1,866 research outputs found
Ionization of Infalling Gas
H-alpha emission from neutral halo clouds probes the radiation and
hydrodynamic conditions in the halo. Armed with such measurements, we can
explore how radiation escapes from the Galactic plane and how infalling gas can
survive a trip through the halo. The Wisconsin H-Alpha Mapper (WHAM) is one of
the most sensitive instruments for detecting and mapping optical emission from
the ISM. Here, we present recent results exploring the ionization of two
infallling high-velocity complexes. First, we report on our progress mapping
H-alpha emission covering the full extent of Complex A. Intensities are faint
(<100 mR; EM <0.2 pc cm^-6 but correlate on the sky and in velocity with 21-cm
emission. Second, we explore the ionized component of some Anti-Center Complex
clouds studied by Peek et al. (2007) that show dynamic shaping from interaction
with the Galactic halo.Comment: 4 pages, 2 figures; to appear in proceedings of "The Role of
Disk-Halo Interaction in Galaxy Evolution: Outflow vs Infall?" held in
Espinho, Portugal during 2008 Augus
Equiangular tight frames and fourth root seidel matrices
AbstractIn this paper we construct complex equiangular tight frames (ETFs). In particular, we study the grammian associated with an ETF whose off-diagonal entries consist entirely of fourth roots of unity. These ETFs are classified, and we also provide some computational techniques which give rise to previously undiscovered ETFs
A computer vision model for visual-object-based attention and eye movements
This is the post-print version of the final paper published in Computer Vision and Image Understanding. The published article is available from the link below. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. Copyright @ 2008 Elsevier B.V.This paper presents a new computational framework for modelling visual-object-based attention and attention-driven eye movements within an integrated system in a biologically inspired approach. Attention operates at multiple levels of visual selection by space, feature, object and group depending on the nature of targets and visual tasks. Attentional shifts and gaze shifts are constructed upon their common process circuits and control mechanisms but also separated from their different function roles, working together to fulfil flexible visual selection tasks in complicated visual environments. The framework integrates the important aspects of human visual attention and eye movements resulting in sophisticated performance in complicated natural scenes. The proposed approach aims at exploring a useful visual selection system for computer vision, especially for usage in cluttered natural visual environments.National Natural Science of Founda-
tion of Chin
LEDA 074886: A remarkable rectangular-looking galaxy
We report the discovery of an interesting and rare, rectangular-shaped
galaxy. At a distance of 21 Mpc, the dwarf galaxy LEDA 074886 has an absolute
R-band magnitude of -17.3 mag. Adding to this galaxy's intrigue is the presence
of an embedded, edge-on stellar disk (of extent 2R_{e,disk} = 12 arcsec = 1.2
kpc) for which Forbes et al. reported V_rot/sigma ~ 1.4. We speculate that this
galaxy may be the remnant of two (nearly edge-one) merged disk galaxies in
which the initial gas was driven inward and subsequently formed the inner disk,
while the stars at larger radii effectively experienced a dissipationless
merger event resulting in this `emerald cut galaxy' having very boxy isophotes
with a_4/a = -0.05 to -0.08 from 3 to 5 kpc. This galaxy suggests that
knowledge from simulations of both `wet' and `dry' galaxy mergers may need to
be combined to properly understand the various paths that galaxy evolution can
take, with a particular relevance to blue elliptical galaxies.Comment: To appear in ApJ. Six pages including references and figure
The contribution of executive control to semantic cognition : Convergent evidence from semantic aphasia and executive dysfunction
Semantic cognition, as described by the controlled semantic cognition (CSC) framework (Rogers et al., Neuropsychologia, 76, 220), involves two key components: activation of coherent, generalizable concepts within a heteromodal 'hub' in combination with modality-specific features (spokes), and a constraining mechanism that manipulates and gates this knowledge to generate time- and task-appropriate behaviour. Executive-semantic goal representations, largely supported by executive regions such as frontal and parietal cortex, are thought to allow the generation of non-dominant aspects of knowledge when these are appropriate for the task or context. Semantic aphasia (SA) patients have executive-semantic deficits, and these are correlated with general executive impairment. If the CSC proposal is correct, patients with executive impairment should not only exhibit impaired semantic cognition, but should also show characteristics that align with those observed in SA. This possibility remains largely untested, as patients selected on the basis that they show executive impairment (i.e., with 'dysexecutive syndrome') have not been extensively tested on tasks tapping semantic control and have not been previously compared with SA cases. We explored conceptual processing in 12 patients showing symptoms consistent with dysexecutive syndrome (DYS) and 24 SA patients, using a range of multimodal semantic assessments which manipulated control demands. Patients with executive impairments, despite not being selected to show semantic impairments, nevertheless showed parallel patterns to SA cases. They showed strong effects of distractor strength, cues and miscues, and probe-target distance, plus minimal effects of word frequency on comprehension (unlike semantic dementia patients with degradation of conceptual knowledge). This supports a component process account of semantic cognition in which retrieval is shaped by control processes, and confirms that deficits in SA patients reflect difficulty controlling semantic retrieval
Nucleosynthesis-relevant conditions in neutrino-driven supernova outflows. II. The reverse shock in two-dimensional simulations
After the initiation of the explosion of core-collapse supernovae, neutrinos
emitted from the nascent neutron star drive a supersonic baryonic outflow. This
neutrino-driven wind interacts with the more slowly moving, earlier supernova
ejecta forming a wind termination shock (or reverse shock), which changes the
local wind conditions and their evolution. Important nucleosynthesis processes
(alpha-process, charged-particle reactions, r-process, and vp-process) occur or
might occur in this environment. The nucleosynthesis depends on the long-time
evolution of density, temperature, and expansion velocity. Here we present
two-dimensional hydrodynamical simulations with an approximate description of
neutrino-transport effects, which for the first time follow the post-bounce
accretion, onset of the explosion, wind formation, and the wind expansion
through the collision with the preceding supernova ejecta. Our results
demonstrate that the anisotropic ejecta distribution has a great impact on the
position of the reverse shock, the wind profile, and the long-time evolution.
This suggests that hydrodynamic instabilities after core bounce and the
consequential asymmetries may have important effects on the
nucleosynthesis-relevant conditions in the neutrino-heated baryonic mass flow
from proto-neutron stars.Comment: 13 pages, 8 figures, accepted for publication in A&
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