1,989 research outputs found
The History of Galaxy Formation in Groups: An Observational Perspective
We present a pedagogical review on the formation and evolution of galaxies in
groups, utilizing observational information from the Local Group to galaxies at
z~6. The majority of galaxies in the nearby universe are found in groups, and
galaxies at all redshifts up to z~6 tend to cluster on the scale of nearby
groups (~1 Mpc). This suggests that the group environment may play a role in
the formation of most galaxies. The Local Group, and other nearby groups,
display a diversity in star formation and morphological properties that puts
limits on how, and when, galaxies in groups formed. Effects that depend on an
intragroup medium, such as ram-pressure and strangulation, are likely not major
mechanisms driving group galaxy evolution. Simple dynamical friction arguments
however show that galaxy mergers should be common, and a dominant process for
driving evolution. While mergers between L_* galaxies are observed to be rare
at z < 1, they are much more common at earlier times. This is due to the
increased density of the universe, and to the fact that high mass galaxies are
highly clustered on the scale of groups. We furthermore discus why the local
number density environment of galaxies strongly correlates with galaxy
properties, and why the group environment may be the preferred method for
establishing the relationship between properties of galaxies and their local
density.Comment: Invited review, 16 pages, to be published in ESO Astrophysics
Symposia: "Groups of Galaxies in the Nearby Universe", eds. I. Saviane, V.
Ivanov, J. Borissov
Optimal Sizes of Dielectric Microspheres for Cavity QED with Strong Coupling
The whispering gallery modes (WGMs) of quartz microspheres are investigated
for the purpose of strong coupling between single photons and atoms in cavity
quantum electrodynamics (cavity QED). Within our current understanding of the
loss mechanisms of the WGMs, the saturation photon number, n, and critical atom
number, N, cannot be minimized simultaneously, so that an "optimal" sphere size
is taken to be the radius for which the geometric mean, (n x N)^(1/2), is
minimized. While a general treatment is given for the dimensionless parameters
used to characterize the atom-cavity system, detailed consideration is given to
the D2 transition in atomic Cesium (852nm) using fused-silica microspheres, for
which the maximum coupling coefficient g/(2*pi)=750MHz occurs for a sphere
radius a=3.63microns corresponding to the minimum for n=6.06x10^(-6). By
contrast, the minimum for N=9.00x10^(-6) occurs for a sphere radius of
a=8.12microns, while the optimal sphere size for which (n x N)^(1/2) is
minimized occurs at a=7.83microns. On an experimental front, we have fabricated
fused-silica microspheres with radii a=10microns and consistently observed
quality factors Q=0.8x10^(7). These results for the WGMs are compared with
corresponding parameters achieved in Fabry-Perot cavities to demonstrate the
significant potential of microspheres as a tool for cavity QED with strong
coupling.Comment: 12 pages, 14 figure
Real space renormalization group approach to the 2d antiferromagnetic Heisenberg model
The low energy behaviour of the 2d antiferromagnetic Heisenberg model is
studied in the sector with total spins by means of a renormalization
group procedure, which generates a recursion formula for the interaction matrix
of 4 neighbouring " clusters" of size ,
from the corresponding quantities . Conservation
of total spin is implemented explicitly and plays an important role. It is
shown, how the ground state energies , approach each
other for increasing , i.e. system size. The most relevant couplings in the
interaction matrices are generated by the transitions
between the ground states
() on an -cluster of size , mediated
by the staggered spin operator Comment: 18 pages, 8 figures, RevTe
Conformations of closed DNA
We examine the conformations of a model for a short segment of closed DNA.
The molecule is represented as a cylindrically symmetric elastic rod with a
constraint corresponding to a specification of the linking number. We obtain
analytic expressions leading to the spatial configuration of a family of
solutions representing distortions that interpolate between the circular form
of DNA and a figure-eight form that represents the onset of interwinding. We
are also able to generate knotted loops. We suggest ways to use our approach to
produce other configurations relevant to studies of DNA structure. The
stability of the distorted configurations is assessed, along with the effects
of fluctuations on the free energy of the various configurations.Comment: 39 pages in REVTEX with 14 eps figures. Submitted to Phys. Rev. E.
This manuscript updates, expands and revises, to a considerable extent, a
previously posted manuscript, entitled "Conformations of Circular DNA," which
appeared as cond-mat/970104
On Relativistic Material Reference Systems
This work closes certain gaps in the literature on material reference systems
in general relativity. It is shown that perfect fluids are a special case of
DeWitt's relativistic elastic media and that the velocity--potential formalism
for perfect fluids can be interpreted as describing a perfect fluid coupled to
a fleet of clocks. A Hamiltonian analysis of the elastic media with clocks is
carried out and the constraints that arise when the system is coupled to
gravity are studied. When the Hamiltonian constraint is resolved with respect
to the clock momentum, the resulting true Hamiltonian is found to be a
functional only of the gravitational variables. The true Hamiltonian is
explicitly displayed when the medium is dust, and is shown to depend on the
detailed construction of the clocks.Comment: 18 pages, ReVTe
Magnetic field diagnostics and spatio-temporal variability of the solar transition region
Magnetic field diagnostics of the transition region from the chromosphere to
the corona faces us with the problem that one has to apply extreme UV
spectro-polarimetry. While for coronal diagnostic techniques already exist
through infrared coronagraphy above the limb and radio observations on the
disk, for the transition region one has to investigate extreme UV observations.
However, so far the success of such observations has been limited, but there
are various projects to get spectro-polarimetric data in the extreme UV in the
near future. Therefore it is timely to study the polarimetric signals we can
expect for such observations through realistic forward modeling.
We employ a 3D MHD forward model of the solar corona and synthesize the
Stokes I and Stokes V profiles of C IV 1548 A. A signal well above 0.001 in
Stokes V can be expected, even when integrating for several minutes in order to
reach the required signal-to-noise ratio, despite the fact that the intensity
in the model is rapidly changing (just as in observations). Often this
variability of the intensity is used as an argument against transition region
magnetic diagnostics which requires exposure times of minutes. However, the
magnetic field is evolving much slower than the intensity, and thus when
integrating in time the degree of (circular) polarization remains rather
constant. Our study shows the feasibility to measure the transition region
magnetic field, if a polarimetric accuracy on the order of 0.001 can be
reached, which we can expect from planned instrumentation.Comment: Accepted for publication in Solar Physics (4.Mar.2013), 19 pages, 9
figure
3D FEM analysis of pounding response of bridge structures at a canyon site to spatially varying ground motions
Previous studies of pounding responses of adjacent bridge structures under seismic excitation were usually based on the simplified lumped mass model or beamcolumn element model. Consequently, only 1D point to point pounding, which is usually in the longitudinal direction of the bridge, could be considered. In reality, pounding could occur along the entire surfaces of the adjacent bridge structures. Moreover, spatially varying transverse ground motions generate torsional responses of bridge decks and these responses may cause eccentric poundings. That is why many pounding damages occurred at corners of the adjacent decks as observed in almost all previous major earthquakes. A simplified 1D model cannot capture torsional response and eccentric poundings. To more realistically investigate pounding between adjacent bridge structures, a two-span simply-supported bridge structure located at a canyon site is established with a detailed 3D finite element model in the present study. Spatially varying ground motions in the longitudinal, transverse and vertical directions at the bridge supports are stochastically simulated as inputs in the analysis. The pounding responses of the bridge structure under multi-component spatially varying ground motions are investigated in detail by using the finite element code LS-DYNA. Numerical results show that the detailed 3D finite element model clearly captures the eccentric poundings of bridge decks, which may induce local damage around the corners of bridge decks. It demonstrates the necessity of detailed 3D modelling for a more realistic simulation of pounding responses of adjacent bridge decks to earthquake excitations
London equation studies of thin-film superconductors with a triangular antidot lattice
We report on a study of vortex pinning in nanoscale antidot defect arrays in
the context of the London Theory. Using a wire network model, we discretize the
array with a fine mesh, thereby providing a detailed treatment of pinning
phenomena. The use of a fine grid has enabled us to examine both circular and
elongated defects, patterned in the form of a rhombus. The latter display
pinning characteristics superior to circular defects constructed with the
similar area. We calculate pinning potentials for defects containing zero and
single quanta, and we obtain a pinning phase diagram for the second matching
field, .Comment: 10 pages and 14 figure
Emergency department visits for acute gastrointestinal illness after a major water pipe break in 2010
Background: When a water pipe breaks, contaminants can enter the drinking water system and cause waterborne illnesses such as acute gastrointestinal illness. In May 2010, a major water pipe broke near Boston, MA, and a boil water order was issued to nearly two million residents. Methods: Using a case-crossover study design, we examined the association between the water pipe break and subsequent emergency department visits for acute gastrointestinal illness. We identified cases of illness according to ICD-9-CM diagnosis codes and selected control dates 2 weeks before and after each case. We estimated the risk of visiting the emergency department during the 0-3 and 4-7 days after the water pipe break using conditional logistic regression models. Results: Our analysis included 5,726 emergency department visits for acute gastrointestinal illness from 3 April 2010 to 5 June 2010. Overall, there was a 1.3-fold increased odds for visiting the emergency department for acute gastrointestinal illness during the 0-3 days after the water pipe break (odds ratio [OR] = 1.3; 95% confidence interval [CI] = 1.1, 1.4) compared with referent dates selected 2 weeks before and after. During the 4-7 days after the break, the association diminished overall (OR = 1.1; 95% CI = 0.96, 1.2). However, in communities over 12 miles from the break, the 4- to 7-day association was elevated (OR = 1.4; 95% CI = 1.1, 1.8). Conclusions: This study suggests that a major water pipe break was associated with emergency department visits for acute gastrointestinal illness, particularly during the 0-3 days after the break, when a boil water order was in effect
Can We Improve the Preprocessing of Photospheric Vector Magnetograms by the Inclusion of Chromospheric Observations?
The solar magnetic field is key to understanding the physical processes in
the solar atmosphere. Nonlinear force-free codes have been shown to be useful
in extrapolating the coronal field upward from underlying vector boundary data.
However, we can only measure the magnetic field vector routinely with high
accuracy in the photosphere, and unfortunately these data do not fulfill the
force-free condition. We must therefore apply some transformations to these
data before nonlinear force-free extrapolation codes can be self-consistently
applied. To this end, we have developed a minimization procedure that yields a
more chromosphere-like field, using the measured photospheric field vectors as
input. The procedure includes force-free consistency integrals, spatial
smoothing, and -- newly included in the version presented here -- an improved
match to the field direction as inferred from fibrils as can be observed in,
e.g., chromospheric H images. We test the procedure using a model
active-region field that included buoyancy forces at the photospheric level.
The proposed preprocessing method allows us to approximate the chromospheric
vector field to within a few degrees and the free energy in the coronal field
to within one percent.Comment: 22 pages, 6 Figur
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