16,882 research outputs found
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
Dark and Baryonic Matter in Bright Spiral Galaxies: I.Near-infrared and Optical Broadband Surface Photometry of 30 Galaxies
We present photometrically calibrated images and surface photometry in the B,
V, R, J, H, and K-bands of 25, and in the g, r, and K-bands of 5 nearby bright
(Bo_T<12.5 mag) spiral galaxies with inclinations between 30-65 degrees
spanning the Hubble Sequence from Sa to Scd. Data are from The Ohio State
University Bright Spiral Galaxy Survey, the Two Micron All Sky Survey, and the
Sloan Digital Sky Survey Second Data Release. Radial surface brightness
profiles are extracted, and integrated magnitudes are measured from the
profiles. Axis ratios, position angles, and scale lengths are measured from the
near-infrared images. A 1-dimensional bulge/disk decomposition is performed on
the near-infrared images of galaxies with a non-negligible bulge component, and
an exponential disk is fit to the radial surface brightness profiles of the
remaining galaxies.Comment: 28 page
Role of electrostatics in the texture of islands in free standing ferroelectric liquid crystal films
Curved textures of ferroelectric smectic C* liquid crystals produce space
charge when they involve divergence of the spontaneous polarization field.
Impurity ions can partially screen this space charge, reducing long range
interactions to local ones. Through studies of the textures of islands on very
thin free-standing smectic films, we see evidence of this effect, in which
materials with a large spontaneous polarization have static structures
described by a large effective bend elastic constant. To address this issue, we
calculated the electrostatic free energy of a free standing film of
ferroelectric liquid crystal, showing how the screened coulomb interaction
contributes a term to the effective bend elastic constant, in the static long
wavelength limit. We report experiments which support the main features of this
model
The cholesterol-raising diterpenes from coffee beans increase serum lipid transfer protein activity levels in humans
Cafestol and kahweolâditerpenes present in unfiltered coffeeâ strongly raise serum VLDL and LDL cholesterol and slightly reduce HDL cholesterol in humans. The mechanism of action is unknown. We determined whether the coffee diterpenes may affect lipoprotein metabolism via effects on lipid transfer proteins and lecithin:cholesterol acyltransferase in a randomized, double-blind cross-over study with 10 healthy male volunteers. Either cafestol (61â64 mg/day) or a mixture of cafestol (60 mg/day) and kahweol (48â54 mg/day) was given for 28 days. Serum activity levels of cholesterylester transfer protein, phospholipid transfer protein and lecithin:cholesterol acyltransferase were measured using exogenous substrate assays. Relative to baseline values, cafestol raised the mean (±S.D.) activity of cholesterylester transfer protein by 18±12% and of phospholipid transfer protein by 21±14% (both P<0.001). Relative to cafestol alone, kahweol had no significant additional effects. Lecithin:cholesterol acyltransferase activity was reduced by 11±12% by cafestol plus kahweol (P=0.02). It is concluded that the effects of coffee diterpenes on plasma lipoproteins may be connected with changes in serum activity levels of lipid transfer proteins
VLT/VIMOS Observations of an Occulting Galaxy Pair: Redshifts and Effective Extinction Curve
We present VLT/VIMOS IFU observations of an occulting galaxy pair previously
discovered in HST observations. The foreground galaxy is a low-inclination
spiral disk, which causes clear attenuation features seen against the bright
bulge and disk of the background galaxy. We find redshifts of and z=0.065 for the foreground and background galaxy respectively.
This relatively small difference does not rule out gravitational interaction
between the two galaxies. Emission line ratios point to a star-forming, not
AGN-dominated foreground galaxy.
We fit the Cardelli, Clayton & Mathis (CCM) extinction law to the spectra of
individual fibres to derive slope () and normalization (). The
normalization agrees with the HST attenuation map and the slope is lower than
the Milky Way relation (), which is likely linked to the spatial
sampling of the disk. We speculate that the values of point to either
coherent ISM structures in the disk larger than usual ( kpc) or higher
starting values of , indicative of recent processing of the dust.
The foreground galaxy is a low stellar mass spiral () with a high dust content (). The dust disk geometry visible in the HST image would explain the
observed SED properties of smaller galaxies: a lower mean dust temperature, a
high dust-to-stellar mass ratio but relatively little optical attenuation.
Ongoing efforts to find occulting pairs with a small foreground galaxies will
show how common this geometry is.Comment: 16 pages, 3 tables, 13 figures, accepted for publication in MNRA
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
Regulatory Dynamics on Random Networks: Asymptotic Periodicity and Modularity
We study the dynamics of discrete-time regulatory networks on random
digraphs. For this we define ensembles of deterministic orbits of random
regulatory networks, and introduce some statistical indicators related to the
long-term dynamics of the system. We prove that, in a random regulatory
network, initial conditions converge almost surely to a periodic attractor. We
study the subnetworks, which we call modules, where the periodic asymptotic
oscillations are concentrated. We proof that those modules are dynamically
equivalent to independent regulatory networks.Comment: 23 pages, 3 figure
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.
Relativistic Brueckner-Hartree-Fock calculations with explicit intermediate negative energy states
In a relativistic Brueckner-Hartree-Fock calculation we include explicit
negative-energy states in the two-body propagator. This is achieved by using
the Gross spectator-equation, modified by medium effects. Qualitatively our
results compare well with other RBHF calculations. In some details significant
differences occur, e.g, our equation of state is stiffer and the momentum
dependence of the self-energy components is stronger than found in a reference
calculation without intermediate negative energy states.Comment: 13 pages Revtex, 5 figures included seperatel
A skewer survey of the Galactic halo from deep CFHT and INT images
We study the density profile and shape of the Galactic halo using deep
multicolour images from the MENeaCS and CCCP projects, over 33 fields selected
to avoid overlap with the Galactic plane. Using multicolour selection and PSF
homogenization techniques we obtain catalogues of F stars (near-main sequence
turnoff stars) out to Galactocentric distances up to 60kpc. Grouping nearby
lines of sight, we construct the stellar density profiles through the halo in
eight different directions by means of photometric parallaxes. Smooth halo
models are then fitted to these profiles. We find clear evidence for a
steepening of the density profile power law index around R=20 kpc, from -2.50
+- 0.04 to -4.85 +- 0.04, and for a flattening of the halo towards the poles
with best-fit axis ratio 0.63 +- 0.02. Furthermore, we cannot rule out a mild
triaxiality (w>=0.8). We recover the signatures of well-known substructure and
streams that intersect our lines of sight. These results are consistent with
those derived from wider but shallower surveys, and augur well for upcoming,
wide-field surveys of comparable depth to our pencil beam surveys.Comment: 14 pages, 8 figures, 6 table
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