97 research outputs found
Interstellar Turbulence: II. Energy Spectra of Molecular Regions in the Outer Galaxy
The multivariate tool of Principal Component Analysis (PCA) is applied to 23
fields in the FCRAO CO Survey of the Outer Galaxy. PCA enables the
identification of line profile differences which are assumed to be generated
from fluctuations within a turbulent velocity field. The variation of these
velocity differences with spatial scale within a molecular region is described
by a singular power law, delta v= c L^alpha which can be used as a powerful
diagnostic to turbulent motions. For the ensemble of 23 fields, we find a mean
value alpha = 0.62 +- 0.11. From a recent calibration of this method using
fractal Brownian motion simulations (Brunt & Heyer 2001), the measured velocity
difference-size relationship corresponds to an energy spectrum, E(k), which
varies as k^-beta, where beta = 2.17 +- 0.31. We compare our results to both
decaying and forced hydrodynamic simulations of turbulence. We conclude that
energy must be continually injected into the regions to replenish that lost by
dissipative processes such as shocks. The absence of large, widely distributed
shocks within the targeted fields suggests that the energy is injected at
spatial scales less than several pc.Comment: 24 pages, 10 figures, accepted by Ap
The Global Evolution of Giant Molecular Clouds II: The Role of Accretion
We present virial models for the global evolution of giant molecular clouds.
Focusing on the presence of an accretion flow, and accounting for the amount of
mass, momentum, and energy supplied by accretion and star formation feedback,
we are able to follow the growth, evolution, and dispersal of individual giant
molecular clouds. Our model clouds reproduce the scaling relations observed in
both galactic and extragalactic clouds. We find that accretion and star
formation contribute contribute roughly equal amounts of turbulent kinetic
energy over the lifetime of the cloud. Clouds attain virial equilibrium and
grow in such a way as to maintain roughly constant surface densities, with
typical surface densities of order 50 - 200 Msun pc^-2, in good agreement with
observations of giant molecular clouds in the Milky Way and nearby external
galaxies. We find that as clouds grow, their velocity dispersion and radius
must also increase, implying that the linewidth-size relation constitutes an
age sequence. Lastly, we compare our models to observations of giant molecular
clouds and associated young star clusters in the LMC and find good agreement
between our model clouds and the observed relationship between H ii regions,
young star clusters, and giant molecular clouds.Comment: 23 Pages, 9 Figures. Accepted to Ap
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Post-carotid endarterectomy neurocognitive decline is associated with cerebral blood flow asymmetry on post-operative magnetic resonance perfusion brain scans
Objective: Up to 25% of patients experience subtle declines in post-operative neurocognitive function following, otherwise uncomplicated, carotid endarterectomy (CEA). We sought to determine if post-CEA neurocognitive deficits are associated with cerebral blood flow (CBF) abnormalities on post-operative MR perfusion brain scans. Methods: We enrolled 22 CEA patients to undergo a battery of neuropsychometric tests pre-operatively and on post-operative day 1 (POD 1). Neurocognitive dysfunction was defined as a two standard deviation decline in performance in comparison to a similarly aged control group of lumbar laminectomy patients. All patients received MR perfusion brain scans on POD 1 that were analysed for asymmetries in CBF distribution. One patient experienced a transient ischemic attack within 24 hours before the procedure and was excluded from our analysis. Results: Twenty-nine percent of CEA patients demonstrated neurocognitive dysfunction on POD 1. One hundred percent of those patients with cognitive deficits demonstrated CBF asymmetry, in contrast to only 27% of those patients without cognitive impairment. Post-CEA cognitive dysfunction was significantly associated with CBF abnormalities (RR=3.75, 95% CI: 1.62-8.67, p=0.004). Conclusion: Post-CEA neurocognitive dysfunction is significantly associated with post-operative CBF asymmetry. These results support the hypothesis that post-CEA cognitive impairment is caused by cerebral hemodynamic changes. Further work exploring the relationship between CBF and post-CEA cognitive dysfunction is needed
Turbulent Gas Flows in the Rosette and G216-2.5 Molecular Clouds: Assessing Turbulent Fragmentation Descriptions of Star Formation
The role of turbulent fragmentation in regulating the efficiency of star
formation in interstellar clouds is examined from new wide field imaging of
12CO and 13CO J=1-0 emission from the Rosette and G216-2.5 molecular clouds.
The Rosette molecular cloud is a typical star forming giant molecular cloud and
G215-2.5 is a massive molecular cloud with no OB stars and very little low mass
star formation. The properties of the turbulent gas flow are derived from the
set of eigenvectors and eigenimages generated by Principal Component Analysis
of the spectroscopic data cubes. While the two clouds represent quite divergent
states of star formation activity, the velocity structure functions for both
clouds are similar. The sonic scale, lambda_S, defined as the spatial scale at
which turbulent velocity fluctuations are equivalent to the local sound speed,
and the turbulent Mach number evaluated at 1 pc, M_{1pc}, are derived for an
ensemble of clouds including the Rosette and, G216-2.5 regions that span a
large range in star formation activity. We find no evidence for the positive
correlations between these quantities and the star formation efficiency, that
are predicted by turbulent fragmentation models. A correlation does exist
between the star formation efficiency and the sonic scale for a subset of
clouds with L_{FIR}/M(H_2) > 1 that are generating young stellar clusters.
Turbulent fragmentation must play a limited and non-exclusive role in
determining the yield of stellar masses within interstellar clouds.Comment: Accepted by ApJ, 22 pages, 7 figure
Resolving Giant Molecular Clouds in NGC 300: : A First Look with the Submillimeter Array
Christopher M. Faesi, et al, 'RESOLVING GIANT MOLECULAR CLOUDS IN NGC 300: A FIRST LOOK WITH THE SUBMILLIMETER ARRAY', The Astrophysical Journal, Vol. 821(2) (16 pp), April 2016. doi:10.3847/0004-637X/821/2/125. © 2016. The American Astronomical Society. All rights reserved.We present the first high angular resolution study of giant molecular clouds (GMCs) in the nearby spiral galaxy NGC 300, based on observations from the Submillimeter Array (SMA). We target eleven 500 pc-sized regions of active star formation within the galaxy in the CO(J=2-1) line at 40 pc spatial and 1 km/s spectral resolution and identify 45 individual GMCs. We characterize the physical properties of these GMCs, and find that they are similar to GMCs in the disks of the Milky Way and other nearby spiral galaxies. For example, the GMC mass spectrum in our sample has a slope of 1.80+/-0.07. Twelve clouds are spatially resolved by our observations, of which ten have virial mass estimates that agree to within a factor of two with mass estimates derived directly from CO integrated intensity, suggesting that the majority of these GMCs are bound. The resolved clouds show consistency with Larson's fundamental relations between size, linewidth, and mass observed in the Milky Way. We find that the linewidth scales with the size as DeltaV ~ R^(0.52+/-0.20), and the median surface density in the subsample is 54 Msun/pc^(-2). We detect 13CO in four GMCs and find a mean 12CO/13CO flux ratio of 6.2. Our interferometric observations recover between 30% and 100% of the integrated intensity from the APEX single dish CO observations of Faesi et al. 2014, suggesting the presence of low-mass GMCs and/or diffuse gas below our sensitivity limit. The fraction of APEX emission recovered increases with the SMA total intensity as well as with the star formation rate.Peer reviewe
A Simple Perspective on the Mass-Area Relationship in Molecular Clouds
Despite over 30 years of study, the mass-area relationship within and among
clouds is still poorly understood both observationally and theoretically.
Modern extinction datasets should have sufficient resolution and dynamic range
to characterize this relationship for nearby molecular clouds, although recent
papers using extinction data seem to yield different interpretations regarding
the nature and universality of this aspect of cloud structure. In this paper we
try to unify these various results and interpretations by accounting for the
different ways cloud properties are measured and analyzed. We interpret the
mass-area relationship in terms of the column density distribution function and
its possible variation within and among clouds. We quantitatively characterize
regional variations in the column density PDF. We show that structures both
within and among clouds possess the same degree of "universality", in that
their PDF means do not systematically scale with structure size. Because of
this, mass scales linearly with area.Comment: 10 pages, 8 figures, MNRAS in pres
Bar Evolution Over the Last Eight Billion Years: A Constant Fraction of Strong Bars in GEMS
One third of present-day spirals host optically visible strong bars that
drive their dynamical evolution. However, the fundamental question of how bars
evolve over cosmological times has yet to be addressed, and even the frequency
of bars at intermediate redshifts remains controversial. We investigate the
frequency of bars out to z~1.0 drawing on a sample of 1590 galaxies from the
GEMS survey, which provides morphologies from HST ACS two-color images, and
highly accurate redshifts from the COMBO-17 survey. We identify spiral galaxies
using the Sersic index, concentration parameter, and rest-frame color. We
characterize bars and disks by fitting ellipses to F606W and F850LP images,
taking advantage of the two bands to minimize bandpass shifting. We exclude
highly inclined (i>60 deg) galaxies to ensure reliable morphological
classifications, and apply completeness cuts of M_v <= -19.3 and -20.6. More
than 40% of the bars that we detect have semi major axes a<0.5" and would be
easily missed in earlier surveys without the small PSF of ACS. The bars that we
can reliably detect are fairly strong (with ellipticities e>=0.4) and have a in
the range ~1.2-13 kpc. We find that the optical fraction of such strong bars
remains at ~(30% +- 6%) from the present-day out to look-back times of 2-6 Gyr
(z~0.2-0.7) and 6-8 Gyr (z~0.7-1.0); it certainly shows no sign of a drastic
decline at z>0.7. Our findings of a large and similar bar fraction at these
three epochs favor scenarios in which cold gravitationally unstable disks are
already in place by z~1, and where on average bars have a long lifetime (well
above 2 Gyr). The distributions of structural bar properties in the two slices
are, however, not statistically identical and therefore allow for the
possibility that the bar strengths and sizes may evolve over time.Comment: Accepted by ApJ Letters, to appear in Nov 2004 issue. Minor
revisions,updated reference
Strategy and rationale for urine collection protocols employed in the NEPTUNE study
Abstract
Background
Glomerular diseases are potentially fatal, requiring aggressive interventions and close monitoring. Urine is a readily-accessible body fluid enriched in molecular signatures from the kidney and therefore particularly suited for routine clinical analysis as well as development of non-invasive biomarkers for glomerular diseases.
Methods
The Nephrotic Syndrome Study Network (NEPTUNE; ClinicalTrials.gov Identifier NCT01209000) is a North American multicenter collaborative consortium established to develop a translational research infrastructure for nephrotic syndrome. This includes standardized urine collections across all participating centers for the purpose of discovering non-invasive biomarkers for patients with nephrotic syndrome due to minimal change disease, focal segmental glomerulosclerosis, and membranous nephropathy. Here we describe the organization and methods of urine procurement and banking procedures in NEPTUNE.
Results
We discuss the rationale for urine collection and storage conditions, and demonstrate the performance of three experimental analytes (neutrophil gelatinase-associated lipocalin [NGAL], retinol binding globulin, and alpha-1 microglobulin) under these conditions with and without urine preservatives (thymol, toluene, and boric acid). We also demonstrate the quality of RNA and protein collected from the urine cellular pellet and exosomes.
Conclusions
The urine collection protocol in NEPTUNE allows robust detection of a wide range of proteins and RNAs from urine supernatant and pellets collected longitudinally from each patient over 5Â years. Combined with the detailed clinical and histopathologic data, this provides a unique resource for exploration and validation of new or accepted markers of glomerular diseases.
Trial registration
ClinicalTrials.gov Identifier
NCT01209000http://deepblue.lib.umich.edu/bitstream/2027.42/116023/1/12882_2015_Article_185.pd
A Minimum Column Density of 1 g cm^-2 for Massive Star Formation
Massive stars are very rare, but their extreme luminosities make them both
the only type of young star we can observe in distant galaxies and the dominant
energy sources in the universe today. They form rarely because efficient
radiative cooling keeps most star-forming gas clouds close to isothermal as
they collapse, and this favors fragmentation into stars <~1 Msun. Heating of a
cloud by accreting low-mass stars within it can prevent fragmentation and allow
formation of massive stars, but what properties a cloud must have to form
massive stars, and thus where massive stars form in a galaxy, has not yet been
determined. Here we show that only clouds with column densities >~ 1 g cm^-2
can avoid fragmentation and form massive stars. This threshold, and the
environmental variation of the stellar initial mass function (IMF) that it
implies, naturally explain the characteristic column densities of massive star
clusters and the difference between the radial profiles of Halpha and UV
emission in galactic disks. The existence of a threshold also implies that
there should be detectable variations in the IMF with environment within the
Galaxy and in the characteristic column densities of massive star clusters
between galaxies, and that star formation rates in some galactic environments
may have been systematically underestimated.Comment: Accepted for publication in Nature; Nature manuscript style; main
text: 14 pages, 3 figures; supplementary text: 8 pages, 1 figur
The ALMA view of GMCs in NGC 300 : Physical Properties and Scaling Relations at 10 pc Resolution
This is an author-created, un-copyedited version of an article accepted for published in The Astrophysical Journal. The Version of Record is available online at https://doi.org/10.3847/1538-4357/aaad60We have conducted a 12CO(2-1) survey of several molecular gas complexes in the vicinity of H ii regions within the spiral galaxy NGC 300 using the Atacama Large Millimeter Array (ALMA). Our observations attain a resolution of 10 pc and 1 , sufficient to fully resolve giant molecular clouds (GMCs) and the highest obtained to date beyond the Local Group. We use the CPROPS algorithm to identify and characterize 250 GMCs across the observed regions. GMCs in NGC 300 appear qualitatively and quantitatively similar to those in the Milky Way disk: they show an identical scaling relationship between size R and linewidth ÎV (ÎV â R 0.48±0.05), appear to be mostly in virial equilibrium, and are consistent with having a constant surface density of about 60 pc -2. The GMC mass spectrum is similar to those in the inner disks of spiral galaxies (including the Milky Way). Our results suggest that global galactic properties such as total stellar mass, morphology, and average metallicity may not play a major role in setting GMC properties, at least within the disks of galaxies on the star-forming main sequence. Instead, GMC properties may be more strongly influenced by local environmental factors such as the midplane disk pressure. In particular, in the inner disk of NGC 300, we find this pressure to be similar to that in the local Milky Way but markedly lower than that in the disk of M51, where GMCs are characterized by systematically higher surface densities and a higher coefficient for the size-linewidth relation.Peer reviewedFinal Accepted Versio
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