15,351 research outputs found
UK Centre for Coaching Excellence Scoping Project Report: National and International Best Practice in Level 4 Coach Development
Non-equilibrium Dynamics of Finite Interfaces
We present an exact solution to an interface model representing the dynamics
of a domain wall in a two-phase Ising system. The model is microscopically
motivated, yet we find that in the scaling regime our results are consistent
with those obtained previously from a phenomenological, coarse-grained Langevin
approach.Comment: 12 pages LATEX (figures available on request), Oxford preprint
OUTP-94-07
The Visibility of Galactic Bars and Spiral Structure At High Redshifts
We investigate the visibility of galactic bars and spiral structure in the
distant Universe by artificially redshifting 101 B-band CCD images of local
spiral galaxies from the Ohio State University Bright Spiral Galaxy Survey. Our
artificially redshifted images correspond to Hubble Space Telescope I-band
observations of the local galaxy sample seen at z=0.7, with integration times
matching those of both the very deep Northern Hubble Deep Field data, and the
much shallower Flanking Field observations. The expected visibility of galactic
bars is probed in two ways: (1) using traditional visual classification, and
(2) by charting the changing shape of the galaxy distribution in "Hubble
space", a quantitative two-parameter description of galactic structure that
maps closely on to Hubble's original tuning fork. Both analyses suggest that
over 2/3 of strongly barred luminous local spirals i.e. objects classified as
SB in the Third Reference Catalog) would still be classified as strongly barred
at z=0.7 in the Hubble Deep Field data. Under the same conditions, most weakly
barred spirals (classified SAB in the Third Reference Catalog) would be
classified as regular spirals. The corresponding visibility of spiral structure
is assessed visually, by comparing luminosity classifications for the
artificially redshifted sample with the corresponding luminosity
classifications from the Revised Shapley Ames Catalog. We find that for
exposures times similar to that of the Hubble Deep Field spiral structure
should be detectable in most luminous low-inclination spiral galaxies at z=0.7
in which it is present. [ABRIDGED]Comment: Accepted for publication in The Astronomical Journa
FERENGI: Redshifting galaxies from SDSS to GEMS, STAGES and COSMOS
We describe the creation of a set of artificially "redshifted" galaxies in
the range 0.1<z<1.1 using a set of ~100 SDSS low redshift (v<7000 km/s) images
as input. The intention is to generate a training set of realistic images of
galaxies of diverse morphologies and a large range of redshifts for the GEMS
and COSMOS galaxy evolution projects. This training set allows other studies to
investigate and quantify the effects of cosmological redshift on the
determination of galaxy morphologies, distortions and other galaxy properties
that are potentially sensitive to resolution, surface brightness and bandpass
issues. We use galaxy images from the SDSS in the u, g, r, i, z filter bands as
input, and computed new galaxy images from these data, resembling the same
galaxies as located at redshifts 0.1<z<1.1 and viewed with the Hubble Space
Telescope Advanced Camera for Surveys (HST ACS). In this process we take into
account angular size change, cosmological surface brightness dimming, and
spectral change. The latter is achieved by interpolating a spectral energy
distribution that is fit to the input images on a pixel-to-pixel basis. The
output images are created for the specific HST ACS point spread function and
the filters used for GEMS (F606W and F850LP) and COSMOS (F814W). All images are
binned onto the desired pixel grids (0.03" for GEMS and 0.05" for COSMOS) and
corrected to an appropriate point spread function. Noise is added corresponding
to the data quality of the two projects and the images are added onto empty sky
pieces of real data images. We make these datasets available from our website,
as well as the code - FERENGI: "Full and Efficient Redshifting of Ensembles of
Nearby Galaxy Images" - to produce datasets for other redshifts and/or
instruments.Comment: 11 pages, 10 figures, 3 table
Non-LTE, Relativistic Accretion Disk Fits to 3C~273 and the Origin of the Lyman Limit Spectral Break
We fit general relativistic, geometrically thin accretion disk models with
non-LTE atmospheres to near simultaneous multiwavelength data of 3C~273,
extending from the optical to the far ultraviolet. Our model fits show no flux
discontinuity associated with a hydrogen Lyman edge, but they do exhibit a
spectral break which qualitatively resembles that seen in the data. This break
arises from relativistic smearing of Lyman emission edges which are produced
locally at tens of gravitational radii in the disk. We discuss the possible
effects of metal line blanketing on the model spectra, as well as the
substantial Comptonization required to explain the observed soft X-ray excess.
Our best fit accretion disk model underpredicts the near ultraviolet emission
in this source, and also has an optical spectrum which is too red. We discuss
some of the remaining physical uncertainties, and suggest in particular that an
extension of our models to the slim disk regime and/or including nonzero
magnetic torques across the innermost stable circular orbit may help resolve
these discrepancies.Comment: Accepted for publication in Ap
The Role of the Dielectric Barrier in Narrow Biological Channels: a Novel Composite Approach to Modeling Single-channel Currents
A composite continuum theory for calculating ion current through a protein channel of known structure is proposed, which incorporates information about the channel dynamics. The approach is utilized to predict current through the Gramicidin A ion channel, a narrow pore in which the applicability of conventional continuum theories is questionable. The proposed approach utilizes a modified version of Poisson-Nernst-Planck (PNP) theory, termed Potential-of-Mean-Force-Poisson-Nernst-Planck theory (PMFPNP), to compute ion currents. As in standard PNP, ion permeation is modeled as a continuum drift-diffusion process in a self-consistent electrostatic potential. In PMFPNP, however, information about the dynamic relaxation of the protein and the surrounding medium is incorporated into the model of ion permeation by including the free energy of inserting a single ion into the channel, i.e., the potential of mean force along the permeation pathway. In this way the dynamic flexibility of the channel environment is approximately accounted for. The PMF profile of the ion along the Gramicidin A channel is obtained by combining an equilibrium molecular dynamics (MD) simulation that samples dynamic protein configurations when an ion resides at a particular location in the channel with a continuum electrostatics calculation of the free energy. The diffusion coefficient of a potassium ion within the channel is also calculated using the MD trajectory. Therefore, except for a reasonable choice of dielectric constants, no direct fitting parameters enter into this model. The results of our study reveal that the channel response to the permeating ion produces significant electrostatic stabilization of the ion inside the channel. The dielectric self-energy of the ion remains essentially unchanged in the course of the MD simulation, indicating that no substantial changes in the protein geometry occur as the ion passes through it. Also, the model accounts for the experimentally observed saturation of ion current with increase of the electrolyte concentration, in contrast to the predictions of standard PNP theory
Casimir interactions in Ising strips with boundary fields: exact results
An exact statistical mechanical derivation is given of the critical Casimir
forces for Ising strips with arbitrary surface fields applied to edges. Our
results show that the strength as well as the sign of the force can be
controled by varying the temperature or the fields. An interpretation of the
results is given in terms of a linked cluster expansion. This suggests a
systematic approach for deriving the critical Casimir force which can be used
in more general models.Comment: 10 pages, 4 figure
Zeolite-dye micro lasers
We present a new class of micro lasers based on nanoporous molecular sieve
host-guest systems. Organic dye guest molecules of
1-Ethyl-4-(4-(p-Dimethylaminophenyl)-1,3-butadienyl)-pyridinium Perchlorat were
inserted into the 0.73-nm-wide channel pores of a zeolite AlPO-5 host. The
zeolitic micro crystal compounds where hydrothermally synthesized according to
a particular host-guest chemical process. The dye molecules are found not only
to be aligned along the host channel axis, but to be oriented as well. Single
mode laser emission at 687 nm was obtained from a whispering gallery mode
oscillating in a 8-m-diameter monolithic micro resonator, in which the
field is confined by total internal reflection at the natural hexagonal
boundaries inside the zeolitic microcrystals.Comment: Accepted for publication in Phys. Rev. Let
Prospects for Redshifted 21-cm observations of quasar HII regions
The introduction of low-frequency radio arrays over the coming decade is
expected to revolutionize the study of the reionization epoch. Observation of
the contrast in redshifted 21cm emission between a large HII region and the
surrounding neutral IGM will be the simplest and most easily interpreted
signature. We find that an instrument like the planned Mileura Widefield Array
Low-Frequency Demonstrator (LFD) will be able to obtain good signal to noise on
HII regions around the most luminous quasars, and determine some gross
geometric properties, e.g. whether the HII region is spherical or conical. A
hypothetical follow-up instrument with 10 times the collecting area of the LFD
(MWA-5000) will be capable of mapping the detailed geometry of HII regions,
while SKA will be capable of detecting very narrow spectral features as well as
the sharpness of the HII region boundary. The MWA-5000 will discover
serendipitous HII regions in widefield observations. We estimate the number of
HII regions which are expected to be generated by quasars. Assuming a late
reionization at z~6 we find that there should be several tens of quasar HII
regions larger than 4Mpc at z~6-8 per field of view. Identification of HII
regions in forthcoming 21cm surveys can guide a search for bright galaxies in
the middle of these regions. Most of the discovered galaxies would be the
massive hosts of dormant quasars that left behind fossil HII cavities that
persisted long after the quasar emission ended, owing to the long recombination
time of intergalactic hydrogen. A snap-shot survey of candidate HII regions
selected in redshifted 21cm image cubes may prove to be the most efficient
method for finding very high redshift quasars and galaxies.Comment: 14 pages, 8 figures. Submitted to Ap
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