Seeing the Lyman-Alpha Forest for the Trees: Constraints on the Thermal State of the IGM from SDSS-III/BOSS

Abstract

The Lyman-alpha (Lya) forest in the line-of-sight to distant quasars is an important probe of the intergalactic medium (IGM). The thermal properties of the IGM can provide insight the reionization history of the universe, as well as indirectly constraining energy sources in the universe such as galaxies and quasars. This thesis is concerned primarily with studying the IGM using moderate quality Lya forest data sets from large-scale spectroscopic surveys, such as the Sloan Digital Sky Survey (SDSS) and the Baryon Oscillations Spectroscopic Survey (BOSS). In Chapter 1, we study the potential of SDSS Lya forest data to study the IGM. Using simulated mock spectra, we show that the flux probability distribution function (PDF) of SDSS data can place interesting constraints on the spatially-averaged temperature-density relation (TDR) of the IGM. We also introduce the threshold probability functions, a one-dimensional two-point statistic adapted from material sciences that can be applied to SDSS data to detect &sim 50 Mpc IGM thermal inhomogeneities arising from He II reionization. Chapter 2 discusses the effect of continuum biases in the TDR measured from high-resolution Lya forest spectra in the context of recent evidence for an inverted (gamma < 1) TDR. We argue that forward modeling of continuum errors in mock spectra are necessary to make robust estimates of the TDR. Motivated by the importance of accurate continuum estimation in Lya forest analysis, Chapter 3 introduces the mean-flux regulated/principal component analysis (MF-PCA) continuum estimation technique. We show using mock spectra that this technique can achieve continuum accuracies of < 10% and < 4% in noisy spectra of S/N &sim 2 and S/N &sim 5, respectively. We have also publicly released &sim 13, 000 continua from SDSS Data Release 7. In Chapter 4, we measure the flux PDF from BOSS, drawing from an overall sample of &sim 30, 000 Lya forest sightlines. This uses a novel procedure for optimally combining the multiple BOSS exposures and estimating the spectral noise contribution. Comparing a high signal-to-noise subsample in BOSS with mock spectra generated from detailed hydrodynamical simulations, we find that the flux PDF from BOSS at 2 < z < 3 is consistent with an isothermal TDR, consistent with theoretical expectations from He II reionization.