8,378 research outputs found
Massive and Red Objects predicted by a semianalytical model of galaxy formation
We study whether hierarchical galaxy formation in a concordance CDM
universe can produce enough massive and red galaxies compared to the
observations. We implement a semi-analytical model in which the central black
holes gain their mass during major mergers of galaxies and the energy feedback
from active galaxy nuclei (AGN) suppresses the gas cooling in their host halos.
The energy feedback from AGN acts effectively only in massive galaxies when
supermassive black holes have been formed in the central bulges. Compared with
previous models without black hole formation, our model predicts more massive
and luminous galaxies at high redshift, agreeing with the observations of K20
up to . Also the predicted stellar mass density from massive galaxies
agrees with the observations of GDDS. Because of the energy feedback from AGN,
the formation of new stars is stopped in massive galaxies with the termination
of gas cooling and these galaxies soon become red with color 5 (Vega
magnitude), comparable to the Extremely Red Objects (EROs) observed at redshift
1-2. Still the predicted number density of very EROs is lower than
observed at , and it may be related to inadequate descriptions of dust
extinction, star formation history and AGN feedback in those luminous galaxies.Comment: Accepted for Publication in ApJ, added reference
The First Detailed X-ray Observations of High-Redshift, Optically-Selected Clusters: XMM-Newton Results for Cl 1324+3011 at z = 0.76 and Cl 1604+4304 at z = 0.90
We present the first detailed X-ray observations of optically-selected
clusters at high redshift. Two clusters, Cl 1324+3011 at z = 0.76 and Cl
1604+4304 at z = 0.90, were observed with XMM-Newton. The optical center of
each cluster is coincident with an extended X-ray source whose emission is
detected out to a radius of 0.5 Mpc. The emission from each cluster appears
reasonably circular, with some indication of asymmetries and more complex
morphologies. Similarly to other optically-selected clusters at redshifts of z
> 0.4, both clusters are modest X-ray emitters with bolometric luminosities of
only Lx = 1.4 - 2.0 x 10^(44) erg/s. We measure gas temperatures of T = 2.88
(+0.71/-0.49) keV for Cl 1324+3011 and 2.51 (+1.05/-0.69) keV for Cl 1604+4304.
The X-ray properties of both clusters are consistent with the high-redshift
Lx-T relation measured from X-ray-selected samples at z > 0.5. However, based
on the local relations, their X-ray luminosities and temperatures are low for
their measured velocity dispersions (sigma). The clusters are cooler by a
factor of 2 - 9 compared to the local sigma-T relation. We briefly discuss the
possible explanations for these results.Comment: 14 pages, 4 figures; accepted for publication in Astrophysical
Journal Letters; version with full resolution figures available at
http://bubba.ucdavis.edu/~lubin/xmm.pd
Study of thin film large area photovoltaic solar energy converter Final report
Thin film large area cadmium sulfide solar cell
Study of thin film large area photovoltaic solar energy converter Third quarterly report, 25 Apr. - 24 Jul. 1966
Cadmium sulfide-thin film large area photovoltaic solar energy converter - plastic substrate cell fabrication and stability testing under various conditions of temperature and humidit
Investigation of tidal displacements of the Earth's surface by laser ranging to GEOS-3
An analysis of laser ranging data from three stations was carried out in an attempt to measure the geometric Earth tide. Two different approaches to the problem were investigated. The dynamic method computes pass to pass apparent movements in stations height relative to short arcs fitted to several passes of data from the same station by the program GEODYNE. The quasi-geometric method reduces the dependence on unmodelled satellite dynamics to a knowledge of only the radial position of the satellite by considering two station simultaneous ranging at the precise time that the satellite passes through the plane defined by two stations and the center of mass of the Earth
Colors, magnitudes and velocity dispersions in early-type galaxies: Implications for galaxy ages and metallicities
We present an analysis of the color-magnitude-velocity dispersion relation
for a sample of 39320 early-type galaxies within the Sloan Digital Sky Survey.
We demonstrate that the color-magnitude relation is entirely a consequence of
the fact that both the luminosities and colors of these galaxies are correlated
with stellar velocity dispersions. Previous studies of the color-magnitude
relation over a range of redshifts suggest that the luminosity of an early-type
galaxy is an indicator of its metallicity, whereas residuals in color from the
relation are indicators of the luminosity-weighted age of its stars. We show
that this, when combined with our finding that velocity dispersion plays a
crucial role, has a number of interesting implications. First, galaxies with
large velocity dispersions tend to be older (i.e., they scatter redward of the
color-magnitude relation). Similarly, galaxies with large dynamical mass
estimates also tend to be older. In addition, at fixed luminosity, galaxies
which are smaller, or have larger velocity dispersions, or are more massive,
tend to be older. Second, models in which galaxies with the largest velocity
dispersions are also the most metal poor are difficult to reconcile with our
data. However, at fixed velocity dispersion, galaxies have a range of ages and
metallicities: the older galaxies have smaller metallicities, and vice-versa.
Finally, a plot of velocity dispersion versus luminosity can be used as an age
indicator: lines of constant age run parallel to the correlation between
velocity dispersion and luminosity.Comment: 12 pages, 9 figures. Accepted by A
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