8,304 research outputs found
Optimization and performance of Space Station Freedom solar cells
High efficiency, large area and low cost solar cells are the drivers for Space Station solar array designs. The manufacturing throughput, process complexity, yield of the cells, and array manufacturing technique determine the economics of the solar array design. The cell efficiency optimization of large area (8 x 8 m), dielectric wrapthrough contact solar cells are described. The results of the optimization are reported and the solar cell performance of limited production runs is reported
Gallium arsenide 55Fe X-ray-photovoltaic battery
The effects of temperature on the key parameters of a prototype GaAs 55Fe radioisotope X-ray microbattery were studied over the temperature range -20 °C to 70 °C. A p-i-n GaAs structure was used to collect the photons from a 254 Bq 55Fe radioisotope X-ray source. Experimental results showed that the open circuit voltage and the short circuit current decreased with increased temperature. The maximum output power and the conversion efficiency of the device decreased at higher temperatures. For the reported microbattery, the highest maximum output power (1 pW, corresponding to 0.4 μW/Ci) was observed at -20 °C. A conversion efficiency of 9% was measured at -20 °C
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
The millimetre variability of M81* -- Multi-epoch dual frequency mm-observations of the nucleus of M81
There are still many open questions as to the physical mechanisms at work in
Low Luminosity AGN that accrete in the extreme sub-Eddington regime.
Simultaneous multi-wavelength studies have been very successful in constraining
the properties of SgrA*, the extremely sub-Eddington black hole at the centre
of our Milky Way. M81*, the nucleus of the nearby spiral galaxy M81, is an
ideal source to extend the insights obtained on SgrA* toward higher luminosity
AGN. Here we present observations at 3 and 1 mm that were obtained within the
framework of a coordinated,multi-wavelength campaign on M81*. The continuum
emission from M81* was observed during three epochs with the IRAM Plateau de
Bure Interferometer simultaneously at wavelengths of 3 and 1 mm. We present the
first flux measurements of M81* at wavelengths around 1 mm. We find that M81*
is a continuously variable source with the higher variability observed at the
shorter wavelength. Also, the variability at 3 and 1 mm appears to be
correlated. Like SgrA*, M81* appears to display the strongest flux density and
variability in the mm-to-submm regime. There remains still some ambiguity
concerning the exact location of the turnover frequency from optically thick to
optically thin emission. The observed variability time scales point to an upper
size limit of the emitting region of the order 25 Schwarzschild radii. The data
show that M81* is indeed a system with very similar physical properties to
SgrA* and an ideal bridge toward high luminosity AGN. The data obtained clearly
demonstrate the usefulness and, above all, the necessity of simultaneous
multi-wavelength observations of LLAGN.Comment: accepted for publication in A&
The Mass Assembly History of Spheroidal Galaxies: Did Newly-Formed Systems Arise Via Major Mergers?
We examine the properties of a morphologically-selected sample of 0.4<z<1.0
spheroidal galaxies in the GOODS fields in order to ascertain whether their
increase in abundance with time arises primarily from mergers. To address this
question we determine scaling relations between the dynamical mass determined
from stellar velocity dispersions, and the stellar mass determined from optical
and infrared photometry. We exploit these relations across the larger sample
for which we have stellar masses in order to construct the first statistically
robust estimate of the evolving dynamical mass function over 0<z<1. The trends
observed match those seen in the stellar mass functions of Bundy et al. 2005
regarding the top-down growth in the abundance of spheroidal galaxies. By
referencing our dynamical masses to the halo virial mass we compare the growth
rate in the abundance of spheroidals to that predicted by the assembly of dark
matter halos. Our comparisons demonstrate that major mergers do not fully
account for the appearance of new spheroidals since z~1 and that additional
mechanisms, such as morphological transformations, are required to drive the
observed evolution.Comment: Accepted to ApJL; New version corrects the Millennium merger
predictions--further details at
http://www.astro.utoronto.ca/~bundy/millennium
B3 0003+387: AGN Marked Large-Scale Structure at z=1.47?
We present evidence for a significant overdensity of red galaxies, as much as
a factor of 14 over comparable field samples, in the field of the z=1.47 radio
galaxy B3 0003+387. The colors and luminosities of the brightest red galaxies
are consistent with their being at z>0.8. The radio galaxy and one of the red
galaxies are separated by 5" and show some evidence of a possible interaction.
However, the red galaxies do not show any strong clustering around the radio
galaxy nor around any of the brighter red galaxies. The data suggest that we
are looking at a wall or sheet of galaxies, possibly associated with the radio
galaxy at z=1.47. Spectroscopic redshifts of these red galaxies will be
necessary to confirm this large-scale structure.Comment: 19 pages, 7 figures, LaTeX2e/AASTeX v5.0.2. The full photometric
catalog is included as a separate deluxetable file. To appear in the
Astronomical Journal (~Nov 00
Age, Metallicity and Star Formation History of Cluster Galaxies at z~0.3 F
We investigate the color-magnitude distribution in the rich cluster AC 118 at
z=0.31. The sample is selected by the photometric redshift technique, allowing
to study a wide range of properties of stellar populations, and is complete in
the K-band, allowing to study these properties up to a given galaxy mass. We
use galaxy templates based on population synthesis models to translate the
physical properties of the stellar populations - formation epoch, time-scale of
star formation, and metallicity - into observed magnitudes and colors. In this
way we show that a sharp luminosity-metallicity relation is inferred without
any assumption on the galaxy formation scenario (either monolithic or
hierarchical). Our data exclude significant differences in star formation
histories along the color-magnitude relation, and therefore confirm a pure
metallicity interpretation for its origin, with an early (z~5) formation epoch
for the bulk of stellar populations. The dispersion in the color-magnitude
diagram implies that fainter galaxies in our sample (K~18) ceased to form stars
as late as z~0.5, in agreement with the picture that these galaxies were
recently accreted into the cluster environment. The trend with redshift of the
total stellar mass shows that half of the luminous mass in AC 118 was already
formed at $z~2, but also that 20% of the stars formed at z<1.Comment: 16 pages, 10 figures. ApJ in pres
Signatures of Interstellar-Intracluster Medium Interactions: Spiral Galaxy Rotation Curves in Abell 2029
We investigate the rich cluster Abell 2029 (z~0.08) using optical imaging and
long-slit spectral observations of 52 disk galaxies distributed throughout the
cluster field. No strong emission-line galaxies are present within ~400 kpc of
the cluster center, a region largely dominated by the similarly-shaped X-ray
and low surface brightness optical envelopes centered on the giant cD galaxy.
However, two-thirds of the galaxies observed outside the cluster core exhibit
line emission. H-alpha rotation curves of 14 cluster members are used in
conjunction with a deep I band image to study the environmental dependence of
the Tully-Fisher relation. The Tully-Fisher zero-point of Abell 2029 matches
that of clusters at lower redshifts, although we do observe a relatively larger
scatter about the Tully-Fisher relation. We do not observe any systematic
variation in the data with projected distance to the cluster center: we see no
environmental dependence of Tully-Fisher residuals, R-I color, H-alpha
equivalent width, and the shape and extent of the rotation curves.Comment: 22 pages, 6 figures, 3 tables; to appear in the August 2000
Astronomical Journa
Approaching the event horizon: 1.3mm VLBI of SgrA*
Advances in VLBI instrumentation now allow wideband recording that
significantly increases the sensitivity of short wavelength VLBI observations.
Observations of the super-massive black hole candidate at the center of the
Milky Way, SgrA*, with short wavelength VLBI reduces the scattering effects of
the intervening interstellar medium, allowing observations with angular
resolution comparable to the apparent size of the event horizon of the putative
black hole. Observations in April 2007 at a wavelength of 1.3mm on a three
station VLBI array have now confirmed structure in SgrA* on scales of just a
few Schwarzschild radii. When modeled as a circular Gaussian, the fitted
diameter of SgrA* is 37 micro arcsec (+16,-10; 3-sigma), which is smaller than
the expected apparent size of the event horizon of the Galactic Center black
hole. These observations demonstrate that mm/sub-mm VLBI is poised to open a
new window onto the study of black hole physics via high angular resolution
observations of the Galactic Center.Comment: 6 pages, 4 figures, Proceedings for "The Universe under the
Microscope" (AHAR 2008), held in Bad Honnef (Germany) in April 2008, to be
published in Journal of Physics: Conference Series by Institute of Physics
Publishing, R. Schoedel, A. Eckart, S. Pfalzner, and E. Ros (eds.
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