13,601 research outputs found
The Spatial and Kinematic Distributions of Cluster Galaxies in a LCDM Universe -- Comparison with Observations
We combine dissipationless N-body simulations and semi-analytic models of
galaxy formation to study the spatial and kinematic distributions of cluster
galaxies in a LCDM cosmology. We investigate how the star formation rates,
colours and morphologies of galaxies vary as a function of distance from the
cluster centre and compare our results with the CNOC1 survey of galaxies from
15 X-ray luminous clusters in the redshift range 0.18 to 0.55. In our model,
gas no longer cools onto galaxies after they fall into the cluster and their
star formation rates decline on timescales of 1-2 Gyr. Galaxies in cluster
cores have lower star formation rates and redder colours than galaxies in the
outer regions because they were accreted earlier. Our colour and star formation
gradients agree with those those derived from the data. The difference in
velocity dispersions between red and blue galaxies observed in the CNOC1
clusters is also well reproduced by the model. We assume that the morphologies
of cluster galaxies are determined solely by their merging histories.
Morphology gradients in clusters arise naturally, with the fraction of bulge-
dominated galaxies highest in cluster cores. We compare these gradients with
the CNOC1 data and find excellent agreement for bulge-dominated galaxies. The
simulated clusters contain too few galaxies of intermediate bulge-to-disk
ratio, suggesting that additional processes may influence the morphological
evolution of disk-dominated galaxies in clusters. Although the properties of
the cluster galaxies in our model agree extremely well with the data, the same
is not true of field galaxies. Both the star formation rates and the colours of
bright field galaxies appear to evolve much more strongly from redshift 0.2 to
0.4 in the CNOC1 field sample than in our simulations.Comment: 17 pages, sumitted to MNRAS. Simulation outputs, halo catalogs,
merger trees and galaxy catalogs are now available at
http://www.mpa-garching.mpg.de/GIF
The Savannah River Site: Site Description, Land Use and Management History
The 78,000-ha Savannah River Site, which is located in the upper Coastal Plain of South Carolina along the Savannah River, was established as a nuclear production facility in 1951 by the Atomic Energy Commission. The site’s physical and vegetative characteristics, land use history, and the impacts of management and operations are described. Aboriginal and early European settlement was primarily along streams, where much of the farming and timber cutting have occurred. Woodland grazing occurred in the uplands and lowlands. Land use intensity increased after the Civil War and peaked in the 1920s. Impacts from production of cotton and corn, naval stores, fuelwood, and timber left only scattered patches of relatively untouched land and, coupled with grazing and less-frequent fire, severely reduced the extent of longleaf pine (Pinus palustrus) ecosystems. After 1951, the USDA Forest Service, under the direction of the Atomic Energy Commission, initiated a large-scale reforestation effort and continued to manage the site’s forests. Over the last decade, forest management efforts have shifted to recovering the Red-cockaded Woodpecker (Picoides borealis) and restoring longleaf pine habitat. A research set-aside program was established in the 1950s and is now administered by the Savannah River Ecology Laboratory. Impacts from thermal effluents, fly-ash runoff, construction of radioactive waste facilities, and release of low-level radionuclides and certain metals have been assessed by the Savannah River Ecology Laboratory and other researchers
The Boundary Cosmological Constant in Stable 2D Quantum Gravity
We study further the r\^ole of the boundary operator \O_B for macroscopic
loop length in the stable definition of 2D quantum gravity provided by the
formulation. The KdV flows are supplemented by an additional
flow with respect to the boundary cosmological constant . We
numerically study these flows for the , and models, solving for
the string susceptibility in the presence of \O_B for arbitrary coupling
. The spectrum of the Hamiltonian of the loop quantum mechanics is
continuous and bounded from below by . For large positive , the
theory is dominated by the `universal' topological phase present only in
the formulation. For large negative , the
non--perturbative physics approaches that of the definition, although
there is no path to the unstable solutions of the -even models.Comment: (Plain Tex, 11pp, 4 figures available on request) SHEP 91/92-2
The FIRST-2MASS Red Quasar Survey
Combining radio observations with optical and infrared color selection --
demonstrated in our pilot study to be an efficient selection algorithm for
finding red quasars -- we have obtained optical and infrared spectroscopy for
120 objects in a complete sample of 156 candidates from a sky area of 2716
square degrees. Consistent with our initial results, we find our selection
criteria -- J-K>1.7, R-K>4.0 -- yield a ~50% success rate for discovering
quasars substantially redder than those found in optical surveys. Comparison
with UVX- and optical color-selected samples shows that >~ 10% of the quasars
are missed in a magnitude-limited survey. Simultaneous two-frequency radio
observations for part of the sample indicate that a synchrotron continuum
component is ruled out as a significant contributor to reddening the quasars'
spectra. We go on to estimate extinctions for our objects assuming their red
colors are caused by dust. Continuum fits and Balmer decrements suggest E(B-V)
values ranging from near zero to 2.5 magnitudes. Correcting the K-band
magnitudes for these extinctions, we find that for K <= 14.0, red quasars make
up between 25% and 60% of the underlying quasar population; owing to the
incompleteness of the 2MASS survey at fainter K-band magnitudes, we can only
set a lower limit to the radio-detected red quasar population of >20-30%.Comment: 80 pages (single-column, preprint format) 20 figures, Accepted for
publicated in Ap
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