Brian William O'Shea

A concordance model of the Lyman α forest at z = 1.95

We present 40 fully hydrodynamical numerical simulations of the intergalactic gas that gives rise to the Lyforest. The simulation code, input and output files are available at http://www.cosmos.ucsd.edu/˜gso/index.html. For each simula- tion we predict the observable properties of the H I absorption in QSO spectra. We then find the sets of cosmological and astrophysical parameters that match the QSO spectra. We present our results as scaling relationships between input and output parameters. The input parameters include the main cosmological parameters b, m, �, H0 and �8; and two astrophysical parameters 912 and X228. The parameter 912 controls the rate of ionization of H I, He I and He II and is equivalent to the intensity of the UV background. The second parameter X228 controls the rate of heating from the photoionization of He II and can be related to the shape of the UVB at � < 228 u We show how these input param- eters; especially �8, 912 and X228; effect the output parameters that we measure in simulated spectra. These parameters are the mean flux ¯ F, a measure of the most common Lyline width (b-value) b�, and the 1D power spectrum of the flux on scales from 0.01 - 0.1 s/km. We compare the simulation output to data from Kim et al. (2004) and Tytler et al. (2004) and we give a new measurement of the flux power from HIRES and UVES spectra for the low density IGM alone at z = 1.95. We find that simulations with a wide variety of �8 values, from at least 0.8 - 1.1, can fit the small scale flux power and b-values when we adjust X228 to compensate for the �8 change. We can also use 912 to adjust the H I ionization rate to simultaneously match the mean flux. When we examine only the mean flux, b-values and small scale flux power we can not readily break the strong degeneracy between �8 and X228.

The formation of the first second generation star

While the exact masses of the first stars are still unknown, it remains relatively well accepted that the Pop III initial mass function is not identical to that of stars forming today. This transition in star forming modes is due to the introduction of metals produced in the first supernovae. These metals enhance the radiative cooling efficiency of gas through fine-structure lines, molecular transitions, and thermal emission from dust grains. Simulations have made great contributions to our understanding of the ability of metals to induce fragmentation in collapsing gas, but little attention has been paid the actual conditions present when the Universe becomes enriched with metals for the first time. We present the results from a simulation of the formation of the first metal-enriched stars under realistic conditions. We simulate a Population III star forming halo and assume the formation of a 40 M⊙ metal-free star. Using 3D radiation hydrodynamics, we then compute the evolution of the HII region and the ...