4,127 research outputs found
Detecting Star Formation in Brightest Cluster Galaxies with GALEX
We present the results of GALEX observations of 17 cool core (CC) clusters of
galaxies. We show that GALEX is easily capable of detecting star formation in
brightest cluster galaxies (BCGs) out to and 50-100 kpc. In most of
the CC clusters studied, we find significant UV luminosity excesses and colors
that strongly suggest recent and/or current star formation. The BCGs are found
to have blue UV colors in the center that become increasingly redder with
radius, indicating that the UV signature of star formation is most easily
detected in the central regions. Our findings show good agreement between UV
star formation rates and estimates based on H observations. IR
observations coupled with our data indicate moderate-to-high dust attenuation.
Comparisons between our UV results and the X-ray properties of our sample
suggest clear correlations between UV excess, cluster entropy, and central
cooling time, confirming that the star formation is directly and
incontrovertibly related to the cooling gas.Comment: 39 pages, 11 figures; accepted for publication in The Astrophysical
Journal. Figure quality reduced to comply with arXiv file size requirement
Constraints on Primordial Nongaussiantiy from the High-Redshift Cluster MS1054--03
The implications of the massive, X-ray selected cluster of galaxies
MS1054--03 at are discussed in light of the hypothesis that the
primordial density fluctuations may be nongaussian. We generalize the
Press-Schechter (PS) formalism to the nongaussian case, and calculate the
likelihood that a cluster as massive as MS1054 would appear in the EMSS. The
probability of finding an MS1054-like cluster depends only on \omegam and the
extent of primordial nongaussianity. We quantify the latter by adopting a
specific functional form for the PDF, denoted which tends to
Gaussianity for and show how is related to the more
familiar statistic the probability of fluctuations for a
given PDF relative to a Gaussian. We find that Gaussian initial density
fluctuations are consistent with the data on MS1054 only if \omegam\simlt
0.2. For \omegam\ge 0.25 a significant degree of nongaussianity is required,
unless the mass of MS1054 has been substantially overestimated by X-ray and
weak lensing data. The required amount of nongaussianity is a rapidly
increasing function of \omegam for 0.25 \le \omegam \le 0.45, with (T \simgt 7) at the upper end of this range. For a fiducial
\omegam=0.3, \omegal=0.7 universe, favored by several lines of evidence we
obtain an upper limit corresponding to a This
finding is consistent with the conclusions of Koyama, Soda, & Taruya (1999),
who applied the generalized PS formalism to low (z\simlt 0.1) and
intermediate (z\simlt 0.6) redshift cluster data sets.Comment: 15 pages, 11 figures, submitted to the Astrophysical Journal, uses
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Automotive Stirling engine development program
The study of high power kinematic Stirling engines for transportation use, testing of Mod I and Mod II Stirling engines, and component development activities are summarized. Mod II development testing was performed to complete the development of the basic engine and begin characterization of performance. Mod I engines were used for Mod II component development and to obtain independent party (U.S. Air Force) evaluation of Stirling engine vehicle performance
Simulation Studies of Nanomagnet-Based Architecture
We report a simulation study on interacting ensembles of Co nanomagnets that
can perform basic logic operations and propagate logic signals, where the state
variable is the magnetization direction. Dipole field coupling between
individual nanomagnets drives the logic functionality of the ensemble and
coordinated arrangements of the nanomagnets allow for the logic signal to
propagate in a predictable way. Problems with the integrity of the logic signal
arising from instabilities in the constituent magnetizations are solved by
introducing a biaxial anisotropy term to the Gibbs magnetic free energy of each
nanomagnet. The enhanced stability allows for more complex components of a
logic architecture capable of random combinatorial logic, including horizontal
wires, vertical wires, junctions, fanout nodes, and a novel universal logic
gate. Our simulations define the focus of scaling trends in nanomagnet-based
logic and provide estimates of the energy dissipation and time per nanomagnet
reversal
Bandpass Dependence of X-ray Temperatures in Galaxy Clusters
We explore the band dependence of the inferred X-ray temperature of the
intracluster medium (ICM) for 192 well-observed galaxy clusters selected from
the Chandra Data Archive. If the hot ICM is nearly isothermal in the projected
region of interest, the X-ray temperature inferred from a broad-band (0.7-7.0
keV) spectrum should be identical to the X-ray temperature inferred from a
hard-band (2.0-7.0 keV) spectrum. However, if unresolved cool lumps of gas are
contributing soft X-ray emission, the temperature of a best-fit
single-component thermal model will be cooler for the broad-band spectrum than
for the hard-band spectrum. Using this difference as a diagnostic, the ratio of
best-fitting hard-band and broad-band temperatures may indicate the presence of
cooler gas even when the X-ray spectrum itself may not have sufficient
signal-to-noise to resolve multiple temperature components. To test this
possible diagnostic, we extract X-ray spectra from core-excised annular regions
for each cluster in our archival sample. We compare the X-ray temperatures
inferred from single-temperature fits when the energy range of the fit is
0.7-7.0 keV (broad) and when the energy range is 2.0/(1+z)-7.0 keV (hard). We
find that the hard-band temperature is significantly higher, on average, than
the broad-band temperature. Upon further exploration, we find this temperature
ratio is enhanced preferentially for clusters which are known merging systems.
In addition, cool-core clusters tend to have best-fit hard-band temperatures
that are in closer agreement with their best-fit broad-band temperatures. We
show, using simulated spectra, that this diagnostic is sensitive to secondary
cool components (TX = 0.5-3.0 keV) with emission measures >10-30% of the
primary hot component.Comment: Accepted for publication in Ap
Two Clusters with Radio-quiet Cooling Cores
Radio lobes inflated by active galactic nuclei at the centers of clusters are
a promising candidate for halting condensation in clusters with short central
cooling times because they are common in such clusters. In order to test the
AGN-heating hypothesis, we obtained Chandra observations of two clusters with
short central cooling times yet no evidence for AGN activity: Abell 1650 and
Abell 2244. The cores of these clusters indeed appear systematically different
from cores with more prominent radio emission. They do not have significant
central temperature gradients, and their central entropy levels are markedly
higher than in clusters with stronger radio emission, corresponding to central
cooling times ~ 1 Gigayear. Also, there is no evidence for fossil X-ray
cavities produced by an earlier episode of AGN heating. We suggest that either
(1) the central gas has not yet cooled to the point at which feedback is
necessary to prevent it from condensing, possibly because it is conductively
stabilized, or (2) the gas experienced a major heating event Gyr in
the past and has not required feedback since then. The fact that these clusters
with no evident feedback have higher central entropy and therefore longer
central cooling times than clusters with obvious AGN feedback strongly suggests
that AGNs supply the feedback necessary to suppress condensation in clusters
with short central cooling times.Comment: ApJ Letter, in pres
Field-induced domain wall propagation velocity in magnetic nanowires
A thory of field-induced domain wall (DW) propagation is developed. The
theory not only explains why a DW in a defect-free nanowire must propagate at a
finite velocity, but also provides a proper definition of DW propagation
velocity. This definition, valid for an arbitrary DW structure, allows one to
compute the instantaneous DW velocity in a meaningful way even when the DW is
not moving as a rigid body. A new velocity-field formula beyond the Walker
breakdown field, which is in excellent agreement with both experiments and
numerical simulations, is derived
A Very Hot, High Redshift Cluster of Galaxies: More Trouble for Omega_0 = 1
We have observed the most distant (z=0.829) cluster of galaxies in the
Einstein Extended Medium Sensitivity Survey, with the ASCA and ROSAT
satellites. We find an X-ray temperature of 12.3 +3.1/-2.2 keV for this
cluster, and the ROSAT map reveals significant substructure. The high
temperature of MS1054-0321 is consistent with both its approximate velocity
dispersion, based on the redshifts of 12 cluster members we have obtained at
the Keck and the Canada-France-Hawaii telescopes, and with its weak lensing
signature. The X-ray temperature of this cluster implies a virial mass ~ 7.4 x
10^14 h^-1 solar masses, if the mean matter density in the universe equals the
critical value, or larger if Omega_0 < 1. Finding such a hot, massive cluster
in the EMSS is extremely improbable if clusters grew from Gaussian
perturbations in an Omega_0 = 1 universe. Combining the assumptions that
Omega_0 = 1 and that the intial perturbations were Gaussian with the observed
X-ray temperature function at low redshift, we show that the probability of
this cluster occurring in the volume sampled by the EMSS is less than a few
times 10^{-5}. Nor is MS1054-0321 the only hot cluster at high redshift; the
only two other EMSS clusters already observed with ASCA also have
temperatures exceeding 8 keV. Assuming again that the initial perturbations
were Gaussian and Omega_0 = 1, we find that each one is improbable at the <
10^{-2} level. These observations, along with the fact that these luminosities
and temperatures of the high- clusters all agree with the low-z L_X-T_X
relation, argue strongly that Omega_0 < 1. Otherwise, the initial perturbations
must be non-Gaussian, if these clusters' temperatures do indeed reflect their
gravitational potentials.Comment: 20 pages, 4 figures, To appear in 1 Aug 1998 ApJ (heavily revised
version of original preprint
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