943 research outputs found
Inclination-Dependent Extinction Effects in Disk Galaxies in the Sloan Digital Sky Survey
We analyze the r-band absolute magnitude and u â r color of low-redshift (z 0.9) to ~0.26 for nearly edge-on galaxies (q < 0.3). When the dimming law ÎM_r âș (log q)^2 is used to create an inclination-corrected sample of bright exponential galaxies, their apparent shapes are consistent with a distribution of mildly noncircular disks, with median short-to-long axis ratio Îł approx 0.22 and median disk ellipticity Δ approx 0.08
The Shapes of Dense Cores and Bok Globules
The shapes of isolated Bok globules and embedded dense cores of molecular
clouds are analyzed using a nonparametric method, under the alternate
hypotheses that they are randomly oriented prolate objects or that they are
randomly oriented oblate objects. In all cases, the prolate hypothesis gives a
better fit to the data. If Bok globules are oblate, they must be very flat; the
average axis ratio is b/a = 0.3, and few or no globules can have b/a > 0.7. If
Bok globules are prolate, then the mean axis ratio is b/a = 0.5. For most data
samples of dense cores, the randomly-oriented oblate hypothesis can be rejected
at the 99% confidence level. If the dense cores are prolate, their mean axis
ratio is approximately 0.4 to 0.5. Dense cores are significantly different in
shape from the clouds in which they are embedded; clouds have flatter apparent
shapes, and are inconsistent with a population of randomly oriented
axisymmetric objects.Comment: 26 pages (LaTeX) including 8 postscript figures; to appear in Ap
On formation of domain wall lattices
We study the formation of domain walls in a phase transition in which an
S_5\times Z_2 symmetry is spontaneously broken to S_3\times S_2. In one compact
spatial dimension we observe the formation of a stable domain wall lattice. In
two spatial dimensions we find that the walls form a network with junctions,
there being six walls to every junction. The network of domain walls evolves so
that junctions annihilate anti-junctions. The final state of the evolution
depends on the relative dimensions of the simulation domain. In particular we
never observe the formation of a stable lattice of domain walls for the case of
a square domain but we do observe a lattice if one dimension is somewhat
smaller than the other. During the evolution, the total wall length in the
network decays with time as t^{-0.71}, as opposed to the usual t^{-1} scaling
typical of regular Z_2 networks.Comment: 7 pages, 4 figures. Minor changes, final version accepted for
publication in Phys. Rev.
An entirely analytical cosmological model
The purpose of the present study is to show that in a particular cosmological
model, with an affine equation of state, one can obtain, besides the background
given by the scale factor, Hubble and deceleration parameters, a representation
in terms of scalar fields and, more important, explicit mathematical
expressions for the density contrast and the power spectrum. Although the model
so obtained is not realistic, it reproduces features observed in some previous
numerical studies and, therefore, it may be useful in the testing of numerical
codes and as a pedagogical tool.Comment: 4 pages (revtex4), 4 figure
Embedded disks in Fornax dwarf ellipticals
We present photometric and kinematic evidence for the presence of stellar
disks, seen practically edge-on, in two Fornax dwarf galaxies, FCC204 (dS0(6))
and FCC288 (dS0(7)). This is the first time such structures have been
identified in Fornax dwarfs. FCC2088 has only a small bulge and a bright
flaring and slightly warped disk that can be traced out to 23" from the center
(2.05 kpc for H_0=75 km/s/Mpc). FCC204's disk can be traced out to 20" (1.78
kpc). This galaxy possesses a large bulge. These results can be compared to the
findings of Jerjen et al. (2000) and Barazza et al. (2002) who discovered
nucleated dEs with spiral and bar features in the Virgo Cluster.Comment: 8 pages, 8 figures, accepted for publication in A&
Encounters between spherical galaxies II: systems with dark halo
We performe N-body simulations of encounters between spherical systems
surrounded by a spherical halo. Following a preceding paper with a similar aim,
the initial systems include a spherical Jaffe model for the luminous matter and
a Hernquist model for the halo. The merger remnants from this sample are mainly
slowly rotating, prolate spheroids with a radially anisotropic velocity
distribution. The results are compared with real-life ellipticals and with the
models without halo in paper I. We argue that elliptical galaxies with evidence
of dark matter could be formed in the field via a merger of spheroids
surrounded by a dark matter halo, while ellipticals with no evidence of dark
matter might be formed via a merger of two spheroids in a cluster.Comment: 12 pages, 15 figures, accepted for publication in MNRAS. High res.
version avaible at http://www.iac.es/folleto/research/Publi02/preprints05.ht
"Lattice-Free" Simulations of Topological Defect Formation
We examine simulations of the formation of domain walls, cosmic strings, and
monopoles on a cubic lattice, in which the topological defects are assumed to
lie at the zeros of a piecewise constant 1, 2, or 3 component Gaussian random
field, respectively. We derive analytic expressions for the corresponding
topological defect densities in the continuum limit and show that they fail to
agree with simulation results, even when the fields are smoothed on small
scales to eliminate lattice effects. We demonstrate that this discrepancy,
which is related to a classic geometric fallacy, is due to the anisotropy of
the cubic lattice, which cannot be eliminated by smoothing. This problem can be
resolved by linearly interpolating the field values on the lattice, which gives
results in good agreement with the continuum predictions. We use this procedure
to obtain a lattice-free estimate (for Gaussian smoothing) of the fraction of
the total length of string in the form of infinite strings: .Comment: 12 pages, 9 figures, added acknowledgment of refere
Supermassive black holes do not correlate with dark matter halos of galaxies
Supermassive black holes have been detected in all galaxies that contain
bulge components when the galaxies observed were close enough so that the
searches were feasible. Together with the observation that bigger black holes
live in bigger bulges, this has led to the belief that black hole growth and
bulge formation regulate each other. That is, black holes and bulges
"coevolve". Therefore, reports of a similar correlation between black holes and
the dark matter halos in which visible galaxies are embedded have profound
implications. Dark matter is likely to be nonbaryonic, so these reports suggest
that unknown, exotic physics controls black hole growth. Here we show - based
in part on recent measurements of bulgeless galaxies - that there is almost no
correlation between dark matter and parameters that measure black holes unless
the galaxy also contains a bulge. We conclude that black holes do not correlate
directly with dark matter. They do not correlate with galaxy disks, either.
Therefore black holes coevolve only with bulges. This simplifies the puzzle of
their coevolution by focusing attention on purely baryonic processes in the
galaxy mergers that make bulges.Comment: 12 pages, 9 Postscript figures, 1 table; published in Nature (20
January 2011
Identification of furfural resistant strains of Saccharomyces cerevisiae and Saccharomyces paradoxus from a collection of environmental and industrial isolates
Background Fermentation of bioethanol using lignocellulosic biomass as a raw material provides a sustainable alternative to current biofuel production methods by utilising waste food streams as raw material. Before lignocellulose can be fermented it requires physical, chemical and enzymatic treatment in order to release monosaccharides, a process that causes the chemical transformation of glucose and xylose into the cyclic aldehydes furfural and hydroxyfurfural. These furan compounds are potent inhibitors of Saccharomyces fermentation, and consequently furfural tolerant strains of Saccharomyces are required for lignocellulosic fermentation. Results This study investigated yeast tolerance to furfural and hydroxyfurfural using a collection of 71 environmental and industrial isolates of the bakerâs yeast Saccharomyces cerevisiae and its closest relative Saccharomyces paradoxus. The Saccharomyces strains were initially screened for growth on media containing 100 mM glucose and 1.5 mg ml-1 furfural. Five strains were identified that showed a significant tolerance to growth in the presence of furfural and these were then screened for growth and ethanol production in the presence of increasing amounts (0.1-4 mg ml-1) of furfural. Conclusions Of the five furfural tolerant strains S. cerevisiae NCYC 3451 displayed the greatest furfural resistance, and was able to grow in the presence of up to 3.0 mg ml-1 furfural. Furthermore, ethanol production in this strain did not appear to be inhibited by furfural, with the highest ethanol yield observed at 3.0 mg ml-1 furfural. Although furfural resistance was not found to be a trait specific to any one particular lineage or population, three of the strains were isolated from environments where they might be continually exposed to low levels of furfural through the on-going natural degradation of lignocelluloses, and would therefore develop elevated levels of resistance to these furan compounds. Thus these strains represent good candidates for future studies of genetic variation relevant to understanding and manipulating furfural resistance and in the development of tolerant ethanologenic yeast strains for use in bioethanol production from lignocellulose processing
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