4,974 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
Binding-incompetent adenovirus facilitates molecular conjugate-mediated gene transfer by the receptor-mediated endocytosis pathway
Molecular conjugate vectors may be constructed that accomplish high efficiency gene transfer by the receptor-mediated endocytosis pathway. In order to mediate escape from lysosomal degradation, we have incorporated adenoviruses into the functional design of the conjugate. In doing so, however, we have introduced an additional ligand, which can bind to receptors on the cell surface, undermining the potential for cell specific targeting. To overcome this, we have treated the adenovirus with a monoclonal anti-fiber antibody, which renders the virus incapable of binding to its receptor. The result is a multi-functional molecular conjugate vector, which has preserved its binding specificity while at the same time being capable of preventing lysosomal degradation of endosome-internalized conjugate-DNA complexes. This finding indicates that adenoviral binding is not a prerequisite for adenoviral-mediated endosome disruption
Pairing symmetry of the one-band Hubbard model in the paramagnetic weak-coupling limit: a numerical RPA study
We study the spin-fluctuation-mediated superconducting pairing gap in a
weak-coupling approach to the Hubbard model for a two dimensional square
lattice in the paramagnetic state. Performing a comprehensive theoretical study
of the phase diagram as a function of filling, we find that the superconducting
gap exhibits transitions from p-wave at very low electron fillings to
d_{x^2-y^2}-wave symmetry close to half filling in agreement with previous
reports. At intermediate filling levels, different gap symmetries appear as a
consequence of the changes in the Fermi surface topology and the associated
structure of the spin susceptibility. In particular, the vicinity of a van Hove
singularity in the electronic structure close to the Fermi level has important
consequences for the gap structure in favoring the otherwise sub-dominant
triplet solution over the singlet d-wave solution. By solving the full gap
equation, we find that the energetically favorable triplet solutions are chiral
and break time reversal symmetry. Finally, we also calculate the detailed
angular gap structure of the quasi-particle spectrum, and show how
spin-fluctuation-mediated pairing leads to significant deviations from the
first harmonics both in the singlet d_{x^2-y^2} gap as well as the chiral
triplet gap solution.Comment: 11 pages 11 figure
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
Spectroscopic Observations of Optically Selected Clusters of Galaxies from the Palomar Distant Cluster Survey
We have conducted a redshift survey of sixteen cluster candidates from the
Palomar Distant Cluster Survey (PDCS) to determine both the density of PDCS
clusters and the accuracy of the estimated redshifts presented in the PDCS
catalog (Postman et. al. 1996). We find that the matched-filter redshift
estimate presented in the PDCS has an error sigma_z = 0.06 in the redshift
range 0.1 < z < 0.35 based on eight cluster candidates with three or more
concordant galaxy redshifts.
We measure the low redshift (0.1 < z < 0.35) space density of PDCS clusters
to be 31.3^{+30.5}_{-17.1} * E-06 h^3 Mpc^-3 (68% confidence limits for a
Poisson distribution) for Richness Class 1 systems. We find a tentative space
density of 10.4^{+23.4}_{-8.4}* E-06 h^3 Mpc^-3 for Richness Class 2 clusters.
These densities compare favorably with those found for the whole of the PDCS
and support the finding that the space density of clusters in the PDCS is a
factor of ~5 above that of clusters in the Abell catalog (Abell 1958; Abell,
Corwin, and Olowin 1989). These new space density measurements were derived as
independently as possible from the original PDCS analysis and therefore,
demonstrate the robustness of the original work. Based on our survey, we
conclude that the PDCS matched-filter algorithm is successful in detecting real
clusters and in estimating their true redshifts in the redshift range we
surveyed.Comment: 23 pages with 4 figures and 3 seperate tables. To be published in the
November Issue of the Astronomical Journa
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