Seeing the invisible: tracing cosmic structure across time and space with hydrogen lines

Abstract

In this thesis, we explore astrophysical frontiers along two lines of enquiry, on very different physical scales and separated by ten billion years of cosmic time, yet linked by a focus on the properties of diffuse gas modelled by radiation-hydrodynamic simulations and revealed by spectroscopic observations.

Firstly, we investigate RELHICs, a hypothetical population of low-mass dark matter haloes, predicted by hydrodynamical simulations to remain gas-rich yet star-free. We show that RELHICs illuminated by the extragalactic ultraviolet background (UVB) should produce distinctive "fluorescent" emission, the characterisation of which has the potential to yield constraints on the intensity and spectral shape of the UVB, on the nature of dark matter, and on the primordial abundance of helium. We address the substantial practical challenges inherent in detecting this extremely faint emission.

Secondly, we focus on the damped-Lyman alpha (DLA) absorbers originally detected in the spectra of background quasars, which we study using a novel set of cosmological simulations in conjunction with accurate radiative transfer modelling. We make a detailed investigation of the statistical properties of DLAs, and relate the properties of individual absorbers to the haloes that host them. We present evidence furthering the view that the neutral gas which gives rise to DLAs is primarily composed of circumgalactic material undergoing cosmological accretion onto galaxies, with only the sightlines containing very strong absorbers arising in the galaxies themselves.