13,332 research outputs found
The Kinematic and Spatial Deployment of Compact, Isolated High-Velocity Clouds
We have identified a class of high-velocity clouds which are compact and
apparently isolated. The clouds are compact in that they have angular sizes
less than 2 degrees FWHM. They are isolated in that they are separated from
neighboring emission by expanses where no emission is seen to the detection
limit of the available data. Candidates for inclusion in this class were
extracted from the Leiden/Dwingeloo HI survey of Hartmann & Burton and from the
Wakker & van Woerden catalogue of high-velocity clouds. The candidates were
subject to independent confirmation using either the 25-meter telescope in
Dwingeloo or the 140-foot telescope in Green Bank. We argue that the resulting
list, even if incomplete, is sufficiently representative of the ensemble of
compact, isolated HVCs - CHVCs - that the characteristics of their disposition
on the sky, and of their kinematics, are revealing of some physical aspects of
the class. The CHVCs are in fact distributed quite uniformly across the sky. A
global search for the reference frame which minimizes the velocity dispersion
of the ensemble returns the Local Group Standard of Rest with high confidence.
The CHVCs are not stationary with respect to this reference frame but have a
mean infall velocity of 100 km/s. These properties are strongly suggestive of a
population which has as yet had little interaction with the more massive Local
Group members. At a typical distance of about 1 Mpc these objects would have
sizes of about 15 kpc and gas masses, M_HI, of a few times 10^7 M_Sun,
corresponding to those of (sub-)dwarf galaxies. (abridged)Comment: 13 page LaTeX, requires aa.cls and rotate.sty, 5 GIF figures.
Accepted for publication in A&
HI Observations Towards the Sagittarius Dwarf Spheroidal Galaxy
We have measured the 21-cm line of Galactic HI over more than 50 square
degrees in the direction of the Sagittarius dwarf spheroidal galaxy. The data
show no evidence of HI associated with the dwarf spheroidal which might be
consider analogous to the Magellanic Stream as it is associated in both
position and velocity with the Large Magellanic Cloud. Nor do the HI data show
evidence for any disturbance in the Milky Way disk gas that can be
unambiguously assigned to interaction with the dwarf galaxy. The data shown
here limit the HI mass at the velocity of the Sagittarius dwarf to <7000 solar
masses over some 18 square degrees between Galactic latitudes -13 degrees and
-18 degrees.Comment: 5 pages, 4 figures; accepted for publication in Astronomy &
Astrophysic
Face pairing graphs and 3-manifold enumeration
The face pairing graph of a 3-manifold triangulation is a 4-valent graph
denoting which tetrahedron faces are identified with which others. We present a
series of properties that must be satisfied by the face pairing graph of a
closed minimal P^2-irreducible triangulation. In addition we present
constraints upon the combinatorial structure of such a triangulation that can
be deduced from its face pairing graph. These results are then applied to the
enumeration of closed minimal P^2-irreducible 3-manifold triangulations,
leading to a significant improvement in the performance of the enumeration
algorithm. Results are offered for both orientable and non-orientable
triangulations.Comment: 30 pages, 57 figures; v2: clarified some passages and generalised the
final theorem to the non-orientable case; v3: fixed a flaw in the proof of
the conical face lemm
3D MHD Modeling of the Gaseous Structure of the Galaxy: Synthetic Observations
We generated synthetic observations from the four-arm model presented in
Gomez & Cox (2004) for the Galactic ISM in the presence of a spiral
gravitational perturbation. We found that velocity crowding and diffusion have
a strong effect in the l-v diagram. The v-b diagram presents structures at the
expected spiral arm velocities, that can be explained by the off-the-plane
structure of the arms presented in previous papers of this series. Such
structures are observed in the Leiden/Dwingeloo HI survey. The rotation curve,
as measured from the inside of the modeled galaxy, shows similarities with the
observed one for the Milky Way Galaxy, although it has large deviations from
the smooth circular rotation corresponding to the background potential. The
magnetic field inferred from a synthetic synchrotron map shows a largely
circular structure, but with interesting deviations in the midplane due to
distortion of the field from circularity in the interarm regions.Comment: Accepted for publication in ApJ. Better quality figures in
http://www.astro.umd.edu/~gomez/publica/3d_galaxy-3.pd
Constructing sonified haptic line graphs for the blind student: first steps
Line graphs stand as an established information visualisation and analysis technique taught at various levels of difficulty according to standard Mathematics curricula. It has been argued that blind individuals cannot use line graphs as a visualisation and analytic tool because they currently primarily exist in the visual medium. The research described in this paper aims at making line graphs accessible to blind students through auditory and haptic media. We describe (1) our design space for representing line graphs, (2) the technology we use to develop our prototypes and (3) the insights from our preliminary work
Optimized Synthesis and Structural Characterization of the Borosilicate MCM-70
A structure analysis of the borosilicate zeolite MCM-70, whose synthesis had been patented in 2003, was reported in 2005. Unfortunately, that structure analysis was somewhat ambiguous. Anisotropic line broadening made it difficult to model the peak shape, some peaks in the electron density map could not be interpreted satisfactorily, the framework geometry was distorted, and MAS NMR results were partially contradictory. In an attempt to resolve some of these points, an optimization of the synthesis was undertaken, and the structure was reinvestigated. The structure was solved from synchrotron powder diffraction data collected on an as-synthesized sample (Pmn2_1, a = 13.3167(1) Ă…, b = 4.6604(1) Ă…, c = 8.7000(1) Ă…) using a powder charge-flipping algorithm. The framework topology, with a 1-dimensional, 10-ring channel system, is identical to the one previously reported. However, the B in this new sample was found to be ordered in the framework, fully occupying one of the four tetrahedral sites. Two extra-framework K^+ ion positions, each coordinated to five framework O atoms and one water molecule, were also found. The solid state ^(29)Si, ^(11)B and ^1H NMR results are fully consistent with this ordered structure
Giant Molecular Clouds are More Concentrated to Spiral Arms than Smaller Clouds
From our catalog of Milky Way molecular clouds, created using a temperature
thresholding algorithm on the Bell Laboratories 13CO Survey, we have extracted
two subsets:(1) Giant Molecular Clouds (GMCs), clouds that are definitely
larger than 10^5 solar masses, even if they are at their `near distance', and
(2) clouds that are definitely smaller than 10^5 solar masses, even if they are
at their `far distance'. The positions and velocities of these clouds are
compared to the loci of spiral arms in (l, v) space. The velocity separation of
each cloud from the nearest spiral arm is introduced as a `concentration
statistic'. Almost all of the GMCs are found near spiral arms. The density of
smaller clouds is enhanced near spiral arms, but some clouds (~10%) are
unassociated with any spiral arm. The median velocity separation between a GMC
and the nearest spiral arm is 3.4+-0.6 km/s, whereas the median separation
between smaller clouds and the nearest spiral arm is 5.5+-0.2 km/s.Comment: 11 pages, 3 figure
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