389 research outputs found
Emitted vibration measurement device and method
This invention is directed to a method and apparatus for measuring emitted vibrational forces produced by a reaction wheel assembly due to imbalances, misalignment, bearing defects and the like. The apparatus includes a low mass carriage supported on a large mass base. The carriage is in the form of an octagonal frame having an opening which is adapted for receiving the reaction wheel assembly supported thereon by means of a mounting ring. The carriage is supported on the base by means of air bearings which support the carriage in a generally frictionless manner when supplied with compressed air from a source. A plurality of carriage brackets and a plurality of base blocks provided for physical coupling of the base and carriage. The sensing axes of the load cells are arranged generally parallel to the base and connected between the base and carriage such that all of the vibrational forces emitted by the reaction wheel assembly are effectively transmitted through the sensing axes of the load cells. In this manner, a highly reliable and accurate measurment of the vibrational forces of the reaction wheel assembly can be had. The output signals from the load cells are subjected to a dynamical analyzer which analyzes and identifies the rotor and spin bearing components which are causing the vibrational forces
The Concentration-Density Relation of Galaxies in Las Campanas Redshift Survey
We report the results of the evaluation of the ``concentration-density''
relation of galaxies in the local universe, taking advantage of the very large
and homogeneous data set available from the Las Campanas Redshift Survey
(Shectman et al. 1996). This data set consists of galaxies inhabiting the
entire range of galactic environments, from the sparsest field to the densest
clusters, thus allowing us to study environmental variations without combining
multiple data sets with inhomogeneous characteristics. Concentration is
quantified by the automatically-measured concentration index , which is a
good measure of a galaxy's bulge-to-disk ratio. The environment of the sample
galaxies is characterized both by the three-space local galaxy density and by
membership in groups and clusters. We find that the distribution of C in galaxy
populations varies both with local density and with cluster/group membership:
the fraction of centrally-concentrated galaxies increases with local galaxy
density, and is higher in clusters than in the field. A comparison of the
concentration-local density relation in clusters and the field shows that the
two connect rather smoothly at the intermediate density regime, implying that
the apparent cluster/field difference is only a manifestation of the variation
with the local density. We conclude that the structure of galaxies is
predominantly influenced by the local density and not by the broader
environments characterized by cluster/field memberships.Comment: 11 pages, 4 figures, ApJ in press, uses psfig.st
Disentangling Morphology, Star Formation, Stellar Mass, and Environment in Galaxy Evolution
We present a study of the spectroscopic and photometric properties of
galaxies in six nearby clusters. We perform a partial correlation analysis on
our dataset to investigate whether the correlation between star formation rates
in galaxies and their environment is merely another aspect of correlations of
morphology, stellar mass, or mean stellar age with environment, or whether star
formation rates vary independently of these other correlations. We find a
residual correlation of ongoing star formation with environment, indicating
that even galaxies with similar morphologies, stellar masses, and mean stellar
ages have lower star formation rates in denser environments. Thus, the current
star formation gradient in clusters is not just another aspect of the
morphology-density, stellar mass-density, or mean stellar age-density
relations. Furthermore, the star formation gradient cannot be solely the result
of initial conditions, but must partly be due to subsequent evolution through a
mechanism (or mechanisms) sensitive to environment. Our results constitute a
true ``smoking gun'' pointing to the effect of environment on the later
evolution of galaxies.Comment: 31 pages, including 5 figures; accepted for publication in Ap
Signatures of Interstellar-Intracluster Medium Interactions: Spiral Galaxy Rotation Curves in Abell 2029
We investigate the rich cluster Abell 2029 (z~0.08) using optical imaging and
long-slit spectral observations of 52 disk galaxies distributed throughout the
cluster field. No strong emission-line galaxies are present within ~400 kpc of
the cluster center, a region largely dominated by the similarly-shaped X-ray
and low surface brightness optical envelopes centered on the giant cD galaxy.
However, two-thirds of the galaxies observed outside the cluster core exhibit
line emission. H-alpha rotation curves of 14 cluster members are used in
conjunction with a deep I band image to study the environmental dependence of
the Tully-Fisher relation. The Tully-Fisher zero-point of Abell 2029 matches
that of clusters at lower redshifts, although we do observe a relatively larger
scatter about the Tully-Fisher relation. We do not observe any systematic
variation in the data with projected distance to the cluster center: we see no
environmental dependence of Tully-Fisher residuals, R-I color, H-alpha
equivalent width, and the shape and extent of the rotation curves.Comment: 22 pages, 6 figures, 3 tables; to appear in the August 2000
Astronomical Journa
Wide-Field Chandra X-Ray Observations of AGN in Abell 85 & Abell 754
To better understand the mechanism or mechanisms that lead to AGN activity
today, we measure the X-ray AGN fraction in a new sample of nearby clusters and
examine how it varies with galaxy properties, projected cluster-centric radius,
and cluster velocity dispersion. We present new wide-field Chandra X-ray
Observatory observations of Abell 85, Abell 754 and the background cluster
Abell 89B out to their virial radii. Out of seventeen X-ray sources associated
with galaxies in these clusters, we classify seven as X-ray AGN with L_{X,B} >
10^{41} erg/s. Only two of these would be classified as AGN based on their
optical spectra. We combine these observations with archival data to create a
sample of X-ray AGN from six z < 0.08 clusters and find that 3.4+1.1/-0.8% of
M_R 10^{41} erg/s. We find that
more X-ray AGN are detected in more luminous galaxies and attribute this to
larger spheriods in more luminous galaxies and increased sensitivity to lower
Eddington-rate accretion from black holes in those spheroids. At a given X-ray
luminosity limit, more massive black holes can be accreting less efficiently,
yet still be detected. If interactions between galaxies are the principal
drivers of AGN activity, then the AGN fraction should be higher in lower
velocity dispersion clusters and the outskirts of clusters. However, the
tendency of the most massive and early-type galaxies to lie in the centers of
the richest clusters could dilute such trends. While we find no variation in
the AGN fraction with projected cluster-centric radius, we do find that the AGN
fraction increases significantly from 2.6+1.0/-0.8% in rich clusters to
10.0+6.2/-4.3% in those with lower velocity dispersions.Comment: Accepted by Astrophysical Journal, 17 pages using emulateapj.cls, 10
B & W Figures (degraded): Full resolution paper available at
http://www.astronomy.ohio-state.edu/~sivakoff/AGN/XAGN_A85_A754.pd
Verification and Validation of the RAGE Hydrocode in Preparation for Investigation of Impacts into a Volatile-rich Target
Before a hydrocode is used to investigate a question of scientific interest, it should be tested against analogous laboratory experiments and problems with analytical solutions. The Radiation Adaptive Grid Eulerian (RAGE) hydrocode[1], developed by Los Alamos National Laboratory (LANL) and Science Applications International Corporation (SAIC)[2,3] has been subjected to many tests during its development.[4,5] We extend and review this work, emphasizing tests relevant to impact cratering into volatile-rich targets
The Influence of Environment on the Star Formation Rates of Galaxies
We have used a sample of 15749 galaxies taken from the Las Campanas Redshift
Survey to investigate the effects of environment on the rate of star formation
(SFR) in galaxies. The size and homogeneity of this data set allows us to
sample, for the first time, the entire range of galactic environment, from the
voids to the clusters, in a uniform manner, thus, we could decouple the local
galaxy density from the membership in associations. This decoupling is very
crucial for constraining the physical processes responsible for the
environmental dependencies of SFR. On the other hand, the use of an
automatically-measured concentration index (C), rather than Hubble type, allows
us to cleanly separate the morphological component from the SFR vs. environment
relationship. We find that cluster galaxies exhibit lower SFR for the same C
than field galaxies, while a further division of clusters by `richness' reveals
a new possible excitation of `starbursts' in poor clusters. Meanwhile, a more
general environmental investigation reveals that the SFR of a given C shows a
continuous correlation with the local density. Interestingly, this trend is
also observed both inside and outside of clusters, implying that physical
processes responsible for this correlation might not be intrinsic to the
cluster environment. On the other hand, galaxies with differing levels of SFR
appear to respond differently to the local density. Low levels of SFR are more
sensitive to environment inside than outside of clusters. In contrast, high
levels of SFR, identified as ``starbursts'', are as sensitive to local density
in the field as in clusters. We conclude that at least two separate processes
are responsible for the environmental sensitivity of the SFR.Comment: 25 pages, 10 figures, submitted to Ap
ROTSE All Sky Surveys for Variable Stars I: Test Fields
The ROTSE-I experiment has generated CCD photometry for the entire Northern
sky in two epochs nightly since March 1998. These sky patrol data are a
powerful resource for studies of astrophysical transients. As a demonstration
project, we present first results of a search for periodic variable stars
derived from ROTSE-I observations. Variable identification, period
determination, and type classification are conducted via automatic algorithms.
In a set of nine ROTSE-I sky patrol fields covering about 2000 square degrees
we identify 1781 periodic variable stars with mean magnitudes between m_v=10.0
and m_v=15.5. About 90% of these objects are newly identified as variable.
Examples of many familiar types are presented. All classifications for this
study have been manually confirmed. The selection criteria for this analysis
have been conservatively defined, and are known to be biased against some
variable classes. This preliminary study includes only 5.6% of the total
ROTSE-I sky coverage, suggesting that the full ROTSE-I variable catalog will
include more than 32,000 periodic variable stars.Comment: Accepted for publication in AJ 4/00. LaTeX manuscript. (28 pages, 11
postscript figures and 1 gif
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