4,550 research outputs found
A technique for the direct measurements of ocean currents from aircraft
A method for measuring surface and subsurface ocean currents from aircraft is described. Comparisons of measurements using this technique with those obtained from a ship are given and applications are discussed
Description of a freely dropped instrument for measuring current velocity
A new form of freely dropped instrument for current-velocity measurements is described. Operational procedures for using the instrument and its auxiliary equipment are given
Lensing Effects on the Protogalaxy Candidate cB58 and their Implications for the Cosmological Constant
The amplification of the protogalaxy candidate cB58 due to gravitational
lensing by the foreground cluster of galaxies MS1512.4+3647 is quantified based
on recent ROSAT and ASCA X-ray observations. It is found that the amplification
is at most 25 for any reasonable cosmological model with or without
cosmological constant. It is also argued that the system may be used to place
new constraints on the value of the cosmological constant. The gas mass
fraction for this cluster is found to be about 0.2.Comment: LaTex, 9 pages, 9 figures, uses aas2pp4.sty, Accepted for publication
in Ap
The Large-scale and Small-scale Clustering of Lyman-Break Galaxies at 3.5 < z< 5.5 from the GOODS survey
We report on the angular correlation function of Lyman-break galaxies (LBGs)
at z~4 and 5 from deep samples obtained from the Great Observatories Deep
Origins Survey (GOODS). Similar to LBGs at z~3, the shape of w(theta) of the
GOODS LBGs is well approximated by a power-law with slope beta~0.6 at angular
separation theta > 10 arcsec. The clustering strength of z~4, 5 LBGs also
depends on the rest-frame UV luminosity, with brighter galaxies more strongly
clustered than fainter ones, implying a general correlation between halos' mass
and LBGs' star-formation rate. At smaller separations, w(theta) of deep samples
significantly exceeds the extrapolation of the large-scale power-law fit,
implying enhanced spatial clustering at scales r < 1 Mpc. We also find that
bright LBGs statistically have more faint companions on scales theta < 20
arcsec than fainter ones, showing that the enhanced small-scale clustering is
very likely due to sub-structure, namely the fact that massive halos can host
multiple galaxies. A simple model for the halo occupation distribution and the
CDM halo mass function reproduce well the observed w(theta). The scaling
relationship of the clustering strength with volume density and with redshift
is quantitatively consistent with that of CDM halos. A comparison of the
clustering strength of three samples of equal luminosity limit at z ~ 3, 4 and
5 shows that the LBGs at z~5 are hosted in halos about one order of magnitude
less massive than those in the lower redshift bins, suggesting that
star-formation was more efficient at higher-redshift.Comment: replaced with the version accepted for publication in ApJ. 46 pages,
10 figures; minor changes to text, one subsection adde
Dark Matter and Baryon Fraction at the Virial Radius in Abell 2256
We combine ASCA and ROSAT X-ray data to constrain the radial dark matter
distribution in the primary cluster of A2256, free from the isothermality
assumption. Both instruments indicate that the temperature declines with
radius. The region including the central galaxy has a multicomponent spectrum,
which results in a wide range of allowed central temperatures. We find that the
secondary subcluster has a temperature and luminosity typical of a rich
cluster; however, the ASCA temperature map shows no signs of an advanced
merger. It is therefore assumed that the primary cluster is in hydrostatic
equilibrium. The data then require dark matter density profiles steeper than
rho ~ r^-2.5 in its outer part. Acceptable models have a total mass within
r=1.5 Mpc (the virial radius) of 6.0+-1.5 10^14 Msun at the 90% confidence,
about 1.6 times smaller than the mass derived assuming isothermality. Near the
center, dark matter profiles with and without central cusps are consistent with
the data. Total mass inside the X-ray core (r=0.26 Mpc) is 1.28+-0.08 10^14
Msun, which exceeds the isothermal value by a factor of 1.4. Although the
confidence intervals above may be underestimates since they do not include
possible asymmetry and departures from hydrostatic equilibrium, the behavior of
the mass distribution, if applicable to other clusters, can bring into better
agreement X-ray and lensing mass estimates, but aggravate the ``baryon
catastrophe''. The observed considerable increase in the gas content with
radius, not anticipated by simulations, may imply that a significant fraction
of thermal gas energy comes from sources other than gravity and merger shocks.Comment: Added dynamic argument against advanced merger. Latex, 10 pages, 3
figures; uses emulateapj.sty. ApJ in pres
Is the Butcher-Oemler effect a function of the cluster redshift ?
Using PSPC {\it Rosat} data, we measure x-ray surface brightness profiles,
size and luminosity of the Butcher-Oemler (BO) sample of clusters of galaxies.
The cluster x-ray size, as measured by the Petrosian r_{\eta=2} radius, does
not change with redshift and is independent from x-ray luminosity. On the other
hand, the x-ray luminosity increases with redshift. Considering that fair
samples show no-evolution, or negative luminosity evolution, we conclude that
the BO sample is not formed from the same class of objects observed at
different look-back times. This is in conflict with the usual interpretation of
the Butcher-Oemler as an evolutionary (or redshift-dependent) effect, based on
the assumption that we are comparing the same class of objects at different
redshifts. Other trends present in the BO sample reflect selection criteria
rather than differences in look-back time, as independently confirmed by the
fact that trends loose strength when we enlarge the sample with x-ray selected
sample of clusters. The variety of optical sizes and shapes of the clusters in
the Butcher-Oemler sample, and the Malmquist-like bias, are the reasons for
these selection effects that mimic the trends usually interpreted as changes
due to evolution.Comment: ApJ, in press, scheduled on May, 10 issue. 17 pages & 11 figure
Weak lensing mass reconstructions of the ESO Distant Cluster Survey
We present weak lensing mass reconstructions for the 20 high-redshift
clusters i n the ESO Distant Cluster Survey. The weak lensing analysis was
performed on deep, 3-color optical images taken with VLT/FORS2, using a
composite galaxy catalog with separate shape estimators measured in each
passband. We find that the EDisCS sample is composed primarily of clusters that
are less massive than t hose in current X-ray selected samples at similar
redshifts, but that all of the fields are likely to contain massive clusters
rather than superpositions of low mass groups. We find that 7 of the 20 fields
have additional massive structures which are not associated with the clusters
and which can affect the weak lensing mass determination. We compare the mass
measurements of the remaining 13 clusters with luminosity measurements from
cluster galaxies selected using photometric redshifts and find evidence of a
dependence of the cluster mass-to-light ratio with redshift. Finally we
determine the noise level in the shear measurements for the fields as a
function of exposure time and seeing and demonstrate that future ground-based
surveys which plan to perform deep optical imaging for use in weak lensing
measurements must achieve point-spread functions smaller than a median of 0.6"
FWHM.Comment: 35 pages, 24 figures, accepted to A&A, a version with better figure
resolution can be found at http://www.mpa-garching.mpg.de/ediscs/papers.htm
Radial Temperature Profiles of X-Ray--Emitting Gas Within Clusters of Galaxies
Previous analyses of ASCA data of clusters of galaxies have found conflicting
results regarding the slope of the temperature profile of the hot X-ray gas
within clusters, mainly because of the large, energy-dependent point spread
function (PSF) of the ASCA mirrors. We present a summary of all ASCA-determined
cluster temperature profiles found in the literature, and find a discrepancy in
the radial temperature trend of clusters based on which PSF-correction routine
is used. This uncertainty in the cluster temperature profile in turn can lead
to large uncertainties in the amount of dark matter in clusters. In this study,
we have used ROSAT PSPC data to obtain independent relative temperature
profiles for 26 clusters, most of which have had their temperature profiles
determined by ASCA. Our aim is not to measure the actual temperature values of
the clusters, but to use X-ray color profiles to search for a hardening or
softening of the spectra with radius for comparison to ASCA-derived profiles.
The radial color profiles indicate that outside of the cooling flow region, the
temperature profiles of clusters are in general constant. Within 35% of the
virial radius, we find a temperature drop of 20% at 10 keV and 12% at 5 keV can
be ruled out at the 99% confidence level. A subsample of non-cooling flow
clusters shows that the condition of isothermality applies at very small radii
too, although cooling gas complicates this determination in the cooling flow
subsample. The colors predicted from the temperature profiles of a series of
hydrodynamical cluster simulations match the data very well, although they
cannot be used to discriminate among different cosmologies. An additional
result is that the color profiles show evidence for a central peak in
metallicity in low temperature clusters.Comment: 39 pages, 15 embedded Postscript figures, uses aaspp4.sty, accepted
for publication in Astrophysical Journa
Mass Profiles of the Typical Relaxed Galaxy Clusters A2199 and A496
We present maps and radial profiles of the gas temperature in the nearby
galaxy clusters A2199 and A496, which have the most accurate ASCA spectral data
for all hot clusters. These clusters are relaxed and can provide reliable X-ray
mass measurements under the assumption of hydrostatic equilibrium. The cluster
average temperatures corrected for the presence of cooling flows are 4.8+-0.2
keV and 4.7+-0.2 keV (90% errors), respectively. Outside the central cooling
flow regions, the radial temperature profiles are similar to those of the
majority of nearby relaxed clusters. They are accurately described by
polytropic models with gamma=1.17+-0.07 for A2199 and gamma=1.24+-0.09 for
A496. We use these polytropic models to derive accurate total mass profiles.
Within r=0.5/h Mpc, which corresponds to a radius of overdensity 1000, the
total mass values are 1.45+-0.15 10^14 /h Msun and 1.55+-0.15 10^14 /h Msun.
These values are 10% lower than those obtained assuming constant temperature.
The values inside a gas core radius (0.07-0.13/h Mpc) are a factor of >1.5
higher than the isothermal values. The gas mass fraction increases with radius
(by a factor of 3 between the X-ray core radius and r_1000) and at r_1000
reaches values of 0.057+-0.005 and 0.056+-0.006 h^-3/2 for the two clusters,
respectively. Our mass profiles within r_1000 are remarkably well approximated
by the NFW "universal" profile. Since A2199 and A496 are typical relaxed
clusters, the above findings should be relevant for most such systems. In
particular, the similarity of the temperature profiles in nearby clusters
appears to reflect the underlying "universal" dark matter profile. The upward
revision of mass at small radii will resolve most of the discrepancy between
the X-ray and strong lensing mass estimates. (Abridged)Comment: Latex, 9 pages, 6 figures, uses emulateapj.sty. Submitted to Ap
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