23,459 research outputs found
The Way We Measure: Comparison of Methods to Derive Radial Surface Brightness Profiles
The breaks and truncations in the luminosity profile of face-on spiral
galaxies offer valuable insights in their formation history. The traditional
method of deriving the surface photometry profile for face-on galaxies is to
use elliptical averaging. In this paper, we explore the question whether
elliptical averaging is the best way to do this. We apply two additional
surface photometry methods, one new: principle axis summation, and one old that
has become seldom used: equivalent profiles. These are compared to elliptically
averaged profiles using a set of 29 face-on galaxies. We find that the
equivalent profiles match extremely well with elliptically averaged profiles,
confirming the validity of using elliptical averaging. The principle axis
summation offers a better comparison to edge-on galaxies.Comment: Accepted for publication by Monthly Notices of the R.A.S. A hi-res
version is available at http://www.astro.rug.nl/~vdkruit/Petersetal-VI.pd
Patterns of Striped order in the Classical Lattice Coulomb Gas
We obtain via Monte Carlo simulations the low temperature charge
configurations in the lattice Coulomb gas on square lattices for charge filling
ratio in the range . We find a simple regularity in the low
temperature charge configurations which consist of a suitable periodic
combination of a few basic striped patterns characterized by the existence of
partially filled diagonal channels. In general there exist two separate
transitions where the lower temperature transition () corresponds to the
freezing of charges within the partially filled channels. is found to be
sensitively dependent on through the charge number density within the channels.Comment: 4 pages, 8 figure
Models of Disk Evolution: Confrontation with Observations
We present simple models for disk evolution based on two different
approaches: a forward approach based on predictions generic to hierarchical
models for structure formation (e.g., Mo, Mao, & White 1998) and a backwards
approach based on detailed modeling of the Milky Way galaxy (e.g., Bouwens,
Cayon, & Silk 1997). We normalize these models to local observations and
predict high-redshift luminosities, sizes, circular velocities, and surface
brightnesses. Both approaches yield somewhat similar predictions for size,
surface brightness, and luminosity evolution though they clearly differ in the
amount of number evolution. These predictions seem to be broadly consistent
with the high-redshift observations of Simard et al. (1999), suggesting that
the B-band surface brightness of disks has indeed evolved by ~1.5 mag from z~0
to z~1 similar to the models and is not an artifact of selection effects as
previously claimed. We also find a lack of low surface brightness galaxies in
several high redshift samples relative to model predictions based on local
samples (de Jong & van der Kruit 1994; Mathewson, Ford, & Buchhorn 1992).Comment: 34 pages, 9 figures, accepted to Ap
A Look At Three Different Scenarios for Bulge Formation
In this paper, we present three qualitatively different scenarios for bulge
formation: a secular evolution model in which bulges form after disks and
undergo several central starbursts, a primordial collapse model in which bulges
and disks form simultaneously, and an early bulge formation model in which
bulges form prior to disks. We normalize our models to the local z=0
observations of de Jong & van der Kruit (1994) and Peletier & Balcells (1996)
and make comparisons with high redshift observations. We consider model
predictions relating directly to bulge-to-disk properties. As expected, smaller
bulge-to-disk ratios and bluer bulge colors are predicted by the secular
evolution model at all redshifts, although uncertainties in the data are
currently too large to differentiate strongly between the models.Comment: 19 pages, 6 figures, accepted for publication in the Astrophysical
Journa
High-energy emission from NGC 5506, the brightest hard X-ray Narrow Line Seyfert 1 galaxy
We present results on the hard X-ray emission of NGC 5506, the brightest
narrow line Seyfert 1 galaxy above 20 keV. All the recent observations by
INTEGRAL, Swift and Suzaku have been analysed and spectral analysis during nine
separated time periods has been performed. While flux variations by a factor of
2 were detected during the last 7 years, only moderate spectral variations have
been observed, with the hint of a hardening of the X-ray spectrum and a
decrease of the intrinsic absorption with time. Using Suzaku observations it is
possible to constrain the amount of Compton reflection to R = 0.6-1.0, in
agreement with previous results on the source. The signature of Comptonisation
processes can also be found in the detection of a high-energy cut-off during
part of the observations, at Ec = 40-100 keV. When a Comptonisation model is
applied to the Suzaku data, the temperature and the optical depth of the
Comptonising electron plasma are measured at kT = 60-80 keV and tau = 0.6-1.0,
respectively. The properties inferred for NGC 5506 in this study agree with
those based on other data sets for the same AGN, and fit the picture of NLS1
having in general lower high-energy cut-offs at hard X-rays than their broad
line equivalent.Comment: 8 pages, 3 figures, 1 table. Proceedings of the Workshop "Narrow-Line
Seyfert 1 Galaxies and Their Place in the Universe", Milano, April 4-6, 2011
(Proceedings of Science,
http://pos.sissa.it/cgi-bin/reader/conf.cgi?confid=126
Towards a fully self-consistent spectral function of the nucleon in nuclear matter
We present a calculation of nuclear matter which goes beyond the usual
quasi-particle approximation in that it includes part of the off-shell
dependence of the self-energy in the self-consistent solution of the
single-particle spectrum. The spectral function is separated in contributions
for energies above and below the chemical potential. For holes we approximate
the spectral function for energies below the chemical potential by a
-function at the quasi-particle peak and retain the standard form for
energies above the chemical potential. For particles a similar procedure is
followed. The approximated spectral function is consistently used at all levels
of the calculation. Results for a model calculation are presented, the main
conclusion is that although several observables are affected by the inclusion
of the continuum contributions the physical consistency of the model does not
improve with the improved self-consistency of the solution method. This in
contrast to expectations based on the crucial role of self-consistency in the
proofs of conservation laws.Comment: 26 pages Revtex with 4 figures, submitted to Phys. Rev.
Rotational Widths for Use in the Tully-Fisher Relation. II. The Impact of Surface Brightness
Using a large sample of spiral galaxies for which 21 cm single-dish and/or
long-slit optical spectra are available, we make a detailed comparison between
various estimates of rotational widths. Different optical width estimators are
considered and their limitations discussed, with emphasis on biases associated
with rotation curve properties (shape and extent) and disk central surface
brightness. The best match with HI rotational velocities is obtained with
Polyex widths, which are measured at the optical radius (encompassing a fixed
fraction of the total light of the galaxy) from a model fit to the rotation
curve. In contrast with Polyex widths, optical rotational velocities measured
at 2.15 disk scale lengths r_d deviate from HI widths by an amount that
correlates with the central surface brightness of the disk. This bias occurs
because the rotation curves of galaxies are in general still rising at 2.15
r_d, and the fraction of total mass contained within this radius decreases with
increasing disk surface brightness. Statistical corrections, parameterized by
the radial extent of the observed rotation curve, are provided to reduce Polyex
and HI width measurements into a homogeneous system. This yields a single
robust estimate of rotational velocity to be used for applications of disk
scaling relations.Comment: 13 pages, 8 figures. To appear in the Astronomical Journal (August
2007
Fungal diversity in wounded stems of Aquilaria malaccensis.
Aquilaria malaccensis is a tropical tree which produces agarwood in its trunk often after being wounded and attacked by pathogens or insects. Fungi are generally viewed as the main microbial component responsible for agarwood formation. In this study, isolation of fungi from agarwood in damaged trees was carried out. Culture morphology and microscopic characteristics plus PCR amplification of the internal transcribed spacer (ITS) region from the fungal isolates as well as wood samples, were used to identify the fungal community composition of wounded A. malaccensis trees from a natural forest in West Malaysia. Conventional culture methodology revealed Cunninghamella, Curvularia, Fusarium and Trichoderma species as members of the agarwood community. Analysis of genomic DNA confirmed the identifications. When wood samples were used directly in PCR, an additional Lasiodiplodia species was identified. Neighbor-joining trees were constructed to examine the relationships between the isolates sequence data and reference sequences in GenBank. Five distinct clades resulted, supported with high bootstrap values, indicating the presence of five distinct taxa. The wounded trunks of A. malaccensis in the natural environment harbor multiple fungal taxa that exist in a complex system as a whole or in succession leading to agarwood production in the tree trunk
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