Suarez Noguez, T; (2018) Understanding the distribution of gas in the Universe. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The distribution of gas in the Universe can be observed in absorption in the spectra of quasars. However, interpreting the spectra requires comparison to physical models which map the distribution, temperatures and ionisation states of the gas. First, I focused on understanding the presence of outflowing cold gas around galaxies. I performed numerical simulations to study how outflows, launched from a central galaxy undergoing starbursts, affect the circumgalactic medium. I model an outflow as a rapidly moving bubble of gas above the disk and analyse its evolution. I sampled a distribution of parameters with a grid of two-dimensional hydrodynamical simulations --with and without-- radiative cooling, assuming primordial gas composition. The amount of the cool gas fraction depends strongly on the "burstiness"�� of energy injection; stronger bursts typically lead to a larger fraction of cool gas forming in the outflow. This suggests cool gas is formed in the outflow itself rather than entrained from the interstellar medium, emphasising the importance of cooling. Secondly, I turn to the lower density gas known as the Lyman-alpha forest. This is formed from a trace of HI in a predominantly photoionised intergalactic medium. Existing radiative transfer models show that large-scale ionisation rate fluctuations can have an observable impact on the forest. I extend these models to include beaming of the quasar population. This has two effects: the physical number density of ionising sources is enhanced relative to observations and; the radiative transfer equation is altered. I studied the statistical properties of the resulting ionisation rate and neutral hydrogen density fields at redshift z~2.3. I find that both effects from beaming partially cancel each other. On very large scales (k<0.01 h/Mpc) the source density renormalisation dominates; it can reduce, by an order of magnitude, the contribution of ionising shot-noise to the intergalactic HI power spectrum.

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