4,622 research outputs found
Rotating solenoidal perfect fluids of Petrov type D
We prove that aligned Petrov type D perfect fluids for which the vorticity
vector is not orthogonal to the plane of repeated principal null directions and
for which the magnetic part of the Weyl tensor with respect to the fluid
velocity has vanishing divergence, are necessarily purely electric or locally
rotationally symmetric. The LRS metrics are presented explicitly.Comment: 6 pages, no figure
Spectro-Morphology of Galaxies
We present a quantitative method to classify galaxies, based on
multi-wavelength data and elaborated from the properties of nearby galaxies.
Our objective is to define an evolutionary method that can be used for low and
high redshift objects. We estimate the concentration of light (C) at the galaxy
center and the 180 degree-rotational asymmetry (A), computed at several
wavelengths, from ultraviolet (UV) to I-band. The variation of the indices of
concentration and asymmetry with the wavelength reflects the proportion and the
distribution of young and old stellar populations in galaxies. In general C is
found to decrease from optical to UV, and A is found to increase from optical
to UV: the patchy appearance of galaxies in UV with no bulge is often very
different from their counterpart at optical wavelengths, with prominent bulges
and more regular disks. The variation of C and A with the wavelength is
quantified. By this way, we are able to distinguish five types of galaxies that
we call spectro-morphological types: compact, ringed, spiral, irregular and
central-starburst galaxies, which can be differentiated by the repartition of
their stellar populations. We discuss in detail the morphology of galaxies of
the sample, and describe the morphological characteristics of each
spectro-morphological type. We apply spectro-morphology to three objects at a
redshift z=1 in the Hubble Deep Field North, that gives encouraging results for
applications to large samples of high-redshift galaxies. This method of
morphological classification could be used to study the evolution of the
morphology with the redshift and is expected to bring observational constraints
on scenarios of galaxy evolution.Comment: Accepted for publication in Astronomy & Astrophysic
Political market and regulatory uncertainty: Insights and implications for integrated strategy
Managers can craft effective integrated strategy by properly assessing regulatory uncertainty. Leveraging the existing political markets literature, we predict regulatory uncertainty from the novel interaction of demand and supply side rivalries across a range of political markets. We argue for two primary drivers of regulatory uncertainty: ideology-motivated interests opposed to the firm and a lack of competition for power among political actors supplying public policy. We align three, previously disparate dimensions of nonmarket strategy - profile level, coalition breadth, and pivotal target - to levels of regulatory uncertainty. Through this framework, we demonstrate how and when firms employ different nonmarket strategies. To illustrate variation in nonmarket strategy across levels of regulatory uncertainty, we analyze several market entry decisions of foreign firms operating in the global telecommunications sector
The Grothendieck Group of a Quantum Projective Space Bundle
We compute the Grothendieck group K_0 of non-commutative analogues of quantum
projective space bundles. Our results specialize to give the Grothendieck
groups of non-commutative analogues of projective spaces, and specialize to
recover the Grothendieck group of a usual projective space bundle over a
regular noetherian separated scheme. As an application we develop an
intersection theory for the quantum ruled surfaces defined by Van den Bergh.Comment: This paper is being replaced so I can correct the metadata, the
title! I (Paul) spelled Grothendieck's name incorrectly. The paper is being
reposted with the journal reference and doi added to the metadat
Near-Infrared Adaptive Optics Imaging of the Central Regions of Nearby Sc Galaxies: I. M33
Near-infrared images obtained with the Canada-France-Hawaii Telescope (CFHT)
Adaptive Optics Bonnette (AOB) are used to investigate the stellar content
within 18 arcsec of the center of the Local Group spiral galaxy M33. AGB stars
with near-infrared spectral-energy distributions similar to those of giants in
the solar neighborhood and Baade's Window are detected over most of the field.
The bolometric luminosity function (LF) of these stars has a discontinuity near
M_{bol} = -5.25, and comparisons with evolutionary tracks suggest that most of
the AGB stars formed in a burst of star formation 1 - 3 Gyr in the past. The
images are also used to investigate the integrated near-infrared photometric
properties of the nucleus and the central light concentration. The nucleus is
bluer than the central light concentration, in agreement with previous studies
at visible wavelengths. The CO index of the central light concentration 0.5
arcsec from the galaxy center is 0.05, which corresponds to [Fe/H] = -1.2 for
simple stellar systems. Hence, the central light concentration could not have
formed from the chemically-enriched material that dominates the present-day
inner disk of M33.Comment: 23 pages of text + 11 figures; to appear in A
The spatial and age distribution of stellar populations in DDO 190
The spatial distribution of stellar populations, the star formation history,
and other properties of the dIrr galaxy DDO 190 have been analyzed using
color--magnitude diagrams (CMDs) of about 3900 resolved stars and the Ha fluxes
of HII regions. From the mean color index of the red giant branch, a mean
metallicity [Fe/H]=-2.0 is obtained. The I magnitude of the TRGB has been used
to estimate the distance. DDO 190 is 2.9+/-0.2 Mpc from the Milky Way, 2.1 Mpc
from the M 94 group (CnV-I), 2.4 Mpc from the M 81 group and 2.9 Mpc from the
barycenter of the Local Group, all indicating that it is an isolated, field
galaxy. The surface-brightness distribution of the galaxy is well fitted by
ellipses of ellipticity e=1-a/b=0.1 and P.A.=82deg. The radial star density
distribution follows an exponential law of scale length a=43."4, corresponding
to 611 pc. The Holmberg semi-major axis to mu_B=26.5 is estimated to be
r^B_(26.5)=3.'0. Stellar populations of different ages in DDO 190 show strong
spatial decoupling, the oldest population appearing much more extended than the
youngest. Stars younger than 0.1 Gyr occupy only the central 40'' (0.55 kpc);
stars younger than a few (~4) Gyr extend out to ~80'' (125 kpc), and for larger
galactocentric distances only older stars seem to be present. This behavior is
found in all the dIrr galaxies for which spatially extended studies have been
performed and could be related with the kinematical history of the galaxy.Comment: To be published in the AJ. 29 pages, 13 figure
Long Gamma-Ray Bursts and Type Ic Core Collapse Supernovae Have Similar Locations in Hosts
When the afterglow fades at the site of a long-duration gamma-ray burst
(LGRB), Type Ic supernovae (SN Ic) are the only type of core collapse supernova
observed. Recent work found that a sample of LGRB in high-redshift galaxies had
different environments from a collection of core-collapse environments, which
were identified from their colors and light curves. LGRB were in the brightest
regions of their hosts, but the core-collapse sample followed the overall
distribution of the galaxy light. Here we examine 504 supernovae with types
assigned based on their spectra that are located in nearby (z < 0.06) galaxies
for which we have constructed surface photometry from the Sloan Digital Sky
Survey (SDSS). The distributions of the thermonuclear supernovae (SN Ia) and
some varieties of core-collapse supernovae (SN II and SN Ib) follow the galaxy
light, but the SN Ic (like LGRB) are much more likely to erupt in the brightest
regions of their hosts. The high-redshift hosts of LGRB are overwhelmingly
irregulars, without bulges, while many low redshift SN Ic hosts are spirals
with small bulges. When we remove the bulge light from our low-redshift sample,
the SN Ic and LGRB distributions agree extremely well. If both LGRB and SN Ic
stem from very massive stars, then it seems plausible that the conditions
necessary for forming SN Ic are also required for LGRB. Additional factors,
including metallicity, may determine whether the stellar evolution of a massive
star leads to a LGRB with an underlying broad-lined SN Ic, or simply a SN Ic
without a gamma-ray burst.Comment: Accepted by the Astrophysical Journal, 12 pages, 3 tables, 4 figures,
SN sample size increases from 263 to 504 in v2, varying host magnitude and
distance shown not to introduce systematic error in measurement
Dynamical Mass Estimates for Five Young Massive Stellar Clusters
We have obtained high-dispersion spectra for four massive star clusters in
the dwarf irregular galaxies NGC 4214 and NGC 4449, using the HIRES
spectrograph on the Keck I telescope. Combining the velocity dispersions of the
clusters with structural parameters and photometry from images taken with HST,
we estimate mass-to-light ratios and compare these with simple stellar
population (SSP) models in order to constrain the stellar mass functions (MFs).
For all clusters we find mass-to-light ratios which are similar to or slightly
higher than for a Kroupa MF, and thereby rule out any MF which is deficient in
low-mass stars compared to a Kroupa-type MF. The four clusters have virial
masses ranging between 2.1E5 Msun and 1.5E6 Msun, half-light radii between 3.0
and 5.2 pc, estimated core densities in the range 2E3 Msun pc^-3 to 2E5 Msun
pc^-3 and ages between 200 Myr and 800 Myr. We also present new high-dispersion
near-infrared spectroscopy for a luminous young (about 15 Myr) cluster in the
nearby spiral galaxy NGC 6946, which we have previously observed with HIRES.
The new measurements in the infrared agree well with previous estimates of the
velocity dispersion, yielding a mass of about 1.7E6 Msun. The properties of the
clusters studied here are all consistent with the clusters being young versions
of the old globular clusters found around all major galaxies.Comment: 30 pages, including 7 figures and 9 tables. Corrected an error in
Table 2: The colors listed for N6946-1447 were not reddening corrected. This
also affected Table 9 and Fig 2, 6 and
Some Musings on Galaxy Morphology
Careful inspection of large-scale photographs of Shapley-Ames galaxies seems
to show a smooth transition between the morphological characteristics of
galaxies located on the narrow red, and on the broad blue, sequences in the
galaxian color-magnitude diagram. In other words there does not appear to be a
dichotomy between blue and red galaxies. Both the colors and the morphologies
of galaxies are found to correlate strongly with their environments. Red and
early-type Shapley-Ames galaxies are dominant in clusters, whereas blue
late-type star forming objects dominate the general field. Interestingly the
colors and morphologies of galaxies in small groups resemble the field and
differ from those in clusters. As noted by Baade the presence of dust and star
formation are very closely correlated, except in a few galaxies that probably
had unusual evolutionary histories. Over the entire range from S0 to Sc there
is no significant difference between the integrated colors of normal and barred
objects suggesting that the formation of a bar does not significantly affect
the stellar evolutionary history of a galaxy.Comment: 13 figures; Astronomical Journal in pres
Reduced mortality and subsequent fracture risk associated with oral bisphosphonate recommendation in a fracture liaison service setting: A prospective cohort study
Objective: Osteoporotic fragility fractures, that are common in men and women, signal increased risk of future fractures and of premature mortality. Less than one-third of postmenopausal women and fewer men are prescribed active treatments to reduce fracture risk. Therefore, in this study the association of oral bisphosphonate recommendation with subsequent fracture and mortality over eight years in a fracture liaison service setting was analysed.
Materials and methods: In this prospective cohort study, 5011 men and women aged \u3e50 years, who sustained a clinical fracture, accepted the invitation to attend the fracture liaison service of the West Glasgow health service between 1999 and 2007. These patients were fully assessed and all were recommended calcium and vitamin D. Based on pre-defined fracture risk criteria, 2534 (50.7%) patients were additionally also recommended oral bisphosphonates. Mortality and subsequent fracture risk were the pre-defined outcomes analysed using Cox proportional hazard models.
Results: Those recommended bisphosphonates were more often female (82.9 vs. 72.4%), were older (73.4 vs. 64.4 years), had lower bone mineral density T-score (-3.1 vs. -1.5) and more had sustained hip fractures (21.7 vs. 6.2%; p \u3c 0.001). After adjustments, patients recommended bisphosphonates had lower subsequent fracture risk (Hazard Ratio (HR): 0.60; 95% confidence interval (CI): 0.49±0.73) and lower mortality risk (HR: 0.79, 95%CI: 0.64±0.97).
Conclusion: Of the patients, who are fully assessed after a fracture at the fracture liaison service, those with higher fracture risk and a recommendation for bisphosphonates had worse baseline characteristics. However, after adjusting for these differences, those recommended bisphosphonate treatment had a substantially lower risk for subsequent fragility fracture and lower risk for mortality. These community-based data indicate the adverse public health outcomes and mortality impacts of the current low treatment levels post fracture could be improved by bisphosphonate recommendation for both subsequent fracture and mortality
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