5,817 research outputs found
X-ray Observations of Distant Optically Selected Cluster
We have measured fluxes or flux limits for 31 of the 79 cluster candidates in
the Palomar Distant Cluster Survey (PDCS) using archival ROSAT/PSPC pointed
observations. Our X-ray survey reaches a flux limit of erg s cm (0.4 - 2.0 keV), which corresponds to
luminosities of erg s ( = 50 km
s Mpc, = ), if we assume the PDCS estimated
redshifts. Of the 31 cluster candidates, we detect six at a signal-to-noise
greater than three. We estimate that (90% confidence
limits) of these six detections are a result of X-ray emission from objects
unrelated to the PDCS cluster candidates. The net surface density of X-ray
emitting cluster candidates in our survey, clusters
deg, agrees with that of other, X-ray selected, surveys. It is possible,
given the large error on our contamination rate, that we have not detected
X-ray emission from any of our observed PDCS cluster candidates. We find no
statistically significant difference between the X-ray luminosities of PDCS
cluster candidates and those of Abell clusters of similar optical richness.
This suggests that the PDCS contains objects at high redshift similar to the
low redshift clusters in the Abell catalogs. We show that the PDCS cluster
candidates are not bright X-ray sources, the average luminosity of the six
detected candidates is only erg s (0.4-2.0
keV). This finding is in agreement with previous X-ray studies of high
redshift, optically selected, rich clusters of galaxies.Comment: 19 pages, LaTeX with AAS Preprint Macros (v. 4), 3 embedded
postscript figures, 3 Seperate Tables using aj_pt4.sty, Accepted by the
Astronomical Journal for November 199
On the equivalence of Eulerian and Lagrangian variables for the two-component Camassa-Holm system
The Camassa-Holm equation and its two-component Camassa-Holm system
generalization both experience wave breaking in finite time. To analyze this,
and to obtain solutions past wave breaking, it is common to reformulate the
original equation given in Eulerian coordinates, into a system of ordinary
differential equations in Lagrangian coordinates. It is of considerable
interest to study the stability of solutions and how this is manifested in
Eulerian and Lagrangian variables. We identify criteria of convergence, such
that convergence in Eulerian coordinates is equivalent to convergence in
Lagrangian coordinates. In addition, we show how one can approximate global
conservative solutions of the scalar Camassa-Holm equation by smooth solutions
of the two-component Camassa-Holm system that do not experience wave breaking
Spectroscopic Confirmation of a Radio-Selected Galaxy Overdensity at z=1.11
We report the discovery of a galaxy overdensity at z=1.11 associated with the
z=1.110 high-redshift radio galaxy MG0442+0202. The group, CL0442+0202, was
found in a near-infrared survey of z>1 radio galaxies undertaken to identify
spatially-coincident regions with a high density of objects red in I-K' color,
typical of z>1 elliptical galaxies. Spectroscopic observations from the Keck
telescope reveal five galaxies within 35" of MG0442+0202 at 1.10<z<1.11. These
member galaxies have broad-band colors and optical spectra consistent with
passively-evolving elliptical galaxies formed at high redshift. A 45ks Chandra
X-Ray Observatory observation detects the radio galaxy and four point sources
within 15" of the radio galaxy, corresponding to a surface density two orders
of magnitude higher than average for X-ray sources at these flux levels,
S(0.5-2keV) > 5e-16 erg/cm2/s. One of these point sources is identified with a
radio-quiet, typeII quasar at z=1.863, akin to sources recently reported in
deep Chandra surveys. The limit on an extended hot intracluster medium in the
Chandra data is S(1-6keV) < 1.9e-15 erg/cm2/s (3-sigma, 30" radius aperture).
Though the X-ray observations do not confirm the existence of a massive, bound
cluster at z>1, the success of the optical/near-infrared targeting of
early-type systems near the radio galaxy validates searches using radio
galaxies as beacons for high-redshift large-scale structure. We interpret
CL0442+0202 to be a massive cluster in the process of formation.Comment: 23 pages, 7 figure
Independent Orbiter Assessment (IOA): Analysis of the orbiter main propulsion system
The results of the Independent Orbiter Assessment (IOA) of the Failure Modes and Effects Analysis (FMEA) and Critical Items List (CIL) are presented. The IOA approach features a top-down analysis of the hardware to determine failure modes, criticality, and potential critical items (PCIs). To preserve independence, this analysis was accomplished without reliance upon the results contained within the NASA FMEA/CIL documentation. The independent analysis results for the Orbiter Main Propulsion System (MPS) hardware are documented. The Orbiter MPS consists of two subsystems: the Propellant Management Subsystem (PMS) and the Helium Subsystem. The PMS is a system of manifolds, distribution lines and valves by which the liquid propellants pass from the External Tank (ET) to the Space Shuttle Main Engines (SSMEs) and gaseous propellants pass from the SSMEs to the ET. The Helium Subsystem consists of a series of helium supply tanks and their associated regulators, check valves, distribution lines, and control valves. The Helium Subsystem supplies helium that is used within the SSMEs for inflight purges and provides pressure for actuation of SSME valves during emergency pneumatic shutdowns. The balance of the helium is used to provide pressure to operate the pneumatically actuated valves within the PMS. Each component was evaluated and analyzed for possible failure modes and effects. Criticalities were assigned based on the worst possible effect of each failure mode. Of the 690 failure modes analyzed, 349 were determined to be PCIs
Controls on the spatial distribution of oceanic <i>δ</i><sup>13</sup>C<sub>DIC</sub>
We describe the design and evaluation of a large ensemble of coupled climate–carbon cycle simulations with the Earth system model of intermediate complexity GENIE. This ensemble has been designed for application to a range of carbon cycle questions, including the causes of late- Quaternary fluctuations in atmospheric CO2. Here we evaluate the ensemble by applying it to a transient experiment over the recent industrial era (1858 to 2008 AD). We employ singular vector decomposition and principal component emulation to investigate the spatial modes of ensemble variability of oceanic dissolved inorganic carbon (DIC) δ13C, considering both the spun-up pre-industrial state and the transient change. These analyses allow us to separate the natural (preindustrial) and anthropogenic controls on the δ13CDIC distribution. We apply the same dimensionally reduced emulation techniques to consider the drivers of the spatial uncertainty in anthropogenic DIC. We show that the sources of uncertainty related to the uptake of anthropogenic δ13CDIC and DIC are quite distinct. Uncertainty in anthropogenic δ13C uptake is controlled by air–sea gas exchange, which explains 63% of modelled variance. This mode of variability is largely absent from the ensemble variability in CO2 uptake, which is rather driven by uncertainties in thermocline ventilation rates. Although the need to account for air–sea gas exchange is well known, these results suggest that, to leading order, uncertainties in the ocean uptake of anthropogenic 13C and CO2 are governed by very different processes. This illustrates the difficulties in reconstructing one from the other, and furthermore highlights the need for careful targeting of both δ13CDIC and DIC observations to better constrain the ocean sink of anthropogenic CO2
A Turn-over in the Galaxy Luminosity Function of the Coma Cluster Core?
Our previous study of the faint end (R21.5) of the galaxy luminosity
function (GLF) was based on spectroscopic data in a small region near the Coma
cluster center. In this previous study Adami et al. (1998) suggested, with
moderate statistical significance, that the number of galaxies actually
belonging to the cluster was much smaller than expected. This led us to
increase our spectroscopic sample. Here, we have improved the statistical
significance of the results of the Coma GLF faint end study (R22.5) by
using a sample of 85 redshifts. This includes both new spectroscopic data and a
literature compilation. The relatively small number of faint galaxies belonging
to Coma that was suggested by Adami et al. (1998) and Secker et al. (1998) has
been confirmed with these new observations. We also confirm that the
color-magnitude relation is not well suited for finding the galaxies inside the
Coma cluster core, close to the center at magnitudes fainter than R19. We
show that there is an enhancement in the Coma line of sight of field galaxies
compared to classical field counts. This can be explained by the contribution
of groups and of a distant cluster along the line of sight. The
result is that the Coma GLF appears to turn-over or at least to become flat for
the faint galaxies. We suggest that this is due to environmental effects.Comment: 8 pages, 6 postscript figures, accepted in A&A, new table 1, updated
figure
Coupling transport and biodegradation of VOCs in surface and subsurface soils.
Volatile organic chemicals present at Superfund sites preferentially partition into the soil gas and may be available for microbial degradation. A simple mass transfer model for biodegradation for volatile substrates has been developed for the aerobic decomposition of aromatic and aliphatic hydrocarbons. The mass transfer analysis calculates diffusive fluxes from soil gas through water and membrane films and into the cell. This model predicts an extreme sensitivity of potential biodegradation rates to the air-water partition coefficients of the compounds. Aromatic hydrocarbons are removed rapidly while the aliphatic hydrocarbons are much slower by orders of magnitude. Furthermore, oxygen transfer is likely to limit aromatic hydrocarbon degradation rates. The model presents results that cast doubt on the practicality of using methane or propane for the co-metabolic destruction of trichloroethylene in a gas phase bioreactor. Toluene as a primary substrate has better mass transfer characteristics to achieve more efficient trichloroethylene degradation. Hence, in sites where these contaminants coexist, bioremediation could be improved
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