Talks & Events
Ph.D. Thesis Defenses: 2002
The line of sight toward the SMC star Sk 108
I present the complete FUSE spectrum of the SMC Wolf-Rayet binary star Sk 108. The spectrum covers the 905 1186 Å wavelength range at a resolution of about 12,500 and a S/N of 25 50. I used detailed component information from higher resolution near-UV and optical spectra to model the far- UV lines. Although, both Galactic and SMC gas toward Sk 108 seem to be predominantly neutral, a significant fraction of the SMC gas is ionized. The Fe III/Fe II ratio as well as the column densities of O VI and P V are higher in the SMC than the Galaxy. The total H I column density is the same in the Galactic and SMC components. However, the column density of H2 is over an order of magnitude lower in the SMC. The excitation temperatures for the SMC are higher than the Galaxy, consistent with a stronger radiation field in the SMC. In addition to H2 measurements, I extend the abundance studies in this sight-line, to include Ar, P, N, O and S. The overall relative abundances agree with a lower dust-to-gas-metal ratio in the SMC than the Galaxy. The SMC clouds resemble typical Galactic halo abundance patterns, except for oxygen. Si, O and S are at roughly solar levels, relative to zinc, which might imply material enriched by massive stars or Type II SNe. N appears to be deficient in the SMC. In addition, I empirically determine oscillator strengths for a series of Fe II lines, using the pre-determined column densities for the Fe II components from the GHRS data. There are very few measurements of the elements seen here in QSOALS. More such measurements are needed at a range of redshifts. Similarly, abundances for MC targets are also at low numbers but the archival data of FUSE , in conjunction with high resolution ground- based spectra of the same targets, can be used to expand and more accurately determine depletions in the MC.
Testing the halo model against the SDSS photometric survey
We present halo model predictions for the expected angular clustering and associated errors from the completed Sloan Digital Sky Survey (SDSS) photometric galaxy sample. These results are used to constrain halo model parameters under the assumption of a fixed ΛCDM cosmology using standard Fisher matrix techniques. Given the ability of the five-color SDSS photometry to separate galaxies into sub-populations by intrinsic color, we also use extensions of the standard halo model formalism to calculate the expected clustering of red and blue galaxy sub-populations as a further test of the galaxy evolution included in the semi-analytic methods for populating dark matter halos with galaxies. The extremely small sample variance and Poisson errors from the completed SDSS survey should result in very impressive constraints (˜1 10%) on the halo model parameters for a simple magnitude-limited sample and should provide an extremely useful check on the behavior of current and future N-body simulations and semi-analytic techniques. We also show that similar constraints are possible using a narrow selection function, as would be possible using photometric redshifts, without making linear assumptions regarding the evolution of the underlying power spectra. In both cases, we explore the effects of uncertainty in the selection function on the resulting constraints and the degeneracies between various combinations of parameters.
Particle dark matter constraints from the Draco dwarf galaxy
It is widely thought that neutralinos, the lightest supersymmetric particles, could comprise most of the dark matter. If so, then dark halos will emit radio and gamma ray signals initiated by neutralino annihilation. A particularly promising place to look for these indicators is at the center of the local group dwarf spheroidal galaxy Draco, and recent measurements of the motion of its stars have revealed it to be an even better target for dark matter detection than previously thought. We compute limits on WIMP properties for various models of Draco's dark matter halo. We find that if the halo is nearly isothermal, as the new measurements indicate, then current gamma ray flux limits prohibit much of the neutralino parameter space. If Draco has a moderate magnetic field, then current radio limits can rule out more of it. These results are appreciably stronger than other current constraints, and so acquiring more detailed data on Draco's density profile may become one of the most promising avenues for identifying dark matter.