414 research outputs found
LOFAR: A new radio telescope for low frequency radio observations: Science and project status
LOFAR, the Low Frequency Array, is a large radio telescope consisting about
100 soccer field sized antenna stations spread over a region of 400 km in
diameter. It will operate in the frequency range from ~10 to 240 MHz, with a
resolution at 240 MHz of better than an arcsecond. Its superb sensitivity will
allow for a broad range of astrophysical studies. In this contribution we first
discuss four major areas of astrophysical research in which LOFAR will
undoubtedly make important contributions: reionisation, distant galaxies and
AGNs, transient radio sources and cosmic rays. Subsequently, we will discuss
the technical concept of the instrument and the status of the LOFAR projectComment: 8 pages, 2 figures, to appear in the proceedings of the XXI Texas
Symposium on Relativistic Astrophysics held on December 9--13 2002, in
Florence, Ital
Completeness in Photometric and Spectroscopic Searches for Clusters
We investigate, using simulated galaxy catalogues, the completeness of
searches for massive clusters of galaxies in redshift surveys or imaging
surveys with photometric redshift estimates, i.e. what fraction of clusters
(M>10^14/h Msun) are found in such surveys. We demonstrate that the matched
filter method provides an efficient and reliable means of identifying massive
clusters even when the redshift estimates are crude. In true redshift surveys
the method works extremely well. We demonstrate that it is possible to
construct catalogues with high completeness, low contamination and both varying
little with redshift.Comment: ApJ in press, 15 pages, 10 figure
A Redshift Survey of Nearby Galaxy Groups: the Shape of the Mass Density Profile
We constrain the mass profile and orbital structure of nearby groups and
clusters of galaxies. Our method yields the joint probability distribution of
the density slope n, the velocity anisotropy beta, and the turnover radius r0
for these systems. The measurement technique does not use results from N-body
simulations as priors. We incorporate 2419 new redshifts in the fields of 41
systems of galaxies with z < 0.04. The new groups have median velocity
dispersion sigma=360 km/s. We also use 851 archived redshifts in the fields of
8 nearly relaxed clusters with z < 0.1. Within R < 2 r200, the data are
consistent with a single power law matter density distribution with slope n =
1.8-2.2 for systems with sigma < 470 km/s, and n = 1.6-2.0 for those with sigma
> 470 km/s (95% confidence). We show that a simple, scale-free phase space
distribution function f(E,L^2) ~ (-E)^(alpha-1/2) L^(-2 \beta) is consistent
with the data as long as the matter density has a cusp. Using this DF, matter
density profiles with constant density cores (n=0) are ruled out with better
than 99.7% confidence.Comment: 22 pages; accepted for publication in the Astrophysical Journa
The Effect of the Cosmic Web on Cluster Weak Lensing Mass Estimates
In modern hierarchical theories of structure formation, rich clusters of
galaxies form at the vertices of a weblike distribution of matter, with
filaments emanating from them to large distances and with smaller objects
forming and draining in along these filaments. The amount of mass contained in
structure near the cluster can be comparable to the collapsed mass of the
cluster itself. As the lensing kernel is quite broad along the line of sight
around cluster lenses with typical redshifts near z=0.5, structures many Mpc
away from the cluster are essentially at the same location as the cluster
itself, when considering their effect on the cluster's weak lensing signal. We
use large-scale numerical simulations of structure formation in a
Lambda-dominated cold dark matter model to quantify the effect that large-scale
structure near clusters has upon the cluster masses deduced from weak lensing
analysis. A correction for the scatter in possible observed lensing masses
should be included when interpreting mass functions from weak lensing surveys.Comment: 14 pages, 11 figures. LaTeX2e, uses emulateapj.sty and
onecolfloat.st
Redshifts in the Southern Abell Redshift Survey Clusters. I. The Data
The Southern Abell Redshift Survey contains 39 clusters of galaxies with
redshifts in the range 0.0 < z < 0.31 and a median redshift depth of z =
0.0845. SARS covers the region 0 21h (while
avoiding the LMC and SMC) with b > 40. Cluster locations were chosen from the
Abell and Abell-Corwin-Olowin catalogs while galaxy positions were selected
from the Automatic Plate Measuring Facility galaxy catalog with
extinction-corrected magnitudes in the range 15 <= b_j < 19. SARS utilized the
Las Campanas 2.5 m duPont telescope, observing either 65 or 128 objects
concurrently over a 1.5 sq deg field. New redshifts for 3440 galaxies are
reported in the fields of these 39 clusters of galaxies.Comment: 20 pages, 5 figures, accepted for publication in the Astronomical
Journal, Table 2 can be downloaded in its entirety from
http://trotsky.arc.nasa.gov/~mway/SARS1/sars1-table2.cs
ROSAT PSPC Observations of the Richest () ACO Clusters
We have compiled an X-ray catalog of optically selected rich clusters of
galaxies observed by the PSPC during the pointed GO phase of the ROSAT mission.
This paper contains a systematic X-ray analysis of 150 clusters with an optical
richness classification of from the ACO catalog (Abell, Corwin, and
Olowin 1989). All clusters were observed within 45' of the optical axis of the
telescope during pointed PSPC observations. For each cluster, we calculate: the
net 0.5-2.0 keV PSPC count rate (or upper limit) in a 1 Mpc radius
aperture, 0.5-2.0 keV flux and luminosity, bolometric luminosity, and X-ray
centroid. The cluster sample is then used to examine correlations between the
X-ray and optical properties of clusters, derive the X-ray luminosity function
of clusters with different optical classifications, and obtain a quantitative
estimate of contamination (i.e, the fraction of clusters with an optical
richness significantly overestimated due to interloping galaxies) in the ACO
catalog
Identification of new populations of chicken natural killer (NK) cells
Natural killer (NK) cell activity is conserved throughout vertebrate development, but characterization of non-mammalian NK-cells has been hampered by the absence of specific mAbs for these cells.Monoclonal antibodies were generated against in vitro IL-2 expanded sorted CD3-CD8α+ peripheral blood lymphocytes, previously described to contain chicken NK-cells. Screening of embryonic and adult splenocytes with hybridoma supernatants resulted in five candidate NK markers.Activation of chicken NK-cells with PMA/Ionomycin or with the NK target cell-line LSCC-RP9 resulted in increased expression of CD107 (LAMP-1) and a newly developed flow cytometry based cytotoxicity assay showed that NK-cells were able to kill target cells. Combining NK markers with functional assays indicated that marker positive cells showed NK-cell function.In conclusion, we generated new monoclonal antibodies and developed two functional assays which will enhance our understanding of the role of NK-cells in healthy and diseased chickens
Spectral calibration requirements of radio interferometers for epoch of reionisation science with the SKA
Spectral features introduced by instrumental chromaticity of radio interferometers have the potential to negatively impact the ability to perform Epoch of Reionisation and Cosmic Dawn (EoR/CD) science. We describe instrument calibration choices that influence the spectral characteristics of the science data, and assess their impact on EoR/CD statistical and tomographic experiments. Principally, we consider the intrinsic spectral response of the antennas, embedded within a complete frequency-dependent primary beam response, and instrument sampling. The analysis is applied to the proposed SKA1-Low EoR/CD experiments. We provide tolerances on the smoothness of the SKA station primary beam bandpass, to meet the scientific goals of statistical and tomographic (imaging) of EoR/CD programs. Two calibration strategies are tested: (1) fitting of each fine channel independently, and (2) fitting of an nth-order polynomial for each ~ 1 MHz coarse channel with (n+1)th-order residuals (n = 2, 3, 4). Strategy (1) leads to uncorrelated power in the 2D power spectrum proportional to the thermal noise power, thereby reducing the overall sensitivity. Strategy (2) leads to correlated residuals from the fitting, and residual signal power with (n+1)th-order curvature. For the residual power to be less than the thermal noise, the fractional amplitude of a fourth-order term in the bandpass across a single coarse channel must be <2.5% (50 MHz), <0.5% (150 MHz), <0.8% (200 MHz). The tomographic experiment places constraints on phase residuals in the bandpass. We find that the root-mean-square variability over all stations of the change in phase across any fine channel (4.578 kHz) should not exceed 0.2 degrees
Correlation length of X-ray brightest Abell clusters
We compute the cluster auto-correlation function of an X-ray
flux limited sample of Abell clusters (XBACs, \cite{ebe}). For the total XBACs
sample we find a power-law fit with Mpc
hand consistent with the results of Abell
clusters. We also analyze for subsamples defined by different
X-ray luminosity thresholds where we find a weak tendency of larger values of
with increasing X-ray luminosity although with a low statistical
significance. In the different subsamples analyzed we find Mpc
h and . Our analysis suggests that cluster X-ray
luminosities may be used for a reliable confrontation of cluster spatial
distribution properties in models and observations.Comment: Accepted for publication in Astrophysical Journa
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