65 research outputs found
The properties of the Galactic bar implied by gas kinematics in the inner Milky Way
Longitude-velocity (l-V) diagrams of H I and CO gas in the inner Milky Way
have long been known to be inconsistent with circular motion in an axisymmetric
potential. Several lines of evidence suggest that the Galaxy is barred, and gas
flow in a barred potential could be consistent with the observed ``forbidden''
velocities and other features in the data. We compare the H I observations to
l-V diagrams synthesized from 2-D fluid dynamical simulations of gas flows in a
family of barred potentials. The gas flow pattern is very sensitive to the
parameters of the assumed potential, which allows us to discriminate among
models. We present a model that reproduces the outer contour of the H I l-V
diagram reasonably well; this model has a strong bar with a semimajor axis of
3.6 kpc, an axis ratio of approximately 3:1, an inner Lindblad resonance (ILR),
and a pattern speed of 42 km/s/kpc, and matches the data best when viewed from
34\deg to the bar major axis. The behavior of the models, combined with the
constraint that the shocks in the Milky Way bar should resemble those in
external barred galaxies, leads us to conclude that wide ranges of parameter
space are incompatible with the observations. In particular we suggest that the
bar must be fairly strong, must have an ILR, and cannot be too end-on, with the
bar major axis at 35\deg +/- 5\deg to the line of sight. The H I data exhibit
larger forbidden velocities over a wider longitude range than are seen in
molecular gas; this important difference is the reason our favored model
differs so significantly from other recently proposed models.Comment: 23 pages, 14 figures, 1 table, uses emulateapj and psfig, 640 kb.
Submitted to Ap
HIPASS High-Velocity Clouds: Properties of the Compact and Extended Populations
A catalog of Southern anomalous-velocity HI clouds at Decl. < +2 deg is
presented, based on data from the HI Parkes All-Sky Survey (HIPASS). The
improved sensitivity (5sigma: T_B = 0.04 K) and resolution (15.5') of the
HIPASS data results in a substantial increase in the number of individual
clouds (1956, as well as 41 galaxies) compared to previous surveys. Most
high-velocity emission features, HVCs, have a filamentary morphology and are
loosely organized into large complexes extending over tens of degrees. In
addition, 179 compact and isolated anomalous-velocity objects, CHVCs, are
identified based on their size and degree of isolation. 25% of the CHVCs
originally classified by Braun & Burton (1999) are reclassified. Both the
entire population of high-velocity emission features and the CHVCs alone have
typical HI masses of ~ 4.5 D(kpc)^2 solar masses and have similar slopes for
their column density and flux distributions. On the other hand, the CHVCs
appear to be clustered and the population can be broken up into three spatially
distinct groups, while the entire population of clouds is more uniformly
distributed with a significant percentage aligned with the Magellanic Stream.
The median velocities are V_GSR = -38 km/s for the CHVCs and -30 km/s for all
of the anomalous-velocity clouds. Based on the catalog sizes, high-velocity
features cover 19% of the southern sky and CHVCs cover 1%. (abridged)Comment: 32 pages, 26 figures in gif format, 2 ascii tables, to appear in the
Jan 2002 issue of The Astronomical Journal, high resolution version available
at http://origins.Colorado.EDU/~mputman/pubs.htm
The high-velocity cloud complex Galactic Center Negative as seen by EBHIS and GASS. I. Cloud catalog and global properties
Using Milky Way data of the new Effelsberg-Bonn HI Survey (EBHIS) and the
Galactic All-Sky Survey (GASS), we present a revised picture of the
high-velocity cloud (HVC) complex Galactic Center Negative (GCN). Owing to the
higher angular resolution of these surveys compared to previous studies (e.g.,
the Leiden Dwingeloo Survey), we resolve Complex GCN into lots of individual
tiny clumps, that mostly have relatively broad line widths of more than 15
km/s. We do not detect a diffuse extended counterpart, which is unusual for an
HVC complex. In total 243 clumps were identified and parameterized which allows
us to statistically analyze the data. Cold-line components (i.e., w < 7.5 km/s)
are found in about 5% only of the identified cloudlets. Our analysis reveals
that Complex GCN is likely built up of several subpopulations that do not share
a common origin. Furthermore, Complex GCN might be a prime example for warm-gas
accretion onto the Milky Way, where neutral HI clouds are not stable against
interaction with the Milky Way gas halo and become ionized prior to accretion.Comment: 13 pages, 19 figures; accepted by A&
An automated search for high-velocity clouds in the Leiden/Dwingeloo Survey
We describe an automated search through the Leiden/Dwingeloo HI Survey (LDS)
for high-velocity clouds north of Dec=-28 deg. From the general catalog we
extract a sample of isolated high-velocity clouds, CHVCs: anomalous-velocity HI
clouds which are sharply bounded in angular extent with no kinematic or spatial
connection to other HI features down to a limiting column density of
1.5*10^18cm^-2. This column density is an order of magnitude lower than the
critical HI column density, about 2*10^19cm^-2, where the ionized fraction is
thought to increase dramatically due to the extragalactic radiation field. As
such, these objects are likely to provide their own shielding to ionizing
radiation. Their small median angular size, of about 1 deg. FWHM, might then
imply substantial distances, since the partially ionized HI skin in a power-law
ionizing photon field has a typical exponential scale-length of 1 kpc. The
automated search algorithm has been applied to the HIPASS and to the
Leiden/Dwingeloo data sets. The results from the LDS are described here; Putman
et al. (2002) describe application of this algorithm to the HIPASS material. We
identify 67 CHVCs in the LDS which satisfy stringent requirements on isolation,
and an additional 49 objects which satisfy somewhat less stringent
requirements. Independent confirmation is available for all of these objects,
either from earlier data in the literature or from new observations made with
the Westerbork Synthesis Radio Telescope and reported here. The catalog
includes 54 of the 65 CHVCs listed by Braun and Burton (1999) on the basis of a
visual search of the LDS data.Comment: 17 pages, 19 png/jpeg figures, in review at A&A, (embedded PS figures
removed due to outdated astro-ph size limits
The Maastricht Acquisition Platform for Studying Mechanisms of Cell-Matrix Crosstalk (MAPEX): An Interdisciplinary and Systems Approach towards Understanding Thoracic Aortic Disease
Current management guidelines for ascending thoracic aortic aneurysms (aTAA) recommend intervention once ascending or sinus diameter reaches 5-5.5 cm or shows a growth rate of \u3e0.5 cm/year estimated from echo/CT/MRI. However, many aTAA dissections (aTAAD) occur in vessels with diameters below the surgical intervention threshold ofMoreover, during aTAA repair surgeons observe and experience considerable variations in tissue strength, thickness, and stiffness that appear not fully explained by patient risk factors. To improve the understanding of aTAA pathophysiology, we established a multi-disciplinary research infrastructure: The Maastricht acquisition platform for studying mechanisms of tissue-cell crosstalk (MAPEX). The explicit scientific focus of the platform is on the dynamic interactions between vascular smooth muscle cells and extracellular matrix (i.e., cell-matrix crosstalk), which play an essential role in aortic wall mechanical homeostasis. Accordingly, we consider pathophysiological influences of wall shear stress, wall stress, and smooth muscle cell phenotypic diversity and modulation. Co-registrations of hemodynamics and deep phenotyping at the histological and cell biology level are key innovations of our platform and are critical for understanding aneurysm formation and dissection at a fundamental level. The MAPEX platform enables the interpretation of the data in a well-defined clinical context and therefore has real potential for narrowing existing knowledge gaps. A better understanding of aortic mechanical homeostasis and its derangement may ultimately improve diagnostic and prognostic possibilities to identify and treat symptomatic and asymptomatic patients with existing and developing aneurysms
Alkali-activation potential of biomass-coal co-fired fly ash
Co-fired fly ash, derived from the co-combustion of coal and biomass, is examined as a potential precursor for geopolymers. Compared to a coal fly ash, two co-fired fly ashes have a lower vitreous content and higher carbon content, primarily due to differing combustion processing variables. As a result, binders produced with these co-fired fly ashes have reduced reaction potential. Nevertheless, compressive strengths are generally highest for all ashes activated with solutions with a molar ratio of SiOâ‚‚/(Naâ‚‚O + Kâ‚‚O) = 1, and these mixes reach the highest extent of reaction among those studied. Activation with sodium hydroxide solution forms zeolitic phases for all ashes. The thermal and dilatometric behavior of the coal and co-fired fly ash geopolymers is similar between equivalent mix designs. These results indicate that co-fired fly ashes can be viably used to form alkali-activated geopolymers, which is a new beneficial end-use for these emerging waste materials
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