Probing the AGN/galaxy co-evolution in the widest luminosity range ever

Abstract

Active Galactic Nuclei (AGN) play a fundamental role in many aspects of modern astro physics. Recent studies of the local Universe have established the presence of Supermas sive Black Holes (SMBHs) in the nuclei of virtually all galaxies with a bulge/spheroidal component (Kormendy and Richstone, 1995), and one of the first queries people raised up was about the interconnection between the SMBH and its host galaxy. While at low masses the most accepted scenarios deal with either the action of feedback from Super novae (which regulates the formation of stars into galaxies by heating the inter-galactic medium) or the so-called gas starvation or strangulation, at the high mass end there is the need for more disruptive phenomena, commonly termed as feedback, capable to quench the star formation. Both observations and theory suggest that the mass out flows’ rate is proportional to the AGN luminosity; therefore, the best places where to hunt for feedback mechanisms are the sources which lie at the high extreme of the AGN Luminosity Function (LF). The luminosity emitted from the AGN is a key parameter not only to describe the single source itself, but within a wider context of galaxy-BH evolution. Even the famous argument by Soltan (1982), who predicted that the cosmo logical growth of SMBHs is mostly because of the accretion of matter during their active phases, is based on some assumptions on the AGN luminosity and accretion efficiency. The same goes for several theories which have been developed to describe physical pro cesses possibly responsible of a common formation scenario for galaxies and their central black hole (Volonteri et al., 2003; Springel et al., 2005; Vittorini et al., 2005). However, the bolometric luminosity (i.e., the luminosity integrated over the whole electromagnetic spectrum) is not the simplest observable to measure, because it requires a complete and reliable knowledge of the AGN spectral energy distribution (SED), and a good treatment of the contamination by the galaxy. Hence the need to calibrate affordable methods to estimate the AGN luminosity when only a narrow region of their spectrum is available. There is a further point which raises problems in our current knowledge of the SMBH galaxy connection paradigm: all the scaling relations between the SMBH mass and some properties of the galaxy (i.e., mass and luminosity of the bulge, stellar velocity disper sion) have been found using local quiescent galaxies. Shankar et al. (2016) showed that using Monte Carlo simulations, resolution-related selection effects have the potential to increase the normalization of the MBH - σ relation by a factor of a few, and the MBH - M∗ by at least an order of magnitude. The presence of these biases is confirmed when the scaling relations are measured using type 1 AGN where the BLR structure and dynamics have been modelled directly (Pancoast et al., 2014; Grier et al., 2017; Williams et al., 2018). Even more, we must consider the role of type 2 sources in these scaling relations, being however the most predominant fraction of the AGN population. However, due to the difficulty in measuring their BH mass, constraints on the way they move on the local scaling relations are challenging to obtain. The work performed in this thesis is inserted in this scientific framework. We have stud ied two main AGN samples showing complementary properties. The first one belongs to a wide project which consists in the broad-band analysis of an infrared-selected (WISSH, standing for WISE-SDSS Selected Hyper-luminous) sample of the ∼ 90 type 1 most luminous AGN known in the redshift range 2<z<4. Given their extreme luminosities (LBOL > 2 × 1047 erg/s), they represent the perfect place to look for feedback mecha nisms in place. Thanks to the multi-wavelength coverage available, studies focused in different bands of their SED have been performed, leading to the characterization of their X-ray properties (Martocchia et al., 2017) and to the discovery of very powerful outflows (Bischetti et al., 2017; Vietri et al., 2018). For this specific work we analyzed the 16 WISSH sources with Herschel data available in the three SPIRE bands (250, 350 and 500 µm), deriving their main physical properties (i.e., bolometric and monochromatic luminosities, star formation rate and dust masses). We focused on their infrared proper ties, finding extreme (up to thousands of solar masses per year) values of star formation rate (SFR) even properly accounting for the not negligible AGN contribution to the FIR fluxes, estimated to be about the 50%. Their unusual properties make them as witnesses of a very precise phase in the galaxy life cycle, in between the hot dust-obscured galaxies (hot DOGs) and the optically luminous sources. Within this framework we have car ried out the analysis of a Compton thick hyper-luminous AGN at z ∼ 2 observed by NuSTAR, and whose X-ray emission and SED-derived physical properties confirm the dust-enshrouded transitional phase which eventually leads to an optically bright AGN. The second sample of type 1 and type 2 sources, extracted from the SWIFT/BAT 70- month catalog, shows complementary properties both in redshift (being local, at z < 0.1) and luminosity (LBOL ∼ 1043 − 1044 erg/s) if compared with the WISSH one. For a sub-sample of 15 type 2 sources, estimates of the BH mass are available via deep NIR spectroscopy (Onori et al., 2017). Together with the derivation of the physical prop erties via SED-fitting, the knowledge of the BH masses allowed us to characterize the MBH − M∗ plane adding the information about the type 2 population, which was not investigated so far. The combination of the high luminosity WISSH sample and of the low luminosity SWIFT/BAT sample, results in a collection of type 1 and type 2 AGN sources which spans a wide range of AGN luminosity (from 1041 erg/s to 1048 erg/s). We were therefore able to build up a new bolometric correction relation, statistically repre sentative for the whole AGN population, and valid over the most extended luminosity range (∼ 7 decades) ever sampled.