7,225 research outputs found
Investigation of tidal displacements of the Earth's surface by laser ranging to GEOS-3
An analysis of laser ranging data from three stations was carried out in an attempt to measure the geometric Earth tide. Two different approaches to the problem were investigated. The dynamic method computes pass to pass apparent movements in stations height relative to short arcs fitted to several passes of data from the same station by the program GEODYNE. The quasi-geometric method reduces the dependence on unmodelled satellite dynamics to a knowledge of only the radial position of the satellite by considering two station simultaneous ranging at the precise time that the satellite passes through the plane defined by two stations and the center of mass of the Earth
Colors, magnitudes and velocity dispersions in early-type galaxies: Implications for galaxy ages and metallicities
We present an analysis of the color-magnitude-velocity dispersion relation
for a sample of 39320 early-type galaxies within the Sloan Digital Sky Survey.
We demonstrate that the color-magnitude relation is entirely a consequence of
the fact that both the luminosities and colors of these galaxies are correlated
with stellar velocity dispersions. Previous studies of the color-magnitude
relation over a range of redshifts suggest that the luminosity of an early-type
galaxy is an indicator of its metallicity, whereas residuals in color from the
relation are indicators of the luminosity-weighted age of its stars. We show
that this, when combined with our finding that velocity dispersion plays a
crucial role, has a number of interesting implications. First, galaxies with
large velocity dispersions tend to be older (i.e., they scatter redward of the
color-magnitude relation). Similarly, galaxies with large dynamical mass
estimates also tend to be older. In addition, at fixed luminosity, galaxies
which are smaller, or have larger velocity dispersions, or are more massive,
tend to be older. Second, models in which galaxies with the largest velocity
dispersions are also the most metal poor are difficult to reconcile with our
data. However, at fixed velocity dispersion, galaxies have a range of ages and
metallicities: the older galaxies have smaller metallicities, and vice-versa.
Finally, a plot of velocity dispersion versus luminosity can be used as an age
indicator: lines of constant age run parallel to the correlation between
velocity dispersion and luminosity.Comment: 12 pages, 9 figures. Accepted by A
B3 0003+387: AGN Marked Large-Scale Structure at z=1.47?
We present evidence for a significant overdensity of red galaxies, as much as
a factor of 14 over comparable field samples, in the field of the z=1.47 radio
galaxy B3 0003+387. The colors and luminosities of the brightest red galaxies
are consistent with their being at z>0.8. The radio galaxy and one of the red
galaxies are separated by 5" and show some evidence of a possible interaction.
However, the red galaxies do not show any strong clustering around the radio
galaxy nor around any of the brighter red galaxies. The data suggest that we
are looking at a wall or sheet of galaxies, possibly associated with the radio
galaxy at z=1.47. Spectroscopic redshifts of these red galaxies will be
necessary to confirm this large-scale structure.Comment: 19 pages, 7 figures, LaTeX2e/AASTeX v5.0.2. The full photometric
catalog is included as a separate deluxetable file. To appear in the
Astronomical Journal (~Nov 00
Sunyaev - Zel'dovich fluctuations from spatial correlations between clusters of galaxies
We present angular power spectra of the cosmic microwave background radiation
anisotropy due to fluctuations of the Sunyaev-Zel'dovich (SZ) effect through
clusters of galaxies. A contribution from the correlation among clusters is
especially focused on, which has been neglected in the previous analyses.
Employing the evolving linear bias factor based on the Press-Schechter
formalism, we find that the clustering contribution amounts to 20-30% of the
Poissonian one at degree angular scales. If we exclude clusters in the local
universe, it even exceeds the Poissonian noise, and makes dominant contribution
to the angular power spectrum. As a concrete example, we demonstrate the
subtraction of the ROSAT X-ray flux-limited cluster samples. It indicates that
we should include the clustering effect in the analysis of the SZ fluctuations.
We further find that the degree scale spectra essentially depend upon the
normalization of the density fluctuations, i.e., \sigma_8, and the gas mass
fraction of the cluster, rather than the density parameter of the universe and
details of cluster evolution models. Our results show that the SZ fluctuations
at the degree scale will provide a possible measure of \sigma_8, while the
arc-minute spectra a probe of the cluster evolution. In addition, the
clustering spectrum will give us valuable information on the bias at high
redshift, if we can detect it by removing X-ray luminous clusters.Comment: 11 pages, 4 figures, submitted to Astrophysical Journa
Towards a Holistic View of the Heating and Cooling of the Intracluster Medium
(Abridged) X-ray clusters are conventionally divided into two classes: "cool
core" (CC) clusters and "non-cool core" (NCC) clusters. Yet relatively little
attention has been given to the origins of this dichotomy and, in particular,
to the energetics and thermal histories of the two classes. We develop a model
for the entropy profiles of clusters starting from the configuration
established by gravitational shock heating and radiative cooling. At large
radii, gravitational heating accounts for the observed profiles and their
scalings well. However, at small and intermediate radii, radiative cooling and
gravitational heating cannot be combined to explain the observed profiles of
either type of cluster. The inferred entropy profiles of NCC clusters require
that material is preheated prior to cluster collapse in order to explain the
absence of low entropy (cool) material in these systems. We show that a similar
modification is also required in CC clusters in order to match their properties
at intermediate radii. In CC clusters, this modification is unstable, and an
additional process is required to prevent cooling below a temperature of a few
keV. We show that this can be achieved by adding a self-consistent AGN feedback
loop in which the lowest-entropy, most rapidly cooling material is heated so
that it rises buoyantly to mix with material at larger radii. The resulting
model does not require fine tuning and is in excellent agreement with a wide
variety of observational data. Some of the other implications of this model are
briefly discussed.Comment: 27 pages, 13 figures, MNRAS accepted. Discussion of cluster heating
energetics extended, results unchange
A Richness Study of 14 Distant X-ray Clusters From the 160 Square Degree Survey
We have measured the surface density of galaxies toward 14 X-ray-selected
cluster candidates at redshifts greater than z=0.46, and we show that they are
associated with rich galaxy concentrations. We find that the clusters range
between Abell richness classes 0-2, and have a most probable richness class of
one. We compare the richness distribution of our distant clusters to those for
three samples of nearby clusters with similar X-ray luminosities. We find that
the nearby and distant samples have similar richness distributions, which shows
that clusters have apparently not evolved substantially in richness since
redshift z =0.5. We compare the distribution of distant X-ray clusters in the
L_x--richness plane to the distribution of optically-selected clusters from the
Palomar Distant Cluster Survey. The optically-selected clusters appear overly
rich for their X-ray luminosities when compared to X-ray-selected clusters.
Apparently, X-ray and optical surveys do not necessarily sample identical mass
concentrations at large redshifts. This may indicate the existence of a
population of optically rich clusters with anomalously low X-ray emission. More
likely, however, it reflects the tendency for optical surveys to select
unvirialized mass concentrations, as might be expected when peering along
large-scale filaments.Comment: The abstract has been abridged. Accepted for publication in the
Astrophysical Journa
Masses for the Local Group and the Milky Way
We use the very large Millennium Simulation of the concordance CDM
cosmogony to calibrate the bias and error distribution of Timing Argument
estimators of the masses of the Local Group and of the Milky Way. From a large
number of isolated spiral-spiral pairs similar to the Milky Way/Andromeda
system, we find the interquartile range of the ratio of timing mass to true
mass to be a factor of 1.8, while the 5% and 95% points of the distribution of
this ratio are separated by a factor of 5.7. Here we define true mass as the
sum of the ``virial'' masses of the two dominant galaxies. For
current best values of the distance and approach velocity of Andromeda this
leads to a median likelihood estimate of the true mass of the Local Group of
5.27\times 10^{12}\msun, or , with an
interquartile range of and a 5% to 95% range of . Thus a 95% lower confidence limit on the true mass of the Local Group
is 1.81\times 10^{12}\msun. A timing estimate of the Milky Way's mass based
on the large recession velocity observed for the distant satellite Leo I works
equally well, although with larger systematic uncertainties. It gives an
estimated virial mass for the Milky Way of 2.43 \times 10^{12}\msun with a
95% lower confidence limit of 0.80 \times 10^{12}\msun.Comment: 11 pages, 6 figures, MNRAS accepted. Added a new discussion paragraph
and a new figure regarding the relative transverse velocity but conclusions
unchange
The Evolution of X-ray Clusters and the Entropy of the Intra Cluster Medium
The thermodynamics of the diffuse, X-ray emitting gas in clusters of galaxies
is determined by gravitational processes associated with shock heating,
adiabatic compression, and non-gravitational processes such as heating by SNe,
stellar winds, activity in the central galactic nucleus, and radiative cooling.
The effect of gravitational processes on the thermodynamics of the Intra
Cluster Medium (ICM) can be expressed in terms of the ICM entropy S ~
ln(T/\rho^{2/3}). We use a generalized spherical model to compute the X-ray
properties of groups and clusters for a range of initial entropy levels in the
ICM and for a range of mass scales, cosmic epochs and background cosmologies.
We find that the statistical properties of the X-ray clusters strongly depend
on the value of the initial excess entropy. Assuming a constant, uniform value
for the excess entropy, the present-day X-ray data are well fitted for the
following range of values K_* = kT/\mu m_p \rho^{2/3} = (0.4\pm 0.1) \times
10^{34} erg cm^2 g^{-5/3} for clusters with average temperatures kT>2 keV; K_*
= (0.2\pm 0.1) \times 10^{34} erg cm^2 g^{-5/3} for groups and clusters with
average temperatures kT<2 keV. These values correspond to different excess
energy per particle of kT \geq 0.1 (K_*/0.4\times 10^{34}) keV. The dependence
of K_* on the mass scale can be well reproduced by an epoch dependent external
entropy: the relation K_* = 0.8(1+z)^{-1}\times 10^{34} erg cm^2 g^{-5/3} fits
the data over the whole temperature range. Observations of both local and
distant clusters can be used to trace the distribution and the evolution of the
entropy in the cosmic baryons, and ultimately to unveil the typical epoch and
the source of the heating processes.Comment: 53 pages, LateX, 19 figures, ApJ in press, relevant comments and
references adde
On the Number Density of Sunyaev-Zel'dovich Clusters of Galaxies
If the mean properties of clusters of galaxies are well described by the
entropy-driven model, the distortion induced by the cluster population on the
blackbody spectrum of the Cosmic Microwave Background radiation is proportional
to the total amount of intracluster gas while temperature anisotropies are
dominated by the contribution of clusters of about 10^{14} solar masses. This
result depends marginally on cluster parameters and it can be used to estimate
the number density of clusters with enough hot gas to produce a detectable
Sunyaev-Zel'dovich effect. Comparing different cosmological models, the
relation depends mainly on the density parameter Omega_m. If the number density
of clusters could be estimated by a different method, then this dependence
could be used to constrain Omega_m.Comment: 8 pages, 3 figures, submitted to ApJ Letter
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