HIDE AND SEEK: Cosmic evolution of the Inter Stellar Medium and its implications for nuclear obscuration

Recent observations carried out with ALMA show that the density of galaxies'interstellar medium (ISM) increases toward earlier cosmic epochs, evolving approximately as (1+z)^3.3. This implies that at z>3 the gas column density along the nuclear line of sight can exceed by more than two orders of magnitude the typical values observed in local, low-redshift galaxies, approaching the Compton-thick regime at z>=6. This scenario suggests that the host galaxy's ISM may substantially contribute to the obscuration of accreting supermassive black holes in the primordial Universe, hiding them from the seeker. This thesis discusses the evolution of the ISM column density in galaxy samples from z~0 up to z>9. Three methods are investigated: the analysis of sub-millimeter continuum emission from cold dust (Method 1), the use of line tracers such as [CII] (Method 2), and an indirect method based on the Schmidt-Kennicutt relation and its possible evolution with redshift, deriving the gas surface density from the measured star formation rate (Method 3). All three approaches require spatial information on the sources, particularly the size of the gas distribution or star-forming region. Observations at high (sub-arcsecond) angular resolution were therefore considered, including galaxies and high-redshift quasars from ALMA surveys in the literature, and sources selected by JWST within the COSMOS-Web survey area. For each subsample the ISM column density was measured using the available tracers, allowing a comparison between the approaches and an assessment of the methodological assumptions. Our results confirm that the ISM density increases with redshift, suggesting that the host galaxy's ISM may be a dominant component of AGN obscuration. These findings are relevant to the growth history of hidden supermassive black holes, and to the processes regulating the evolution of galaxies and their interstellar medium in the early Universe.