6,394 research outputs found
IMPROVING PROJECT MANAGEMENT FOR SUSTAINABLE DEVELOPMENT
Institutional and Behavioral Economics,
X-ray Properties of the Abell 644 Cluster of Galaxies
We use new ASCA observations and archival ROSAT Position Sensitive
Proportional Counter (PSPC) data to determine the X-ray spectral properties of
the intracluster gas in Abell 644. From the overall spectrum, we determine the
average gas temperature to be 8.64 (+0.67,-0.56) keV, and an abundance of 0.32
(+/-0.04) . The global ASCA and ROSAT spectra imply a cooling rate
of 214 (+100,-91) yr. The PSPC X-ray surface brightness
profile and the ASCA data suggest a somewhat higher cooling rate. We determine
the gravitational mass and gas mass as a function of radius. The total
gravitating mass within 1.2 Mpc is , of which 20%
is in the form of hot gas. There is a region of elevated temperature 1.5-5
arcmin to the west of the cluster center. The south-southwest region of the
cluster also shows excess emission in the ROSAT PSPC X-ray image, aligned with
the major axis of the optical cD galaxy in the center of the cluster. We argue
that the cluster is undergoing or has recently undergone a minor merger. The
combination of a fairly strong cooling flow and evidence for a merger make this
cluster an interesting case to test the disruption of cooling flow in mergers.Comment: 26 pages LaTeX including 9 eps figures + 4 pages LaTeX tables
(landscape); accepted to ApJ, uses aaspp
Recommended from our members
Texas Shoreline Change Project Gulf of Mexico Shoreline Change from the Brazos River to Pass Cavallo
In June, Governor Bush signed into law the Coastal Erosion Planning and Response Act (CEPRA). This act provides $15 million over the next 2 years for coastal erosion projects. It authorizes the Texas General Land Office (GLO) to implement a comprehensive coastal erosion response program that can include designing, funding, building, and maintaining erosion projects. The GLO is named in the act as the entity that will monitor shoreline change rates with the assistance of the Bureau of Economic Geology and local governments. Through the Texas Shoreline Change Project (TSCP), the Bureau is working with the GLO to identify and quantify eroding areas. The TSCP is addressing requirements of the CEPRA regarding (1) identification of "critical coastal erosion areas," (2) monitoring of historical shoreline erosion rates, (3) making data accessible on the Internet, and (4) increasing public awareness of coastal erosion issues. The TSCP will be completed in stages according to Gulf of Mexico shoreline segment and bay systems. This report presents an analysis of shoreline change along the Gulf of Mexico Shoreline between the Brazos River and Pass Cavallo (Fig. 1). Other resources provided by the TSCP, including all data used in this report, may be found on the Internet at http://www.beg.utexas.edu/Bureau of Economic Geolog
A Two-Fluid Thermally-Stable Cooling Flow Model
A new model for cooling flows in X-ray clusters, capable of naturally
explaining salient features observed, is proposed. The only requirement is that
a significant relativistic component, in the form of cosmic rays (CR), be
present in the intra-cluster medium and significantly frozen to the thermal
gas. Such an addition qualitatively alters the conventional isobaric thermal
instability criterion such that a fluid parcel becomes thermally stable when
its thermal pressure drops below a threshold fraction of its CR pressure.
Consequently, the lowest possible temperature at any radius is about one third
of the ambient temperature {\it at that radius}, exactly as observed, In
addition, we suggest that dissipation of internal gravity waves, excited by
radial oscillatory motions of inward drifting cooling clouds about their radial
equilibrium positions, may be responsible for heating up cooling gas. With the
ultimate energy source for powering the cooling X-ray luminosity and heating up
cooling gas being gravitational due to inward drifting cooling clouds as well
as the general inward flow, heating is spatially distributed and energetically
matched with cooling. One desirable property of this heating mechanism is that
heating energy is strongly centrally concentrated, providing the required
heating for emission-line nebulae.Comment: 13 pages, submitted to ApJ
A numerical method for detecting incommensurate correlations in the Heisenberg zigzag ladder
We study two Heisenberg spin-1/2 chains coupled by a frustrating ``zigzag''
interaction. We are particularly interested in the regime of weak interchain
coupling, which is difficult to analyse by either numerical or analytical
methods. Previous density matrix renormalisation group (DMRG) studies of the
isotropic model with open boundary conditions and sizeable interchain coupling
have established the presence of incommensurate correlations and of a spectral
gap. By using twisted boundary conditions with arbitrary twist angle, we are
able to determine the incommensurabilities both in the isotropic case and in
the presence of an exchange anisotropy by means of exact diagonalisation of
relatively short finite chains of up to 24 sites. Using twisted boundary
conditions results in a very smooth dependence of the incommensurabilities on
system size, which makes the extrapolation to infinite systems significantly
easier than for open or periodic chains.Comment: 6 pages, including 7 figure
Deep images of cluster radio halos
New radio data are presented for the clusters A401, A545, A754, A1914, A2219
and A2390, where the presence of diffuse radio emission was suggested from the
images of the NRAO VLA Sky Survey. Sensitive images of these clusters, obtained
with the Very Large Array (VLA)at 20 cm confirm the existence of the diffuse
sources and allow us to derive their fluxes and intrinsic parameters.The
correlation between the halo radio power and cluster X-ray luminosity is
derived for a large sample of halo clusters, and is briefly discussed.Comment: 9 pages, 13 figures, Astron. Astrophys. in pres
X-Ray Spectral Properties of the Cluster Abell 2029
We have analyzed ASCA and ROSAT PSPC spectra and images of the galaxy cluster
Abell 2029. The ASCA spectra of the cluster indicate that the gas temperature
declines with radius. The PSPC image shows that the cluster is very regular and
smooth. Also, there is no significant evidence for any irregularities in the
temperature distribution in the cluster, as would be produced by a subcluster
merger. These results suggest that A2029 is a relaxed cluster, and that the gas
is in hydrostatic equilibrium. We use the assumption of equilibrium to
determine the gravitational mass of the cluster as a function of radius. At a
radius of 16' (1.92 h_50^-1 Mpc), the gravitational mass is M_tot =
(9.42+-4.22) x 10^14 h_50^-1 Msun, while the mass of gas is M_gas =
(2.52+-0.77) x 10^14 h_50^-5/2 Msun. The gas fraction is found to increase with
radius; within a spherical radius of 16', the fraction is (M_gas/M_tot) =
(0.26+-0.14) h_50^-3/2. The iron abundance in the gas is found to be
(0.40+-0.04) of solar. There is no significant evidence for any variation in
the abundance with position in the cluster. The global X-ray spectra, central
X-ray spectra, and ROSAT surface brightness all require a cooling flow at the
cluster center. The global X-ray spectrum implies that the total cooling rate
is 363^+79_-96 h_50^-2 Msun/yr. The global X-ray spectra are consistent with
the Galactic value for the soft X-ray absorption toward the cluster. The PSPC
spectra of the central regions of the cluster are inconsistent with the large
value of foreground excess absorption found by White et al. (1991) based on the
Einstein SSS spectrum. The upper limit on excess foreground absorption is 7.3 x
10^19 cm^-2. However, the spectra do not rule a significant amount of intrinsic
absorbing gas located within the cooling flow region.Comment: ApJ, in press, 16 pages including 13 figures, formatted with
emulateapj Latex styl
A Deep Look at the Emission-Line Nebula in Abell 2597
The close correlation between cooling flows and emission-line nebulae in
clusters of galaxies has been recognized for over a decade and a half, but the
physical reason for this connection remains unclear. Here we present deep
optical spectra of the nebula in Abell 2597, one of the nearest strong
cooling-flow clusters. These spectra reveal the density, temperature, and metal
abundances of the line-emitting gas. The abundances are roughly half-solar, and
dust produces an extinction of at least a magnitude in V. The absence of [O
III] 4363 emission rules out shocks as a major ionizing mechanism, and the
weakness of He II 4686 rules out a hard ionizing source, such as an active
galactic nucleus or cooling intracluster gas. Hot stars are therefore the best
candidate for producing the ionization. However, even the hottest O stars
cannot power a nebula as hot as the one we see. Some other nonionizing source
of heat appears to contribute a comparable amount of power. We show that the
energy flux from a confining medium can become important when the ionization
level of a nebula drops to the low levels seen in cooling-flow nebulae. We
suggest that this kind of phenomenon, in which energy fluxes from the
surrounding medium augment photoelectric heating, might be the common feature
underlying the diverse group of objects classified as LINERS.Comment: 33 Latex pages, including 16 Postscript figures, to appear in 1997
September 1 Astrophysical Journa
Magneto-optical spectra of closely spaced magnetite nanoparticles
The Faraday rotation spectrum of composites containing magnetite nanoparticles is found to be dependent on the interparticle spacing of the constituent nanoparticles. The composite materials are prepared by combining chemically synthesized Fe
3O4 smagnetited nanoparticles s8-nm diameterd and polysmethylmethacrylated . Composites are made containing a range of nanoparticle concentrations. The peak of the main spectral feature depends on nanoparticle concentration; this peak is observed to shift from approximately 470 nm for sdilute compositesd to 540 nm concentrated . We present a theory based on the discrete-dipole approximation which accounts for
optical coupling between magnetite particles. Qualitative correlations between theoretical calculations and experimental data suggest that the shifts in spectral peak position depend on both interparticle distance and geometrical configuratio
Electronic thermal conductivity at high temperatures: Violation of the Wiedemann-Franz law in narrow band metals
We study the electronic part of the thermal conductivity kappa of metals. We
present two methods for calculating kappa, a quantum Monte-Carlo (QMC) method
and a method where the phonons but not the electrons are treated
semiclassically (SC). We compare the two methods for a model of alkali-doped
C60, A3C60, and show that they agree well. We then mainly use the SC method,
which is simpler and easier to interpret. We perform SC calculations for Nb for
large temperatures T and find that kappa increases with T as kappa(T)=a+bT,
where a and b are constants, consistent with a saturation of the mean free
path, l, and in good agreement with experiment. In contrast, we find that for
A3C60, kappa(T) decreases with T for very large T. We discuss the reason for
this qualitatively in the limit of large T. We give a quantum-mechanical
explanation of the saturation of l for Nb and derive the Wiedemann-Franz law in
the limit of T much smaller than W, where W is the band width. In contrast, due
to the small W of A3C60, the assumption T much smaller than W can be violated.
We show that this leads to kappa(T) \sim T^{-3/2} for very large T and a strong
violation of the Wiedemann-Franz law.Comment: 8 pages, 4 figure
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