57,177 research outputs found
Monte Carlo Predictions of Far-Infrared Emission from Spiral Galaxies
We present simulations of Far Infrared (FIR) emission by dust in spiral
galaxies, based on the Monte Carlo radiative transfer code of Bianchi, Ferrara
& Giovanardi (1996). The radiative transfer is carried out at several
wavelength in the Ultraviolet, optical and Near Infrared, to cover the range of
the stellar Spectral Energy Distribution (SED). Together with the images of the
galactic model, a map of the energy absorbed by dust is produced. Using
Galactic dust properties, the spatial distribution of dust temperature is
derived under the assumption of thermal equilibrium. A correction is applied
for non-equilibrium emission in the Mid Infrared. Images of dust emission can
then be produced at any wavelength in the FIR.
We show the application of the model to the spiral galaxy NGC 6946. The
observed stellar SED is used as input and models are produced for different
star-dust geometries. It is found that only optically thick dust disks can
reproduce the observed amount of FIR radiation. However, it is not possible to
reproduce the large FIR scalelength suggested by recent observation of spirals
at 200 um, even when the scalelength of the dust disk is larger than that for
stars. Optically thin models have ratios of optical/FIR scalelengths closer to
the 200um observations, but with smaller absolute scalelengths than optically
thick cases. The modelled temperature distributions are compatible with
observations of the Galaxy and other spirals. We finally discuss the
approximations of the model and the impact of a clumpy stellar and dust
structure on the FIR simulations.Comment: 19 pages, 6 figures, accepted by A&
Gravitational Entropy and Quantum Cosmology
We investigate the evolution of different measures of ``Gravitational
Entropy'' in Bianchi type I and Lema\^itre-Tolman universe models.
A new quantity behaving in accordance with the second law of thermodynamics
is introduced. We then go on and investigate whether a quantum calculation of
initial conditions for the universe based upon the Wheeler-DeWitt equation
supports Penrose's Weyl Curvature Conjecture, according to which the Ricci part
of the curvature dominates over the Weyl part at the initial singularity of the
universe. The theory is applied to the Bianchi type I universe models with dust
and a cosmological constant and to the Lema\^itre-Tolman universe models. We
investigate two different versions of the conjecture. First we investigate a
local version which fails to support the conjecture. Thereafter we construct a
non-local entity which shows more promising behaviour concerning the
conjecture.Comment: 20 pages, 7 ps figure
The Energy-Momentum Tensor in Fulling-Rindler Vacuum
The energy density in Fulling-Rindler vacuum, which is known to be negative
"everywhere" is shown to be positive and singular on the horizons in such a
fashion as to guarantee the positivity of the total energy. The mechanism of
compensation is displayed in detail.Comment: 9 pages, ULB-TH-15/9
Optical/near-infrared colours of early-type galaxies and constraints on their star formation histories
(abridged) We introduce and discuss the properties of a theoretical
(B-K)-(J-K) integrated colour diagram for single-age, single-metallicity
stellar populations. This combination of integrated colours is able to largely
disentangle the well known age-metallicity degeneracy when the age of the
population is greater than ~300 Myr. We discuss in detail the effect on this
colour-colour diagram of alpha-enhanced metal abundance ratios, the presence of
blue horizontal branch stars unaccounted for in the theoretical calibration,
and of statistical colour fluctuations in low mass stellar systems. In the case
of populations with multiple stellar generations, the luminosity-weighted mean
age obtained from this diagram is shown to be heavily biased towards the
youngest stellar components. We apply this method to several datasets for which
optical and near-IR photometry are available in the literature. For the two
Local Group dwarf galaxies NGC185 and NGC6822, the mean ages derived from the
integrated colours are consistent with the star formation histories inferred
independently from photometric observations of their resolved stellar
populations. A sample of bright field and Virgo cluster elliptical galaxies is
found to exhibit a range of luminosity-weighted mean ages from 3 to 14 Gyr,
with a mean of 8 Gyr, independent of environment, and mean metallicities at or
just above the solar value. Colour gradients are found in all of the galaxies
studied, in the sense that central regions are redder. Aperture data for five
Virgo early-type dwarf galaxies show that these galaxies appear to be shifted
to lower mean metallicities and lower mean ages (range 1 to 6 Gyr) than their
higher luminosity counterparts.Comment: (1) Liverpool John Moores University, UK; (2) University of Cardiff,
UK; (3) University of Bristol, UK; (4) INAF-Osservatorio Astronomico di
Collurania, I; 12 pages, 9 figures, MNRAS in pres
SCUBA imaging of NGC 7331 dust ring
We present observations of the spiral galaxy NGC 7331 using the
Sub-millimetre Common User Bolometer Array (SCUBA) on the James Clark Maxwell
Telescope. We have detected a dust ring of 45 arcsec radius (3.3 kpc) at
wavelengths of 450 and 850-micron. The dust ring is in good correspondence with
other observations of the ring in the mid-infrared (MIR), CO and
radio-continuum, suggesting that the observed dust is associated with the
molecular gas and star formation. A B-K colour map shows an analogous ring
structure with an asymmetry about the major axis, consistent with the
extinction being produced by a dust ring. The derived temperature of the dust
lies between 16 and 31 K and the gas-to-dust ratio between 150 and 570,
depending on the assumed dust emission efficiency index (beta=1.5 or 2.).Comment: 5 pages, 6 figures, to be published in MNRA
Sharp two-sided heat kernel estimates for critical Schr\"odinger operators on bounded domains
On a smooth bounded domain \Omega \subset R^N we consider the Schr\"odinger
operators -\Delta -V, with V being either the critical borderline potential
V(x)=(N-2)^2/4 |x|^{-2} or V(x)=(1/4) dist (x,\partial\Omega)^{-2}, under
Dirichlet boundary conditions. In this work we obtain sharp two-sided estimates
on the corresponding heat kernels. To this end we transform the Scr\"odinger
operators into suitable degenerate operators, for which we prove a new
parabolic Harnack inequality up to the boundary. To derive the Harnack
inequality we have established a serier of new inequalities such as improved
Hardy, logarithmic Hardy Sobolev, Hardy-Moser and weighted Poincar\'e. As a
byproduct of our technique we are able to answer positively to a conjecture of
E.B.Davies.Comment: 40 page
Range safety signal propagation through the SRM exhaust plume of the space shuttle
Theoretical predictions of plume interference for the space shuttle range safety system by solid rocket booster exhaust plumes are reported. The signal propagation was calculated using a split operator technique based upon the Fresnel-Kirchoff integral, using fast Fourier transforms to evaluate the convolution and treating the plume as a series of absorbing and phase-changing screens. Talanov's lens transformation was applied to reduce aliasing problems caused by ray divergence
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