7,540 research outputs found
Cohomology at infinity and the well-rounded retract for general Linear Groups
Let be a reductive algebraic group defined over \Q, and let
be an arithmetic subgroup of \bold G(\Q). Let be the symmetric
space for , and assume is contractible. Then the cohomology
(mod torsion) of the space is the same as the cohomology of
. In turn, will have the same cohomology as , if
is a ``spine'' in . This means that (if it exists) is a deformation
retract of by a -equivariant deformation retraction, that
is compact, and that equals the virtual cohomological
dimension (vcd) of . Then can be given the structure of a cell
complex on which acts cellularly, and the cohomology of can
be found combinatorially
Radial Trends in IMF-Sensitive Absorption Features in Two Early-Type Galaxies: Evidence for Abundance-Driven Gradients
Samples of early-type galaxies show a correlation between stellar velocity
dispersion and the stellar initial mass function (IMF) as inferred from
gravity-sensitive absorption lines in the galaxies' central regions. To search
for spatial variations in the IMF, we have observed two early-type galaxies
with Keck/LRIS and measured radial gradients in the strengths of absorption
features from 4000-5500 \AA and 8000-10,000 \AA. We present spatially
resolved measurements of the dwarf-sensitive spectral indices NaI (8190 \AA)
and Wing-Ford FeH (9915 \AA), as well as indices for species of H, C, CN,
Mg, Ca, TiO, and Fe. Our measurements show a metallicity gradient in both
objects, and Mg/Fe consistent with a shallow gradient in \alpha-enhancement,
matching widely observed trends for massive early-type galaxies. The NaI index
and the CN index at 4160 \AA exhibit significantly steeper gradients,
with a break at ( pc). Inside this radius
NaI strength increases sharply toward the galaxy center, consistent with a
rapid central rise in [Na/Fe]. In contrast, the ratio of FeH to Fe index
strength decreases toward the galaxy center. This behavior cannot be reproduced
by a steepening IMF inside if the IMF is a single power law.
While gradients in the mass function above may occur,
exceptional care is required to disentangle these IMF variations from the
extreme variations in individual element abundances near the galaxies' centers.Comment: Accepted for publication in ApJ. Updates from v1 include an expanded
comparison of measured index strengths to SPS models. 20 page body + 7 page
appendix + references. Includes 25 figure
The 2000 Periastron Passage of PSR B1259-63
We report here on a sequence of 28 observations of the binary pulsar system
PSR B1259-63/SS2883 at four radio frequencies made with the Australia Telescope
Compact Array around the time of the 2000 periastron passage. Observations made
on 2000 Sep 1 show that the pulsar's apparent rotation measure (RM) reached a
maximum of rad m, some 700 times the value measured
away from periastron, and is the largest astrophysical RM measured. This value,
combined with the dispersion measure implies a magnetic field in the Be star's
wind of 6 mG. We find that the light curve of the unpulsed emission is similar
to that obtained during the 1997 periastron but that differences in detail
imply that the emission disc of the Be star is thicker and/or of higher
density. The behaviour of the light curve at late times is best modelled by the
adiabatic expansion of a synchrotron bubble formed in the pulsar/disc
interaction. The expansion rate of the bubble km s is
surprisingly low but the derived magnetic field of 1.6 G close to that
expected.Comment: 8 pages, 6 figures, 3 tables, LaTeX (mn.sty). Accepted for
publication in the Monthly Notices of the Royal Astronomical Society. Also
available at http://astronomy.swin.edu.au/staff/tconnors/publications.htm
A Different Kind of Restructuring: Forty Years of Debate and the Prospect of a Formal International Sovereign Debt Regime
This thesis will examine how the organization of creditors and debtors within an ad hoc sovereign debt framework affects the prospect of establishing a formal international mechanism for debt disputes. Since sovereign debtors are not bounded by the same constraints and guarantees as domestic actors, crisis-driven political battles and case-by-case compromises between creditor interests and indebted countries are the ideal building blocks for constructing a picture of the contemporary debt regime. A review of sovereign debt disputes between the 1970s and the present day – corresponding to the North-South Dialogue, the Latin American debt crisis of the 1980s, the International Monetary Fund’s Sovereign Debt Restructuring Mechanism proposal, and Argentina’s 2014 default – indicates that a formal sovereign debt regime is unlikely to arise in the current bond-dominated atmosphere. In particular, the dominance of retail investors and other private bondholders, combined with stagnated or waning influence among public entities such as the IMF and the Paris Club, relegates the concept of a formal, permanent debt arbiter to an aspirational status
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