340 research outputs found
Hierarchical models of high redshift galaxies with thermally pulsing asymptotic giant branch stars: comparison with observations
In a recent paper we presented the first semi-analytic model of galaxy
formation in which the Thermally-Pulsing Asymptotic Giant Branch phase of
stellar evolution has been fully implemented. Here we address the comparison
with observations, and show how the TP-AGB recipe affects the performance of
the model in reproducing the colours and near-IR luminosities of high-redshift
galaxies. We find that the semi-analytic model with the TP-AGB better matches
the colour-magnitude and colour-colour relations at z ~ 2, both for
nearly-passive and for star-forming galaxies. The model with TP-AGB produces
star-forming galaxies with red V-K colours, thus revising the unique
interpretation of high-redshift red objects as 'red & dead'. We also show that
without the TP-AGB the semi-analytic model fails at reproducing the observed
colours, a situation that cannot be corrected by dust reddening. We also
explore the effect of nebular emission on the predicted colour-magnitude
relation of star-forming galaxies, to conclude that it does not play a
significant role in reddening their colours, at least in the range of
star-formation rates covered by the model. Finally, the rest-frame K-band
luminosity function at z ~ 2.5 is more luminous by almost 1 magnitude. This
indicates that the AGN feedback recipe that is adopted to regulate the
high-mass end of the luminosity function should be sophisticated to take the
effect of the stellar populations into account at high redshifts.Comment: 10 pages, 8 figures; effects of nebular emission included; accepted
for publication on MNRA
Development of a valid simulation assessment for a military dismounted assault task
The Australian Defence Force is currently developing physical standards commensurate with job demands. Vital to this development process has been the accurate profiling of common military tasks. One such task required of all dismounted combat soldiers, an offensive assault on an enemy force, was the subject of in-depth profiling. In addition to overall assault performance, potential differences among patrol roles (scout, gunner, and flank) were investigated. Three different mock assaults of 100 to 150 m were performed by three patrols comprising qualified experienced infantry soldiers. Each soldier was fitted with a heart rate monitor and wore a global positioning device. Average assault duration was 6.5 minutes and required nineteen 7-m bounds performed on a 22-seconds duty cycle at 75% heart rate reserve and a work to rest ratio 1:4. Assaults conducted in more densely vegetated terrain resulted in significantly reduced (p \u3c 0.05) bound distance, bound duration, and movement velocity. Results indicated significant performance differences (p \u3c 0.05) among patrol roles for external load carried, heart rate response, bound duration, and distance covered while movement velocity was not different (p \u3e 0.05). As a result of profiling the assault task, a valid simulation capable of assessing soldiers\u27 physical capacity to perform this task was developed
Dynamics of Rotating Accretion Flows Irradiated by a Quasar
We study the axisymmetric, time-dependent hydrodynamics of rotating flows
that are under the influence of supermassive black hole gravity and radiation
from an accretion disk surrounding the black hole. This work is an extension of
the earlier work presented by Proga, where nonrotating flows were studied.
Here, we consider effects of rotation, a position-dependent radiation
temperature, density at large radii, and uniform X-ray background radiation. As
in the non-rotating case, the rotating flow settles into a configuration with
two components (1) an equatorial inflow and (2) a bipolar inflow/outflow with
the outflow leaving the system along the pole. However, with rotation the flow
does not always reach a steady state. In addition, rotation reduces the outflow
collimation and the outward flux of mass and kinetic energy. Moreover rotation
increases the outward flux of the thermal energy and can lead to fragmentation
and time-variability of the outflow. We also show that a position-dependent
radiation temperature can significantly change the flow solution. In
particular, the inflow in the equatorial region can be replaced by a thermally
driven outflow. Generally, as it have been discussed and shown in the past, we
find that self-consistently determined preheating/cooling from the quasar
radiation can significantly reduce the rate at which the central BH is fed with
matter. However, our results emphasize also a little appreciated feature.
Namely, quasar radiation drives a non-spherical, multi-temperature and very
dynamic flow. These effects become dominant for luminosities in excess of 0.01
of the Eddington luminosity.Comment: accepted for publication in Ap
Solving the cosmic lithium problems with primordial late-decaying particles
We investigate the modifications to predictions for the abundances of light
elements from standard Big-Bang nucleosynthesis when exotic late-decaying
particles with lifetimes exceeding ~1 sec are prominent in the early Universe.
Utilising a model-independent analysis of the properties of these long-lived
particles, we identify the parameter space associated with models that are
consistent with all observational data and hence resolve the much discussed
discrepancies between observations and theoretical predictions for the
abundances of Li^7 and Li^6.Comment: 6 pages, 3 figures, submitted to Physical Review D; minor changes to
reference
Can Baryonic Features Produce the Observed 100 Mpc Clustering?
We assess the possibility that baryonic acoustic oscillations in adiabatic
models may explain the observations of excess power in large-scale structure on
100h^-1 Mpc scales. The observed location restricts models to two extreme areas
of parameter space. In either case, the baryon fraction must be large
(Omega_b/Omega_0 > 0.3) to yield significant features. The first region
requires Omega_0 < 0.2h to match the location, implying large blue tilts
(n>1.4) to satisfy cluster abundance constraints. The power spectrum also
continues to rise toward larger scales in these models. The second region
requires Omega_0 near 1, implying Omega_b well out of the range of big bang
nucleosynthesis constraints; moreover, the peak is noticeably wider than the
observations suggest. Testable features of both solutions are that they require
moderate reionization and thereby generate potentially observable (about 1 uK)
large-angle polarization, as well as sub-arc-minute temperature fluctuations.
In short, baryonic features in adiabatic models may explain the observed excess
only if currently favored determinations of cosmological parameters are in
substantial error or if present surveys do not represent a fair sample of
100h^-1 Mpc structures.Comment: LaTeX, 7 pages, 5 Postscript figures, submitted to ApJ Letter
Difficulties for Compact Composite Object Dark Matter
It has been suggested ``that DM particles are strongly interacting composite
macroscopically large objects ... made of well known light quarks (or ...
antiquarks)." In doing so it is argued that these compact composite objects
(CCOs) are ``natural explanations of many observed data, such as [the] 511 keV
line from the bulge of our galaxy" observed by INTEGRAL and the excess of
diffuse gamma-rays in the 1-20 MeV band observed by COMPTEL. Here we argue that
the atmospheres of positrons that surround CCOs composed of di-antiquark pairs
in the favoured Colour-Flavour-Locked superconducting state are sufficiently
dense as to stringently limit the penetration of interstellar electrons
incident upon them, resulting in an extreme suppression of previously estimated
rates of positronium, and hence the flux of 511 keV photons resulting from
their decays, and also in the rate of direct electron-positron annihilations,
which yield the MeV photons proposed to explain the 1-20 MeV excess. We also
demonstrate that even if a fraction of positrons somehow penetrated to the
surface of the CCOs, the extremely strong electric fields generated from the
bulk antiquark matter would result in the destruction of positronium atoms long
before they decay.Comment: 7 Pages, 4 Figures. Major changes invoke
Model selection in systems biology depends on experimental design.
Experimental design attempts to maximise the information available for modelling tasks. An optimal experiment allows the inferred models or parameters to be chosen with the highest expected degree of confidence. If the true system is faithfully reproduced by one of the models, the merit of this approach is clear - we simply wish to identify it and the true parameters with the most certainty. However, in the more realistic situation where all models are incorrect or incomplete, the interpretation of model selection outcomes and the role of experimental design needs to be examined more carefully. Using a novel experimental design and model selection framework for stochastic state-space models, we perform high-throughput in-silico analyses on families of gene regulatory cascade models, to show that the selected model can depend on the experiment performed. We observe that experimental design thus makes confidence a criterion for model choice, but that this does not necessarily correlate with a model's predictive power or correctness. Finally, in the special case of linear ordinary differential equation (ODE) models, we explore how wrong a model has to be before it influences the conclusions of a model selection analysis
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