Understanding the black hole - galaxy connection: a multi-X-ray observatory approach

An accurate census and characterization of accreting supermassive black holes (SMBHs) over cosmic time, both in the local Universe and at very high redshift, can be efficiently achieved through a synergy between the different X-ray facilities currently available. For example, deep surveys with Chandra in the 0.5–7 keV band, such as the one performed over the 2.2 deg2 COSMOS field, allow one to detect thousands of active galactic nuclei (AGN) at luminosities that are almost completely below the knee of the luminosity function. Consequently, such X-ray surveys can sample the typical AGN population up to z ∼6, detecting a large number of obscured AGN that would otherwise be missed by optical surveys. Thanks to X-ray surveys, it is well known that the vast majority of SMBHs in the center of galaxies accrete material in an obscured phase, where the obscuration is caused by a nuclear-scale distribution of matter with column density NH,z>1022 cm−2 which is possibly a reservoir of gas for the SMBH accretion. For this reason, a full understanding of SMBH accretion and of his effects on the host galaxy requires i) a detailed study of the obscuring material, i.e., a complete characterization of its geometrical, chemical and kinematical properties, and ii) a systematic comparison between observations and hydrodynamical simulations of active galactic nuclei (AGN) feeding and feedback. These goals can be best addressed studying a large sample of nearby, heavily obscured AGN with high-quality NuSTAR data in the 3–70 keV band, ideally combined with infrared/mm observations to complement the X-ray data. Next generation X-ray facilities, such as Athena and the proposed Lynx and AXIS missions, will build on the results obtained by current X-ray facilities. In particular, these new observatories will discover a new population of z >6 sources, thus providing key constraints to theoretical models of black hole seeds and early SMBH growth.