7,041 research outputs found
Bar pattern speed evolution over the last 7 Gyr
The tumbling pattern of a bar is the main parameter characterising its
dynamics. From numerical simulations, its evolution since bar formation is
tightly linked to the dark halo in which the bar is formed through dynamical
friction and angular momentum exchange. Observational measurements of the bar
pattern speed with redshift can restrict models of galaxy formation and bar
evolution. We aim to determine, for the first time, the bar pattern speed
evolution with redshift based on morphological measurements. We have selected a
sample of 44 low inclination ringed galaxies from the SDSS and COSMOS surveys
covering the redshift range 0 <z< 0.8 to investigate the evolution of the bar
pattern speed. We have derived morphological ratios between the deprojected
outer ring radius (R_{ring}) and the bar size (R_{bar}). This quantity is
related to the parameter {\cal R}=R_{CR}/R_{bar} used for classifiying bars in
slow and fast rotators, and allow us to investigate possible differences with
redshift. We obtain a similar distribution of at all redshifts. We do not
find any systematic effect that could be forcing this result. The results
obtained here are compatible with both, the bulk of the bar population (~70%)
being fast-rotators and no evolution of the pattern speed with redshift. We
argue that if bars are long-lasting structures, the results presented here
imply that there has not been a substantial angular momentum exchange between
the bar and halo, as predicted by numerical simulations. In consequence, this
might imply that the discs of these high surface-brightness galaxies are
maximal.Comment: Accepted for publication in A&
The Design of Random Surfaces with Specified Scattering Properties: Surfaces that Suppress Leakage
We present a method for generating a one-dimensional random metal surface of
finite length L that suppresses leakage, i.e. the roughness-induced conversion
of a surface plasmon polariton propagating on it into volume electromagnetic
waves in the vacuum above the surface. Perturbative and numerical simulation
calculations carried out for surfaces generated in this way show that they
indeed suppress leakage.Comment: Revtex 6 pages (including 4 figures
Evaluation of performance impairment by spacecraft contaminants
The environmental contaminants (isolated as off-gases in Skylab and Apollo missions) were evaluated. Specifically, six contaminants were evaluated for their effects on the behavior of juvenile baboons. The concentrations of contaminants were determined through preliminary range-finding studies with laboratory rats. The contaminants evaluated were acetone, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), trichloroethylene (TCE), heptane and Freon 21. When the studies of the individual gases were completed, the baboons were also exposed to a mixture of MEK and TCE. The data obtained revealed alterations in the behavior of baboons exposed to relatively low levels of the contaminants. These findings were presented at the First International Symposium on Voluntary Inhalation of Industrial Solvents in Mexico City, June 21-24, 1976. A preprint of the proceedings is included
An Archaeological Assessment for the San Antonio Municipal Stadium
The Center for Archaeological Research, The University of Texas at San Antonio was contracted by the Parks and Recreation Department, City of San Antonio to conduct archival and survey testing on 43 acres of land in western San Antonio at Leon Creek prior to establishing a municipal baseball stadium. The past history of the site was investigated and a survey and shovel testing was conducted on July 8-9, 1993. No indications of archaeological resources were discovered, therefore, no further investigations were recommended
Infrared Emission from Clusters in the Starforming Disk of He2-10
We have made subarcsecond-resolution images of the central 10" of the
Wolf-Rayet dwarf galaxy He 2-10 at 11.7 microns, using the Long Wavelength
Spectrometer on the Keck Telescope. The spatial distribution of the infrared
emission roughly agrees with that of the rising spectrum radio sources seen by
Kobulnicky & Johnson (1999) and confirms that those sources are compact HII
regions rather than SNR or other objects. The infrared sources are more
extended than the subarcsecond rising spectrum radio sources, although the
entire complex is still less than 5" in extent. On sizescales of 1" the
infrared and radio emission are in excellent agreement, with each source
requiring several hundred to a thousand O stars for excitation. The nebulae lie
in a flattened disk-like distribution about 240 by 100 pc and provide all of
the flux measured by IRAS for the entire galaxy in the 12 micron band; 30% of
the total IRAS flux from the galaxy emanates from one 15-30 pc source. In this
galaxy, intense star formation, probably triggered by an accretion event, is
confined to a central disk which breaks up into distinct nebulae which
presumably mark the sites of young super star clusters.Comment: Accepted for Publication in the Astronomical Journa
A Lloyd-model generalization: Conductance fluctuations in one-dimensional disordered systems
We perform a detailed numerical study of the conductance through
one-dimensional (1D) tight-binding wires with on-site disorder. The random
configurations of the on-site energies of the tight-binding
Hamiltonian are characterized by long-tailed distributions: For large
, with . Our
model serves as a generalization of 1D Lloyd's model, which corresponds to
. First, we verify that the ensemble average is proportional to the length of the wire for all values of
, providing the localization length from . Then, we show that the probability distribution
function is fully determined by the exponent and
. In contrast to 1D wires with standard
white-noise disorder, our wire model exhibits bimodal distributions of the
conductance with peaks at and . In addition, we show that
is proportional to , for , with , in
agreement to previous studies.Comment: 5 pages, 5 figure
- …