Exploring the High Redshift Universe: Spectral Analysis Methods along Quasar Sight Lines

Abstract

Luminous quasars (QSOs) serve as powerful beacons for probing the intergalactic medium (IGM). Lyman-alpha (Lyα) forests – absorption lines attributed to the presence of neutral hydrogen in the IGM – are regularly observed in QSO spectra. Lyα lines provide critical cosmological information regarding both, the distribution of gas clouds in the IGM at different redshifts, and the Epoch of Reionization (EoR) following the formation of the universe’s first luminous energy sources. IGM opacity (and thus the fraction of hydrogen that is neutral) is estimated as a function of redshift by measuring how much of the intrinsic emission of QSOs are absorbed along each of their sight lines. This measurement requires estimation of the intrinsic emission of each QSO, which is found by fitting their continua. For very high redshift QSOs, there is so much absorption along their sight lines, only the continuum redward of each QSO’s Lyα peak can be fitted. This thesis proposes a method for a first-look estimation of relative neutral hydrogen absorption in the absence of a complex continuum fitting model. Capitalizing on public datasets from Sloan Digital Sky Survey (SDSS) and QSO spectra database igmspec, high redshift QSO spectra are fit with Gaussians where Lyα absorption is estimated by subtracting the fitted flux levels at wavelengths directly preceding the Lyα emission line with the unabsorbed flux levels after. The proposed method provides a rapid method for a statistical assessment of IGM opacity during the EoR complementary to more sophisticated PCA-based continuum fitting models. Looking forward, the spectroscopic advances of the James Webb Space Telescope (JWST) and the telescope’s potential to address tensions between novel, extremely-high redshift QSO discoveries and Big Bang cosmology are discussed.