lyman alpha forest, which encounters difficulty in the
saturated region. With a Gaussian density profile and the scaling relation, a
simple inversion of the simulated Lyman alpha forests shows that the
one-dimensional mass power spectrum is well recovered on scales above 2 Mpc/h,
or roughly k < 0.03 s/km, at z=3. The recovery underestimates the power on
small scales, but improvement is possible with a more sophisticated algorithm.Comment: 7 pages, 9 figures, accepted for publication in MNRAS, replaced by
the version after proo
Measuring the Deviation from the Linear and Deterministic Bias through Cosmic Gravitational Lensing Effects
Since gravitational lensing effects directly probe inhomogeneities of dark
matter, lensing-galaxy cross-correlations can provide us important information
on the relation between dark matter and galaxy distributions, i.e., the bias.
In this paper, we propose a method to measure the stochasticity/nonlinearity of
the galaxy bias through correlation studies of the cosmic shear and galaxy
number fluctuations. Specifically, we employ the aperture mass statistics
to describe the cosmic shear. We divide the foreground galaxy redshift
into several bins, where is the redshift of the source
galaxies, and calculate the quantity for
each redshift bin. Then the ratio of the summation of over the bins to gives a measure of the
nonlinear/stochastic bias. Here is the projected surface number
density fluctuation of foreground galaxies at redshift , and is
the aperture mass from the cosmic-shear analysis. We estimate that for a
moderately deep weak-lensing survey with , source galaxy surface number
density and a survey area of , the effective -parameter that represents the deviation from the
linear and deterministic bias is detectable in the angular range of 1'-10' if
|r-1|\gsim 10%. For shallow, wide surveys such as the Sloan Digital Sky
Survey with , , and a survey area
of , a 10% detection of is possible over the angular
range .Comment: ApJ in pres
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