Galaxies and the intergalactic medium at redshift 5.7: the environment of metal absorption systems

Metal absorption line systems observed in the spectra of background quasars (QSOs) contribute information on two crucial processes of galaxy evolution: the reionization of intergalactic hydrogen and the chemical enrichment of the intergalactic medium (IGM). The aim of this thesis is to study the environment (at both small and large scales) of triply ionized carbon (Civ) absorption systems at redshift z ∼ 5.7 in two independent fields of view, using Lyman alpha emitters (LAEs) and Lyman break galaxies (LBGs) as tracers of the mass distribution and star formation. These two fields are centred on QSOs SDSS J1030+0524 (zem = 6.309) and SDSS 1137+3549 (zem = 5.962). Using wide field photometry (∼80×60h−1 comoving Mpc), we study the impact of the Lyman α line on the broad-band colours of LBGs to develop a set of conditions that aim to select bright galaxies (MUV(1350˚A) <∼ −21.0 mag) in a narrow redshift slice of Δz ∼ 0.2 at z ∼ 5.7. The LBGs’ projected distribution is compared to the distribution of z ∼ 5.7 narrow-band selected LAEs (Δz ∼ 0.08). We find that the Civ systems are located more than 10h−1 projected comoving Mpc from the main concentrations of LBGs and no candidate is closer than ∼5h−1 projected comoving Mpc. In contrast, an excess of LAEs is found on scales of ∼10h−1 comoving Mpc, suggesting that LAEs are the primary candidates for the source of the Civ systems. Our spectroscopic data confirm the excess of line emitters in the field J1030+0524 within 10h−1 comoving Mpc of the highest column density Civ system at z ≥ 5.5 known to date at zCiv = 5.7242 ± 0.0001. Furthermore, the closest galaxy to this Civ absorption system, LAE 103027+052419, is confirmed at zLyα = 5.724±0.001 which results in a separation of 212.8h−1 physical kpc. We compare the observed properties of LAE 103027+052419 with 25 spectroscopically confirmed z∼5.7 LAEs. Our results suggest that, if the Civ system is produced by a galactic outflow from LAE 103027+052419, the gas should have departed at z ≥ 8, which agrees with the pre-galactic enrichment scenario and implies that outflows at z >∼6 can enrich larger volumes than outflows at z <∼ 3.5. Moreover, if an undetected dwarf galaxy population is the source of the carbon, then the enrichment caused by these galaxies would have extended beyond the reach of a typical star-formation driven outflow. Therefore, Civ systems at the tail-end of the epoch of reionization may be produced by star-forming galaxies that have been injecting metals into the IGM since very early times (z > 6). In summary, we conclude that, in contrast with lower redshift (z <∼ 3.5) systems, strong Civ absorption systems at z ∼ 5.7 trace low-to-intermediate density environments dominated by faint galaxies. Moreover, the excess of LAEs associated with a high ionization metal absorption system at z ∼ 5.7 agrees with the idea that faint galaxies dominate the ionizing photon budget at this redshift.