Speaker
Description
Recent observations carried out with the Atacama Large Millimeter/submillimeter Array (ALMA) show that the density of the galaxies' interstellar medium (ISM) increases toward earlier cosmic epochs, evolving approximately as $(1+z)^{3.3}$. 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 galaxies, reaching conditions close to the Compton-thick regime at $z\geq6$. This scenario suggests that the ISM of the host galaxy may substantially contribute to the obscuration of accreting supermassive black holes in the primordial Universe, hiding them and their properties from the seeker.
Here I will discuss the evolution of the ISM column density in galaxy samples at different redshifts, extending the analysis up to $z>9$ thanks to observations with the James Webb Space Telescope (JWST).
A number of probes have been investigated to estimate the ISM column density in these samples, including the analysis of the sub-millimeter continuum emission from cold dust, the use of line tracers such as [CII], and an indirect method based on the Schmidt–Kennicutt relation and its possible evolution with redshift, which allows the gas surface density to be derived from the measured star formation rate. In all three approaches, the estimation of the surface gas density requires spatial information about the sources, in particular the characteristic size of the gas distribution or of the star-forming region.
Observations of objects at high (sub-arcsec) angular resolution were therefore considered, including galaxies and high-redshift quasars from ALMA surveys reported in the literature, and sources selected by JWST within the COSMOS-Web survey area. For each subsample, the ISM density was measured according to the available tracers, allowing a comparison between the different approaches.
The comparative analysis made it possible to assess the robustness of the estimates, the main sources of uncertainty, and the impact of the methodological assumptions.
Preliminary results confirm that the gas column density tends to increase with redshift, suggesting that the ISM of the host galaxy may represent a dominant component of AGN obscuration. These findings have important implications not only for understanding the growth history of hidden supermassive black holes, but also for constraining the processes that regulate the evolution of galaxies and their interstellar medium in the early Universe.