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Luz Angela Garcia

After the recombination epoch, all the Hydrogen in the Universe was neutral and the cosmic gas was filled of HI and other primordial elements. The hot plasma was essentially neutral and opaque (period commonly known as Dark Ages), but the formation and evolution of the first stars produced emission of ultraviolet photons, which ionized the Hydrogen, ending up with Reionization era. Despite the fact that the Universe was again transparent to be observed, the galaxy formation and enrichment of the ISM had proceeded, so there is a huge gap of the cosmic history that has been lost and must be studied by indirect observations in the sky.

One of best ways to study the early universe when Reionization occurred is using the QSO spectra. The Lyman absorption lines (and other metals) are the imprint of the absorbed incoming radiation from the quasar by the IGM and other structures. QSOs are the brightest objects in the early Universe, therefore the study of their spectra is very important in Cosmology in order to understand the high redshift Universe.

In this scenario, the main goal of my research is to make theoretical predictions on how Reionization took place in the early Universe: was it an instantaneous (as a phase transition) or a continuous process? How did it happen? What was the topology of the the IGM sources during Reionization? The first question that the thesis will address is: how well is the fraction of neutral-to-ionized hydrogen represented by the fraction of low-to high ionisation metals? For instance, is there a tight relation of the fraction CII/CIV with HI/HII taking into account some physical conditions in the IGM?

Besides, the ionization potential of OI is similar to HI, hence, the ratio OI/OVI is expected to be a good proxy the neutral-to-ionized hydrogen ratio after Reionization?

These questions will be addressed by the use of hydrodynamical simulations with the SPH approach at redshift z~6 and will allow us to derive some constrains on the mass fraction of metals (OI, SiII, CIV, MgII, and FeII) with different ionizing UV backgrounds and compare the numerical results with observational measurements of metal absorption lines of QSOs spectrum.

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