2,267 research outputs found
Are spiral galaxies optically thin or thick?
The opacity of spiral galaxies is examined by modelling the dust and stellar
content of individual galaxies. The model is applied to five late-type spiral
galaxies (NGC 4013, IC 2531, UGC 1082, NGC 5529 and NGC 5907). Having analyzed
a total of seven galaxies thus far, the five galaxies mentioned above plus UGC
2048 and NGC 891 presented in (Xilouris et al. 1997, 1998), we are able to draw
some general conclusions, the most significant of which are: 1) The face-on
central optical depth is less than one in all optical bands indicating that
typical spiral galaxies like the ones that we have modelled would be completely
transparent if they were to be seen face-on. 2) The dust scaleheight is about
half that of the stars, which means that the dust is more concentrated near the
plane of the disk. 3) The dust scalelength is about 1.4 times larger than that
of the stars and the dust is more radially extended than the stars. 4) The dust
mass is found to be about an order of a magnitude more than previously measured
using the IRAS fluxes, indicating the existence of a cold dust component. The
gas-to-dust mass ratio calculated is close to the value derived for our Galaxy.
5) The derived extinction law matches quite well the Galactic extinction law,
indicating a universal dust behaviour.Comment: 13 pages. Accepted for publication in A&
Scaling laws for the photo-ionisation cross section of two-electron atoms
The cross sections for single-electron photo-ionisation in two-electron atoms
show fluctuations which decrease in amplitude when approaching the
double-ionisation threshold. Based on semiclassical closed orbit theory, we
show that the algebraic decay of the fluctuations can be characterised in terms
of a threshold law as with exponent
obtained as a combination of stability exponents of the triple-collision
singularity. It differs from Wannier's exponent dominating double ionisation
processes. The details of the fluctuations are linked to a set of infinitely
unstable classical orbits starting and ending in the non-regularisable triple
collision. The findings are compared with quantum calculations for a model
system, namely collinear helium.Comment: 4 pages, 1 figur
An Atlas of Monte Carlo Models of Dust Extinction in Galaxies for Cosmological Applications
We present an extensive study of the radiative transfer in dusty galaxies
based on Monte Carlo simulations. The main output of these simulations are the
attenuation curves (i.e. the ratio between the observed,
dust extinguished, total intensity to the intrinsic unextinguished one of the
galaxy as a function of wavelength). We have explored the dependence of on a conspicuous set of quantities (Hubble type, inclination, dust
optical thickness, dust distribution and extinction properties) for a large
wavelength interval, ranging from 1250\AA to the K band, thus finally providing
a comprehensive atlas of dust extinction in galaxies, which is electronically
available. This study is particularly suitable for inclusion into galaxy
formation evolution models and to directly interpret observational data on high
redshift galaxies.Comment: 29 pages, aasms4.sty, LaTeX, 5 figures. ApJSS, accepte
Soft X-Ray Sources at the Centers of the Elliptical Galaxies NGC 4472 and NGC 4649
Analysis of recent Chandra observations of the elliptical galaxies NGC 4472
and NGC 4649 has revealed faint soft X-ray sources at their centers. The
sources are located to within 1'' of the optical centers of the galaxies. They
are most likely associated with the central supermassive black holes. Interest
in these and several other similar objects stems from the unusually low
luminosity of the supermassive black holes embedded in dense interstellar
medium. Our Chandra sources have very soft spectra. They are detectable only
below ~0.6 keV and have luminosities in the 0.2-2.5 keV energy band of ~ 6 *
10^{37} erg/s and ~1.7 * 10^{38} erg/s in NGC 4649 and NGC 4472, respectively.Comment: Shortened version of the paper published in Astronomy Letter
The Taiwanese-American Occultation Survey: The Multi-Telescope Robotic Observatory
The Taiwanese-American Occultation Survey (TAOS) operates four fully
automatic telescopes to search for occultations of stars by Kuiper Belt
Objects. It is a versatile facility that is also useful for the study of
initial optical GRB afterglows. This paper provides a detailed description of
the TAOS multi-telescope system, control software, and high-speed imaging.Comment: 11 pages, 11 figure
The TAOS Project: Statistical Analysis of Multi-Telescope Time Series Data
The Taiwanese-American Occultation Survey (TAOS) monitors fields of up to
~1000 stars at 5 Hz simultaneously with four small telescopes to detect
occultation events from small (~1 km) Kuiper Belt Objects (KBOs). The survey
presents a number of challenges, in particular the fact that the occultation
events we are searching for are extremely rare and are typically manifested as
slight flux drops for only one or two consecutive time series measurements. We
have developed a statistical analysis technique to search the multi-telescope
data set for simultaneous flux drops which provides a robust false positive
rejection and calculation of event significance. In this paper, we describe in
detail this statistical technique and its application to the TAOS data set.Comment: 15 pages, 14 figures. Submitted to PAS
The Centers of Early-Type Galaxies with HST II: Empirical Models and Structural Parameters
We present a set of structural parameters for the central parts of 57
early-type galaxies observed with the Planetary Camera of the Hubble Space
Telescope. These parameters are based on a new empirical law that successfully
characterizes the centers of early type galaxies. This empirical law assumes
that the surface brightness profile is a combination of two power laws with
different slopes gamma and beta for the inner and outer regions. Conventional
structural parameters such as core radius and central surface brightness are
replaced by break radius r_b, where the transition between power-law slopes
takes place, and surface brightness mu_b at that radius. An additional
parameter alpha describes the sharpness of the break. The structural parameters
are derived using a chi-squared minimization process applied to the mean
surface brightness profiles. The resulting model profiles generally give very
good agreement to the observed profiles out to the radius of 10 arcseconds
imaged by the Planetary Camera. Exceptions include galaxies which depart from
pure power-laws at large radius, those with strong nuclear components, and
galaxies partly obscured by dust. The uncertainties in the derived parameters
are estimated using Monte-Carlo simulations which test the stability of
solutions in the face of photon noise and the effects of the deconvolution
process. The covariance of the structural parameters is examined by computing
contours of constant chi squared in multi-dimensional parameter space.Comment: 14 pages, 1 table, 8 postscript figures, aaspp4.sty included,
accepted for publication in the Astronomical Journa
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