14,982 research outputs found
The topological period-index problem over 6-complexes
By comparing the Postnikov towers of the classifying spaces of projective
unitary groups and the differentials in a twisted Atiyah-Hirzebruch spectral
sequence, we deduce a lower bound on the topological index in terms of the
period, and solve the topological version of the period-index problem in full
for finite CW complexes of dimension at most 6. Conditions are established
that, if they were met in the cohomology of a smooth complex 3-fold variety,
would disprove the ordinary period-index conjecture. Examples of
higher-dimensional varieties meeting these conditions are provided. We use our
results to furnish an obstruction to realizing a period-2 Brauer class as the
class associated to a sheaf of Clifford algebras, and varieties are constructed
for which the total Clifford invariant map is not surjective. No such examples
were previously known.Comment: To appear in J. To
Detection of H-alpha emission from the Magellanic Stream: evidence for an extended gaseous Galactic halo
We have detected faint, diffuse H\alpha emission of surface brightness Rayleighs, R, and R respectively,
corresponding to emission measures of 1.0 to 0.5 \cmsixpc. We have observed
several positions near the MS IV concentration, and find that the strongest
emission is on the sharp leading-edge density gradient. There is less emission
at points away from the gradient, and halfway between MS III and MS IV the
H< 0.04\alpha emission at cloud leading edges to heating of the
Stream clouds by ram pressure from ionized gas in the halo of the Galaxy. These
observations suggest that ram pressure from halo gas plays a large role in
stripping the Stream out of the Magellanic Clouds. They also suggest the
presence of a relatively large density of gas, , in the Galactic halo at kpc radius, and far above the
Galactic plane, . This implies that the Galaxy has a very large
baryonic, gaseous extent, and supports models of Lyman-$\alpha and metal-line
QSO absorption lines in which the absorption systems reside in extended
galactic halos.Comment: 15 pages, aaspp latex, + 1 table & 3 figures. Accepted in A.J. Also
available from http://www.physics.rutgers.edu/~bweiner/astro/papers
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