Cohomology at infinity and the well-rounded retract for general Linear Groups

Let $\bold G$ be a reductive algebraic group defined over \Q, and let $\Gamma$ be an arithmetic subgroup of \bold G(\Q). Let $X$ be the symmetric space for $\bold G(\R)$, and assume $X$ is contractible. Then the cohomology (mod torsion) of the space $X/\Gamma$ is the same as the cohomology of $\Gamma$. In turn, $X/\Gamma$ will have the same cohomology as $W/\Gamma$, if $W$ is a ``spine'' in $X$. This means that $W$ (if it exists) is a deformation retract of $X$ by a $\Gamma$-equivariant deformation retraction, that $W/\Gamma$ is compact, and that $\dim W$ equals the virtual cohomological dimension (vcd) of $\Gamma$. Then $W$ can be given the structure of a cell complex on which $\Gamma$ acts cellularly, and the cohomology of $W/\Gamma$ 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$_2$, 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$_1$ index at 4160 \AA $\,$ exhibit significantly steeper gradients, with a break at $r \sim 0.1 r_{\rm eff}$ ($r \sim 300$ 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 $0.1 r_{\rm eff}$ if the IMF is a single power law. While gradients in the mass function above $\sim 0.4 M_\odot$ 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 $-14800 \pm 1800$ rad m$^{-2}$, 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 $\sim 12$ km s$^{-1}$ 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