25,802 research outputs found
Lyman alpha Resonant Scattering in Young Galaxies - Predictions from Cosmological Simulations
We present results obtained with a 3D, Ly alpha radiative transfer code,
applied to a fully cosmological galaxy formation simulation. The developed
Monte Carlo code is capable of treating an arbitrary distribution of source Ly
alpha emission, neutral hydrogen density, temperature, and peculiar velocity of
the interstellar medium. We investigate the influence of resonant scattering on
the appearance and properties of young galaxies by applying the code to a
simulated "Lyman Break Galaxy" at redshift z = 3.6, and of star formation rate
22 M_sun/yr and total Ly alpha luminosity 2.0 X 10^43 erg/s. It is found that
resonant scattering of Ly alpha radiation can explain that young galaxies
frequently are observed to be more extended on the sky in Ly alpha than in the
optical. Moreover, it is shown that, for the system investigated, due to the
anisotropic escape of the photons, the appearent maximum surface brightness can
differ by a factor of ~15, and the total derived luminosity by a factor of ~4,
depending on the orientation of the system relative to the observer.Comment: Letter updated to match version published in Ap
Absolute linear instability in laminar and turbulent gas/liquid two-layer channel flow
We study two-phase stratified flow where the bottom layer is a thin laminar
liquid and the upper layer is a fully-developed gas flow. The gas flow can be
laminar or turbulent. To determine the boundary between convective and absolute
instability, we use Orr--Sommerfeld stability theory, and a combination of
linear modal analysis and ray analysis. For turbulent gas flow, and for the
density ratio r=1000, we find large regions of parameter space that produce
absolute instability. These parameter regimes involve viscosity ratios of
direct relevance to oil/gas flows. If, instead, the gas layer is laminar,
absolute instability persists for the density ratio r=1000, although the
convective/absolute stability boundary occurs at a viscosity ratio that is an
order of magnitude smaller than in the turbulent case. Two further unstable
temporal modes exist in both the laminar and the turbulent cases, one of which
can exclude absolute instability. We compare our results with an
experimentally-determined flow-regime map, and discuss the potential
application of the present method to non-linear analyses.Comment: 33 pages, 20 figure
Impact of stout-link smearing in lattice fermion actions
The impact of stout-link smearing in lattice fermion actions is examined
through the consideration of the mass and renormalization functions of the
overlap quark propagator over a variety of smeared configurations. Up to six
sweeps of stout-link smearing are investigated. For heavy quark masses, the
quark propagator is strongly affected by the smearing procedure. For moderate
masses, the effect appears to be negligible. A small effect is seen for light
quark masses, where dynamical mass generation is suppressed through the
smearing procedure.Comment: 7 pages, 3 figures, presented at the XXVII International Symposium on
Lattice Field Theory - LAT2009, July 26-31 2009, Peking University, Beijing,
Chin
Stout-link smearing in lattice fermion actions
The properties of the momentum space quark propagator in Landau gauge are
studied for the overlap quark action in quenched lattice QCD. Numerical
calculations are performed over four ensembles of gauge configurations, where
three are smeared using either 1, 3, or 6 sweeps of stout-link smearing. We
calculate the non-perturbative wave function renormalization function
and the non-perturbative mass function for a variety of bare quark
masses. We find that the wave-function renormalization function is slightly
sensitive to the number of stout-link smearing sweeps. For the mass function we
find the effect of the stout-link smearing algorithm to be small for moderate
to light bare quark masses. For a heavy bare quark mass we find a strong
dependence on the number of smearing sweeps.Comment: 8 pages, 4 figure
A 33 year constancy of the X-ray coronae of AR Lac and eclipse diagnosis of scale height
Extensive X-ray and extreme ultraviolet (EUV) photometric observations of the
eclipsing RS CVn system AR Lac were obtained over the years 1997 to 2013 with
the Chandra X-ray Observatory Extreme Ultraviolet Explorer. During primary
eclipse, HRC count rates decrease by ~40%. A similar minimum is seen during one
primary eclipse observed by EUVE but not in others owing to intrinsic source
variability. Little evidence for secondary eclipses is present in either the
X-ray or EUV data, reminiscent of earlier X-ray and EUV observations. Primary
eclipses allow us to estimate the extent of a spherically symmetric corona on
the primary G star of about 1.3Rsun, or 0.86Rstar, and indicate the G star is
likely brighter than the K component by a factor of 2-5. Brightness changes not
attributable to eclipses appear to be dominated by stochastic variability and
are generally non-repeating. X-ray and EUV light curves cannot therefore be
reliably used to reconstruct the spatial distribution of emission assuming only
eclipses and rotational modulation are at work. Moderate flaring is observed,
where count rates increase by up to a factor of three above quiescence.
Combined with older ASCA, Einstein, EXOSAT, ROSAT and Beppo-SAX observations,
the data show that the level of quiescent coronal emission at X-ray wavelengths
has remained remarkably constant over 33 years, with no sign of variation due
to magnetic cycles. Variations in base level X-ray emission seen by Chandra
over 13 years are only ~10%, while variations back to pioneering Einstein
observations in 1980 amount to a maximum of 45% and more typically about 15%.Comment: To appear in the Astrophysical Journa
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