Walks in the Lyman and Metal-line Forests.

The aim of this PhD thesis is the study of the properties of the Inter-Galactic Medium (IGM) through Quasi Stellar Objects (QSOs) absorption lines at redshifts z~2-4. The study of the IGM provides a unique picture of the Universe in the early phases of structure formation and is crucial to test cosmological models. Furthermore, the metal content of the IGM can be used to constrain the nature of the enrichment mechanisms at high redshifts. Two main mechanisms have been proposed predicting different distribution of metals in the IGM: a late enrichment, due to the observed galaxies at redshift ~3, and an early enrichment, due to a population of very massive first stars (the so called popIII stars), not yet observed, at higher redshift. The existing observations are not sufficient to discriminate between the two mechanisms. QSO spectra show interesting features to study the IGM: 1. The Lyman-alpha (Lya) forest : the forest of absorption lines blue-ward of the Lya emission arising in large scale neutral hydrogen density fluctuations of moderate amplitude in the warm photo-ionized IGM. Since the dynamical state of the low density IGM is governed mainly by the Hubble expansion, gravitational instabilities and photo-ionization, the involved physics is quite simple and mildly non-linear. The Lyman forest is then a fair tracer of the underlying matter density field. 2. Metal absorptions: many absorption features due to ionic transitions in chemical elements heavier than He (``metals'') are clearly present red-ward of the Lya emission. Some of them are associated with the QSO itself but the majority are tracers of intervening metals belonging to structures of different nature, from diffuse gas to galaxies. Chapter 1 of the thesis presents in detail how the physics of the IGM can be understood studying QSO absorption lines, and the classical results obtained in this field. Then the thesis focuses on the work done during the PhD, which has addressed three research areas: 1. Study of lines of sight (LOSs) to isolated QSOs (Chapter 2,3): (i) a sample of 22 high-resolution QSO spectra has been studied. Classical statistics have been applied: all the absorption features in the spectra have been fit with Voigt profiles to extract physical parameters associated with the absorbing material; quantities like the evolution in redshift of the number density of the lines and the two-point correlation function of the line distribution along the spectra have been studied and compared with measurements available in the literature; (ii) a new method to analyze the Lya forest has been implemented. Traditionally, absorption spectra are resolved in a collection of discrete absorption systems. This method instead reconstructs the underlying density field processing the lines on the basis of the physical properties of the IGM. Therefore, a continuous density field is built and the main drawbacks of the Voigt fitting approach are overcome. (iii) the new algorithm has been tested with N-body hydrodynamical simulations of the IGM; (iiii) the new algorithm has been applied to the observational data sample to study the hydrogen density field ( its evolution with redshift and its clustering properties ) and the so called proximity effect of QSO, estimating the overdensity around the object. 2. Study of lines of sight to multiple QSOs (Chapter 4,5): The thesis presents also the work done studying multiple QSO LOSs, i.e. studying the properties of the IGM not only with the spectrum of a single object, by trying to get transversal informations comparing different LOSs, close both in angular separation and in the emission redshift of the source. A sample of 15 QSO forming 21 pairs have been studied to get the transverse correlation function, using the statistics of the transmitted flux of the objects. Furthermore, we have observed a pair of close QSO during two nights (7-8 august 2007) with UVES, the high resolution spectrograph at the ESO Very Large Telescope (VLT) located at Cerro Paranal, in Chile. These two objects have been observed to study the correspondence between metal absorptions and galaxies in the field; preliminary results and the description of the work in progress are presented in detail in the thesis (Chapter 5). 3. New instrumentation (Chapter 6): Looking ahead to new possibilities of advance in this field of research, an important role will be played by the high sensitivity and medium resolution spectrograph X-shooter. It will receive first light at the ESO VLT in July 2008 and will start operating in early 2009. When in operation, its wide spectral-range observing capability will be unique at very large telescopes and extremely relevant for the study of QSO spectra. The thesis describes the Science Case "Tomography of the IGM" for the instrument, and the work done at ESO within the X-shooter project. For the operation of this instrument we have carried out laboratory measurements of calibration sources for the Near-InfraRed arm and I participated to an observational project to build a spectro-photometric flux catalogue of standard stars for the instrument.