14,783 research outputs found
The Interplay of Cluster and Galaxy Evolution
We review here the interplay of cluster and galaxy evolution. As a case
study, we consider the Butcher-Oemler effect and propose that it is the result
of the changing rate of cluster merger events in a hierarchical universe. This
case study highlights the need for new catalogs of clusters and groups that
possess quantified morphologies. We present such a sample here, namely the
Sloan Digital Sky Survey (SDSS) C4 Catalog, which has been objectively-selected
from the SDSS spectroscopic galaxy sample. We outline here the C4 algorithm and
present first results based on the SDSS Early Data Release, including an X-ray
luminosity-velocity dispersion (L_x-sigma) scaling relationship (as a function
of cluster morphology), and the density-SFR relation of galaxies within C4
clusters (Gomez et al. 2003). We also discuss the merger of Coma and the
NGC4839 group, and its effect on the galaxy populations in these systems. We
finish with a brief discussion of a new sample of Hdelta-selected galaxies
(i.e., k+a, post--starburst galaxies) obtained from the SDSS spectroscopic
survey.Comment: Invited review at the JENAM 2002 Workshop on "Galaxy Evolution in
Groups and Clusters", Porto, Sep 5-7 2002, eds. Lobo, Serote-Roos and
Biviano, Kluwer in pres
Polarized Parton Densities
In this talk we summarize main results of a recent determination of the
polarized deeply inelastic parton distributions to NLO from the world data. In
the analysis the LO and NLO parton densities and their statistical
errors were derived and parameterized. The strong coupling constant
is determined Comparisons of the
low moments of the parton densities with recent lattice results are given. A
detailed error-analysis of the gluon density is performed.Comment: 3 pages LATEX, 1 style file, 1 eps file, to appear in the Proceedings
of PANIC '02, Osaka, Ocrober 200
Numerical Regularization of Electromagnetic Quantum Fluctuations in Inhomogeneous Dielectric Media
Electromagnetic Casimir stresses are of relevance to many technologies based
on mesoscopic devices such as MEMS embedded in dielectric media, Casimir
induced friction in nano-machinery, micro-fluidics and molecular electronics.
Computation of such stresses based on cavity QED generally require numerical
analysis based on a regularization process. A new scheme is described that has
the potential for wide applicability to systems involving realistic
inhomogeneous media. From a knowledge of the spectrum of the stationary modes
of the electromagnetic field the scheme is illustrated by estimating
numerically the Casimir stress on opposite faces of a pair of perfectly
conducting planes separated by a vacuum and the change in this result when the
region between the plates is filled with an incompressible inhomogeneous
non-dispersive dielectric.Comment: 5 pages, 2 figures, submitted to PR
The buckling of thin-walled circular cylinders under axial compression and bending
Bucking of thin-walled electroplated copper and Mylar circular cylinders under axial compression and bendin
Selection and photometric properties of K+A galaxies
Two different simple measurements of galaxy star formation rate with
different timescales are compared empirically on fiber spectra of
galaxies with mag taken from the Sloan Digital Sky Survey in the
redshift range : a ratio \Aamp / \Kamp found by fitting a linear
sum of an average old stellar poplulation spectrum (\Kamp) and average A-star
spectrum (\Aamp) to the galaxy spectrum, and the equivalent width (EW) of the
\Halpha emission line. The two measures are strongly correlated, but there is
a small clearly separated population of outliers from the median correlation
that display excess \Aamp /\Kamp relative to \Halpha EW. These ``K+A'' (or
``E+A'') galaxies must have dramatically decreased their star-formation rates
over the last Gyr. The K+A luminosity distribution is very similar to
that of the total galaxy population. The K+A population appears to be
bulge-dominated, but bluer and higher surface-brightness than normal
bulge-dominated galaxies; it appears that K+A galaxies will fade with time into
normal bulge-dominated galaxies. The inferred rate density for K+A galaxy
formation is at redshift .
These events are taking place in the field; K+A galaxies don't primarily lie in
the high-density environments or clusters typical of bulge-dominated
populations.Comment: submitted to Ap
Chemical Analysis of a Diffuse Cloud along a Line of Sight Toward W51: Molecular Fraction and Cosmic-Ray Ionization Rate
Absorption lines from the molecules OH+, H2O+, and H3+ have been observed in
a diffuse molecular cloud along a line of sight near W51 IRS2. We present the
first chemical analysis that combines the information provided by all three of
these species. Together, OH+ and H2O+ are used to determine the molecular
hydrogen fraction in the outskirts of the observed cloud, as well as the
cosmic-ray ionization rate of atomic hydrogen. H3+ is used to infer the
cosmic-ray ionization rate of H2 in the molecular interior of the cloud, which
we find to be zeta_2=(4.8+-3.4)x10^-16 per second. Combining the results from
all three species we find an efficiency factor---defined as the ratio of the
formation rate of OH+ to the cosmic-ray ionization rate of H---of
epsilon=0.07+-0.04, much lower than predicted by chemical models. This is an
important step in the future use of OH+ and H2O+ on their own as tracers of the
cosmic-ray ionization rate.Comment: 21 pages, 1 figure, 4 table
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