IGM metal enrichment via high-z quasar absorption spectroscopy. A preparatory study for the ANDES@ELT Early Universe science cases

Quasar absorption spectroscopy is a very powerful technique aimed at studying the physical and chemical properties of the gaseous environments along the line of sight, from the Epoch of Reionization to the local Universe. In particular, by taking advantage of bright background quasars, it is possible to put constraints on the metallicity of the inter- and circum-galactic medium (IGM, CGM; e.g. D'Odorico et al. 2016, 2022) through the analysis and modelization of the large number of absorption lines "contaminating" the QSO spectra. The majority of these absorption lines are due to hydrogen (populating the Lyman-alpha forest) but also to ionic transitions of heavier elements such as C, O, Mg, Al, Si, Fe, etc.. In the proposed talk, I will present the analysis of a sample of quasar spectra from the ongoing ESO Large Programme EQUALS, within which we are observing 23 bright quasars at z ≃ 3.3 - 4.2 with the ESPRESSO spectrograph at the VLT. The EQUALS spectra are characterised by a very high resolving power (R ~ 130,000) and high signal-to-noise ratio, which makes them very similar to the spectra we expect to obtain with the ANDES spectrograph foreseen for the ELT. The goal of my work is to investigate the metallicity of the IGM testing different methods (from the direct detection of absorption lines to statistical techniques). The analysis of the spectra is carried out with the Astrocook software, developed by Cupani et al. 2018, which provides tools to fit the lines and obtain key quantities as the column density N, the Doppler broadening parameter b and the redshift z corresponding to each line of interest. I will also discuss some new solutions that I implemented in the software, in terms of new "recipes" e.g. to improve system identification. This work can actually be seen as a training for the science that will be addressed by the forthcoming ANDES spectrograph at the ELT. As a matter of fact, one of the main science cases of ANDES is the study of the chemical enrichment in the Early Universe, exploring regions at z>6, deep into the Epoch of Reionization. ESPRESSO, although sensitive to lower redshift, is the best instrument that we have available today to test both new statistical and analysis approaches to the mentioned studies, and prepare ourselves to the full exploitation of the potentialities of ANDES.