On the dust and gas content of high-redshift galaxies hosting obscured AGN in the CDF-S

Submillimeter Galaxies (SMGs) at high redshift are among the best targets to investigate the early evolutionary phases in the lifetime of massive systems, in which a central Super Massive Black Hole (SMBH) is heavily obscured and the total emission is dominated by the host galaxy light. The early stages of the evolution of these systems are characterized by the presence of large gas and dust reservoirs that sustain (and obscure) a starburst phase during which galaxies behave as bright SMGs, hosting hidden Quasi--Stellar Objects (QSOs). The main purpose of this work is to investigate the contribution of the Inter--Stellar Medium (ISM) to the obscuration of QSOs in distant SMGs, and unveil the morphological and kinematics properties of these objects. The analysis is based on new ALMA band 4 (1.8-2.4 mm) data of six galaxies at redshift > 2.5 in the Chandra Deep Field South (CDF-S). These objects are included in the 7--Ms Chandra exposure which allows an accurate determination of their nuclear properties; they have been selected by their high column density (log N_H > 23), secure spectroscopic redshift and good far--infrared data coverage, resulting in a unique sample with such characteristics. Through the detection of their continuum (i.e., dust-reprocessed) emission at ~ 2.1 mm and of one high-J CO transition per source, we aim at measuring the luminosity and size of dust and molecular gas of these objects to derive their masses and (under simple geometrical assumptions) the ISM column densities. In general, we found that these galaxies are massive (M_H2 ~ 10^10 M_sun) and compact (diameter lower than 3 kpc) systems and that the ISM column densities are of the order of 10^(23-24) cm^-2 This suggests that the galaxy ISM can substantially contribute to the AGN obscuration. In addition, we found indications that two of these sources are well described by a rotating structure and one is possibly undergoing a merger.