688 research outputs found
Spectrophotometry of nearby field galaxies: the data
We have obtained integrated and nuclear spectra, as well as U, B, R surface
photometry, for a representative sample of 196 nearby galaxies. These galaxies
span the entire Hubble sequence in morphological type, as well as a wide range
of luminosities (M_B=-14 to -22). Here we present the spectrophotometry for
these galaxies. The selection of the sample and the U, B, R surface photometry
is described in a companion paper (Paper I). Our goals for the project include
measuring the current star formation rates and metallicities of these galaxies,
and elucidating their star formation histories, as a function of luminosity and
morphology. We thereby extend the work of Kennicutt (1992a) to lower luminosity
systems. We anticipate that our study will be useful as a benchmark for studies
of galaxies at high redshift.
We describe the observing, data reduction and calibration techniques, and
demonstrate that our spectrophotometry agrees well with that of Kennicutt. The
spectra span the range 3550--7250 A at a resolution (FWHM) of ~6 A, and have an
overall relative spectrophotometric accuracy of +/- 6 per cent. We present a
spectrophotometric atlas of integrated and nuclear rest-frame spectra, as well
as tables of equivalent widths and synthetic colors.
We study the correlations of galaxy properties determined from the spectra
and images. Our findings include: (1) galaxies of a given morphological class
display a wide range of continuum shapes and emission line strengths if a broad
range of luminosities are considered, (2) emission line strengths tend to in-
crease and continua tend to get bluer as the luminosity decreases, and (3) the
scatter on the general correlation between nuclear and integrated H_alpha
emission line strengths is large.Comment: Accepted for publication in ApJS (scheduled for Vol.127, 2000 March);
63 pages, LateX, 9 figures and 6 tables included, a spectrophotometric atlas
is provided as GIF images, fig 1 as a JPEG image, in a single tar-file; a
full 600 dpi version is available at http://www.astro.rug.nl/~nfgs
China Maritime Report No. 5: China\u27s Dreadnought? The PLA Navy\u27s Type 055 Cruiser and Its Implications for the Future Maritime Security Environment
China’s naval modernization, a process that has been underway in earnest for three decades, is now hitting its stride. The advent of the Type 055 cruiser firmly places the PLAN among the world’s very top naval services. This study, which draws upon a unique set of Chinese-language writings, offers the first comprehensive look at this new, large surface combatant. It reveals a ship that has a stealthy design, along with a potent and seemingly well-integrated sensor suite. With 112 VLS cells, moreover, China’s new cruiser represents a large magazine capacity increase over legacy surface combatants. Its lethality might also be augmented as new, cutting edge weaponry could later be added to the accommodating design. This vessel, therefore, provides very substantial naval capability to escort Chinese carrier groups, protect Beijing’s long sea lanes, and take Chinese naval diplomacy to an entirely new and daunting level. Even more significant perhaps, the Type 055 will markedly expand the range and firepower of the PLAN and this could substantially impact myriad potential conflict scenarios, from the Indian Ocean to the Korean Peninsula and many in between. This study of Type 055 development, moreover, does yield evidence that Chinese naval strategists are acutely aware of major dilemmas confronting the U.S. Navy surface fleet.https://digital-commons.usnwc.edu/cmsi-maritime-reports/1004/thumbnail.jp
Non-Gaussianity from Self-Ordering Scalar Fields
The Universe may harbor relics of the post-inflationary epoch in the form of
a network of self-ordered scalar fields. Such fossils, while consistent with
current cosmological data at trace levels, may leave too weak an imprint on the
cosmic microwave background and the large-scale distribution of matter to allow
for direct detection. The non-Gaussian statistics of the density perturbations
induced by these fields, however, permit a direct means to probe for these
relics. Here we calculate the bispectrum that arises in models of self-ordered
scalar fields. We find a compact analytic expression for the bispectrum,
evaluate it numerically, and provide a simple approximation that may be useful
for data analysis. The bispectrum is largest for triangles that are aligned
(have edges ) as opposed to the local-model
bispectrum, which peaks for squeezed triangles (), and
the equilateral bispectrum, which peaks at . We
estimate that this non-Gaussianity should be detectable by the Planck satellite
if the contribution from self-ordering scalar fields to primordial
perturbations is near the current upper limit.Comment: 11 pages, 1 figur
Exact Perturbations for inflation with smooth exit
Toy models for the Hubble rate or the scalar field potential have been used
to analyze the amplification of scalar perturbations through a smooth
transition from inflation to the radiation era. We use a Hubble rate that
arises consistently from a decaying vacuum cosmology, which evolves smoothly
from nearly de Sitter inflation to radiation domination. We find exact
solutions for super-horizon perturbations (scalar and tensor), and for
sub-horizon perturbations in the vacuum- and radiation-dominated eras. The
standard conserved quantity for super-horizon scalar perturbations is exactly
constant for growing modes, and zero for the decaying modes.Comment: Minor errors correcte
Frictional cooling of positively charged particles
One of the focuses of research and development towards the construction of a
muon collider is muon beam preparation. Simulation of frictional cooling shows
that it can achieve the desired emittance reduction to produce high-luminosity
muon beams. We show that for positively charged particles, charge exchange
interactions necessitate significant changes to schemes previously developed
for negatively charged particles. We also demonstrate that foil-based schemes
are not viable for positive particles.Comment: 15 pages, 6 figure
Human Cep192 Is Required for Mitotic Centrosome and Spindle Assembly
SummaryAs cells enter mitosis, centrosomes dramatically increase in size and ability to nucleate microtubules. This process, termed centrosome maturation, is driven by the accumulation and activation of γ-tubulin and other proteins that form the pericentriolar material on centrosomes during G2/prophase. Here, we show that the human centrosomal protein, Cep192 (centrosomal protein of 192 kDa), is an essential component of the maturation machinery. Specifically, we have found that siRNA depletion of Cep192 results in a complete loss of functional centrosomes in mitotic but not interphase cells. In mitotic cells lacking Cep192, microtubules become organized around chromosomes but rarely acquire stable bipolar configurations. These cells contain normal numbers of centrioles but cannot assemble γ-tubulin, pericentrin, or other pericentriolar proteins into an organized PCM. Alternatively, overexpression of Cep192 results in the formation of multiple, extracentriolar foci of γ-tubulin and pericentrin. Together, our findings support the hypothesis that Cep192 stimulates the formation of the scaffolding upon which γ-tubulin ring complexes and other proteins involved in microtubule nucleation and spindle assembly become functional during mitosis
SHELS: The Hectospec Lensing Survey
The Smithsonian Hectospec Lensing Survey (SHELS) combines a large deep
complete redshift survey with a weak lensing map from the Deep Lens Survey
(Wittman et al. 2002; 2005). We use maps of the velocity dispersion based on
systems identified in the redshift survey to compare the three-dimensional
matter distribution with the two-dimensional projection mapped by weak lensing.
We demonstrate directly that the lensing map images the three-dimensional
matter distribution obtained from the kinematic data.Comment: Astrophysical Journal Letters 9pages, 3 figures, accepted, corrected
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The K2 Mission: Characterization and Early results
The K2 mission will make use of the Kepler spacecraft and its assets to
expand upon Kepler's groundbreaking discoveries in the fields of exoplanets and
astrophysics through new and exciting observations. K2 will use an innovative
way of operating the spacecraft to observe target fields along the ecliptic for
the next 2-3 years. Early science commissioning observations have shown an
estimated photometric precision near 400 ppm in a single 30 minute observation,
and a 6-hour photometric precision of 80 ppm (both at V=12). The K2 mission
offers long-term, simultaneous optical observation of thousands of objects at a
precision far better than is achievable from ground-based telescopes. Ecliptic
fields will be observed for approximately 75-days enabling a unique exoplanet
survey which fills the gaps in duration and sensitivity between the Kepler and
TESS missions, and offers pre-launch exoplanet target identification for JWST
transit spectroscopy. Astrophysics observations with K2 will include studies of
young open clusters, bright stars, galaxies, supernovae, and asteroseismology.Comment: 25 pages, 11 figures, Accepted to PAS
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