7,706 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
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
Is there a Supermassive Black Hole at the Center of the Milky Way?
This review outlines the observations that now provide an overwhelming
scientific case that the center of our Milky Way Galaxy harbors a supermassive
black hole. Observations at infrared wavelength trace stars that orbit about a
common focal position and require a central mass (M) of 4 million solar masses
within a radius of 100 Astronomical Units. Orbital speeds have been observed to
exceed 5,000 km/s. At the focal position there is an extremely compact radio
source (Sgr A*), whose apparent size is near the Schwarzschild radius
(2GM/c^2). This radio source is motionless at the ~1 km/s level at the
dynamical center of the Galaxy. The mass density required by these observations
is now approaching the ultimate limit of a supermassive black hole within the
last stable orbit for matter near the event horizon.Comment: Invited review submitted to International Journal of Modern Physics
D; 23 pages; 10 figure
Chandra Observations of low velocity dispersion groups
Deviations of galaxy groups from cluster scaling relations can be understood
in terms of an excess of entropy in groups. The main effect of this excess is
to reduce the density and thus luminosity of the intragroup gas. Given this,
groups should also should show a steep relationship between X-ray luminosity
and velocity dispersion. However, previous work suggests that this is not the
case with many measuring slopes flatter than the cluster relation.
Examining the group L_X:\sigma relation shows that much of the flattening is
caused by a small subset of groups which show very high X-ray luminosities for
their velocity dispersions (or vice versa).
Detailed Chandra study of two such groups shows that earlier ROSAT results
were subject to significant (~30-40%) point source contamination, but confirm
that a significant hot IGM is present in these groups, although these are two
of the coolest systems in which intergalactic X-ray emission has been detected.
Their X-ray properties are shown to be broadly consistent with those of other
galaxy groups, although the gas entropy in NGC 1587 is unusually low, and its
X-ray luminosity correspondingly high for its temperature, compared to most
groups.
This leads us to suggest that the velocity dispersion in these systems has
been reduced in some way, and we consider how this might have come about.Comment: Accepted for publication in Ap
Dynamic model of gene regulation for the lac operon
Gene regulatory network is a collection of DNA which interact with each other and with other matter in the cell. The lac operon is an example of a relatively simple genetic network and is one of the best-studied structures in the Escherichia coli bacteria. In this work we consider a deterministic model of the lac operon with a noise term, representing the stochastic nature of the regulation. The model is written in terms of a system of simultaneous first order differential equations with delays. We investigate an analytical and numerical solution and analyse the range of values for the parameters corresponding to a stable solution
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
X-ray Spectra of the RIXOS source sample
We present results of an extensive study of the X-ray spectral properties of
sources detected in the RIXOS survey, that is nearly complete down to a flux
limit of 3e-14 cgs (0.5-2 keV). We show that for X-ray surveys containing
sources with low count rate spectral slopes estimated using simple hardness
ratios in the ROSAT band can be biased. Instead we analyse three-colour X-ray
data using statistical techniques appropriate to the Poisson regime which
removes the effects of this bias. We have then applied this technique to the
RIXOS survey to study the spectral properties of the sample. For the AGN we
find an average energy index of 1.05+-0.05 with no evidence for spectral
evolution with redshift. Individual AGN are shown to have a range of properties
including soft X-ray excesses and intrinsic absorption. Narrow Emission Line
Galaxies also seem to fit to a power-law spectrum, which may indicate a
non-thermal origin for their X-ray emission. We infer that most of the clusters
in the sample have a bremsstrahlung temperature >3 keV, although some show
evidence for a cooling flow. The stars deviate strongly from a power-law model
but fit to a thermal model. Finally, we have analysed the whole RIXOS sample
containing 1762 sources. We find that the mean spectral slope of the sources
hardens at lower fluxes in agreement with results from other samples. However,
a study of the individual sources demonstrates that the hardening of the mean
is caused by the appearance of a population of very hard sources at the lowest
fluxes. This has implications for the nature of the soft X-ray background.Comment: 31,LaTeX file, 2 PS files with Table 2 and 22 PS figures. MNRAS in
pres
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