10,771 research outputs found
On the estimation of galaxy structural parameters: the Sersic Model
This paper addresses some questions which have arisen from the use of the
S\'ersic r^{1/n} law in modelling the luminosity profiles of early type
galaxies. The first issue deals with the trend between the half-light radius
and the structural parameter n. We show that the correlation between these two
parameters is not only real, but is a natural consequence from the previous
relations found to exist between the model-independent parameters: total
luminosity, effective radius and effective surface brightness. We also define a
new galaxy concentration index which is largely independent of the image
exposure depth, and monotonically related with n. The second question concerns
the curious coincidence between the form of the Fundamental Plane and the
coupling between _e and r_e when modelling a light profile. We explain,
through a mathematical analysis of the S\'ersic law, why the quantity
r_e_e^{0.7} appears almost constant for an individual galaxy, regardless of
the value of n (over a large range) adopted in the fit to the light profile.
Consequently, Fundamental Planes of the form r_e_e^{0.7} propto sigma_0^x
(for any x, and where sigma_0 is the central galaxy velocity dispersion) are
insensitive to galaxy structure. Finally, we address the problematic issue of
the use of model-dependent galaxy light profile parameters versus
model-independent quantities for the half-light radii, mean surface brightness
and total galaxy magnitude. The former implicitly assume that the light profile
model can be extrapolated to infinity, while the latter quantities, in general,
are derived from a signal-to-noise truncated profile. We quantify
(mathematically) how these parameters change as one reduces the outer radius of
an r^{1/n} profile, and reveal how these can vary substantially when n>4.Comment: 10 pages, 10 figures, accepted for publication in MNRA
Strain-induced ferroelectricity in simple rocksalt binary oxides
The alkaline earth binary oxides adopt a simple rocksalt structure and form
an important family of compounds because of their large presence in the earth's
mantle and their potential use in microelectronic devices. In comparison to the
class of multifunctional ferroelectric perovskite oxides, however, their
practical applications remain limited and the emergence of ferroelectricity and
related functional properties in simple binary oxides seems so unlikely that it
was never previously considered. Here, we show using first-principles density
functional calculations that ferroelectricity can be easily induced in simple
alkaline earth binary oxides such as barium oxide (BaO) using appropriate
epitaxial strains. Going beyond the fundamental discovery, we show that the
functional properties (polarization, dielectric constant and piezoelectric
response) of such strained binary oxides are comparable in magnitude to those
of typical ferroelectric perovskite oxides, so making them of direct interest
for applications. Finally, we show that magnetic binary oxides such as EuO,
with the same rocksalt structure, behave similarly to the alkaline earth
oxides, suggesting a route to new multiferroics combining ferroelectric and
magnetic properties
Local cytokine transcription in naïve and previously infected sheep and lambs following challenge with Teladorsagia circumcincta
<b>Background</b><p></p>
The abomasal helminth Teladorsagia circumcincta is one of the most economically important parasites affecting sheep in temperate regions. Infection is particularly detrimental to lambs, in which it can cause pronounced morbidity and severe production losses. Due to the spreading resistance of this parasite to all classes of anthelmintic drugs, teladorsagiosis is having an increasingly severe impact on the sheep industry with significant implications for sheep welfare. Protective immunity develops slowly, wanes rapidly and does not appear to be as effective in young lambs. To investigate the development of immunity to T. circumcincta in sheep and lambs, we used cytokine transcript profiling to examine differences in the abomasal mucosa and gastric lymph node of naïve and previously infected sheep and lambs following challenge.<p></p>
<b>Results</b><p></p>
The results of these experiments demonstrated that the abomasal mucosa is a major source of cytokines during abomasal helminth infection. A local Th2-type cytokine response was observed in the abomasal mucosa and gastric lymph node of the previously infected sheep and lambs when compared with those of the naïve during the early stages of infection. In contrast, a pro-inflammatory component more was evident in the abomasal mucosa and gastric lymph node of the naïve sheep when compared with those of the previously infected, which was not observed in the lambs.<p></p>
<b>Conclusions</b><p></p>
The greater levels of Th2-type cytokine transcripts in both the abomasum and gastric lymph node of the previously infected compared with naïve sheep and lambs emphasises the importance of these mechanisms in the immune response to T. circumcincta infection. Younger lambs appear to be able to generate similar Th2-type responses in the abomasum suggesting that the increased morbidity and apparent lack of resistance in younger lambs following continuous or repeated exposure to T. circumcincta is unlikely to be due to a lack of appropriate Th2-type cytokine production
Spatial and temporal localization of light in two dimensions
Quasi-resonant scattering of light in two dimensions can be described either
as a scalar or as a vectorial electromagnetic wave. Performing a scaling
analysis we observe in both cases long lived modes, yet only the scalar case
exhibits Anderson localized modes together with extremely long mode lifetimes.
We show that the localization length of these modes is influenced only by their
position, and not their lifetime. Investigating the reasons for the absence of
localization, it appears that both the coupling of several polarizations and
the presence of near-field terms are able to prevent long lifetimes and
Anderson localization.Comment: 5 pages, 4 figures and Supplementary Informatio
Thematic Review Series: The Immune System and Atherogenesis. Cytokines affecting endothelial and smooth muscle cells in vascular disease
The cellular and extracellular matrix accumulations that comprise the lesions of atherosclerosis are driven by local release of cytokines at sites of predilection for lesion formation, and by the specific attraction and activation of cells expressing receptors for these cytokines. Although cytokines were originally characterized for their potent effects on immune and inflammatory cells, they also promote endothelial cell dysfunction and alter smooth muscle cell (SMC) phenotype and function, which can contribute to or retard vascular pathologies. This review summarizes in vivo studies that have characterized endothelial- and smooth muscle-specific effects of altering cytokine signaling in vascular disease. Although multiple reports have identified cytokines as pivotal players in endothelial and SMC responses in vascular disease, they also have highlighted the need to delineate the critical genes and specific cellular functions regulated by individual cytokine signaling pathways
Synchronization of Bloch oscillations by a ring cavity
We consider Bloch oscillations of ultracold atoms stored in a one-dimensional
vertical optical lattice and simultaneously interacting with a unidirectionally
pumped optical ring cavity whose vertical arm is collinear with the optical
lattice. We find that the feedback provided by the cavity field on the atomic
motion synchronizes Bloch oscillations via a mode-locking mechanism, steering
the atoms to the lowest Bloch band. It also stabilizes Bloch oscillations
against noise, and even suppresses dephasing due to atom-atom interactions.
Furthermore, it generates periodic bursts of light emitted into the
counter-propagating cavity mode, providing a non-destructive monitor of the
atomic dynamics. All these features may be crucial for future improvements of
the design of atomic gravimeters based on recording Bloch oscillations.Comment: 14 pages, 7 figure
Variational approach to gravitational theories with two independent connections
A new variational approach for general relativity and modified theories of
gravity is presented. In addition to the metric tensor, two independent affine
connections enter the action as dynamical variables. In the matter action the
dependence upon one of the connections is left completely unspecified. When the
variation is applied to the Einstein-Hilbert action the Einstein field
equations are recovered. However when applied to and Scalar-Tensor
theories, it yields gravitational field equations which differ from their
equivalents obtained with a metric or Palatini variation and reduce to the
former ones only when no connections appear in the matter action.Comment: 11 pages, no figure
Coherence effects in scattering order expansion of light by atomic clouds
We interpret cooperative scattering by a collection of cold atoms as a
multiple scattering process. Starting from microscopic equations describing the
response of atoms to a probe light beam, we represent the total scattered
field as an infinite series of multiple scattering events. As an application of
the method, we obtain analytical expressions of the coherent intensity in the
double scattering approximation for Gaussian density profiles. In particular,
we quantify the contributions of coherent backward and forward scattering.Comment: 10 pages, 6 figure
A Local Baseline of the Black Hole Mass Scaling Relations for Active Galaxies. I. Methodology and Results of Pilot Study
We present high-quality Keck/LRIS longslit spectroscopy of a pilot sample of
25 local active galaxies selected from the SDSS (0.0210^7 M_sun) to
study the relations between black hole mass (MBH) and host-galaxy properties.
We determine stellar kinematics of the host galaxy, deriving stellar-velocity
dispersion profiles and rotation curves from three spectral regions (including
CaH&K, MgIb triplet, and CaII triplet). In addition, we perform surface
photometry on SDSS images, using a newly developed code for joint multi-band
analysis. BH masses are estimated from the width of the Hbeta emission line and
the host-galaxy free 5100A AGN luminosity. Combining results from spectroscopy
and imaging allows us to study four MBH scaling relations: MBH-sigma,
MBH-L(sph), MBH-M(sph,*), MBH-M(sph,dyn). We find the following results. First,
stellar-velocity dispersions determined from aperture spectra (e.g. SDSS fiber
spectra or unresolved data from distant galaxies) can be biased, depending on
aperture size, AGN contamination, and host-galaxy morphology. However, such a
bias cannot explain the offset seen in the MBH-sigma relation at higher
redshifts. Second, while the CaT region is the cleanest to determine
stellar-velocity dispersions, both the MgIb region, corrected for FeII
emission, and the CaHK region, although often swamped by the AGN powerlaw
continuum and emission lines, can give results accurate to within a few
percent. Third, the MBH scaling relations of our pilot sample agree in slope
and scatter with those of other local active and inactive galaxies. In the next
papers of the series we will quantify the scaling relations, exploiting the
full sample of ~100 objects.Comment: 28 pages, 19 figures. Final version, accepted for publication in The
Astrophysical Journal (ApJ, 726, 59
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