Looking for the broad emission lines in AGN2 with deep NIR spectroscopy

Abstract

Nowadays there is robust evidence that every galaxy hosts a super massive black hole (SMBH; 106-109 M⊙) and the correlations of its mass with the host galaxy properties have been fairly well estabilished. Indeed the accretion of matter on the supermassive black holes and the related radiative and kinetic power output play an important role in the galaxy evolution, by suppressing/ tuning the star formation and feeding the AGN itself (feedback). In this framework, it appears clear that the study of AGN evolution is very important to understand the evolution of the star formation rate and galaxies in the Universe (AGN/galaxy co-evolution). Moreover, the existence of the scaling relationships between the AGN BH mass and the bulge galaxies properties, implies that the evolution of galaxies and the growth of SMBHs are intricately tied together. Thus, in order to obtain a clear picture of the AGN/galaxy co-evolution, it is important to accurately derive the shape and the evolution of both AGNs luminosity and SMBH mass functions. In the last decade, using hard X-ray selected AGN samples, it has been possible to accurately derive the AGN luminosity function up to z∼4 and, recently, using virial based techniques in the optical band on samples of broad line AGN (AGN 1), it has been possible to obtain some estimates of the Super Massive BH Mass Function (SMBHMF). However these measurements are affected by several selection biases against the narrow line AGN (AGN 2), where the Broad Line Region (BLR) is not visible in the optical because of dust extinction. According to the original standard unified model the different observational classes of AGN (AGN1 and AGN2) were believed to be the same kind of objects, observed under different conditions (e.g. different orientations to the observer). Instead, nowadays there are indications that AGN1 and AGN2 have on average different accretion rates (smaller for AGN 2), different host galaxy properties (more early type for AGN 2) and different masses (smaller for AGN2). It is therefore fundamental to measure the BH masses of both AGN 1 and AGN 2 in a consistent way. In the few studies where AGN2 BH masses have been derived the authors used the BH-bulge scale relations which were instead calibrated on AGN1 samples and are unlikely to hold also for all AGN2. It is therefore crucial to directly measure AGN2 BH masses also to verify the validity of the scaling relations for this population of AGNs. The work performed in this thesis is inserted in this scientific framework. Following the studies of Landt et al (2008; 2011b and 2013) we have developed a new virial method that make use of the less affected by dust exstinction Paβ NIR emission line in combination with the hard X-ray continuum luminosity of the AGN, which suffer less from galaxy starlight contamination. Such a relationship represent a usefull tool since it is potentially able to work also with AGN2, allowing us to derive for the first time a reliable measure of the AGN2 BH masses. To this porpuse we have randomly selected a complete sample of ∼40 obscured and intermediate AGN (AGN2, AGN1.9 and AGN1.8) from the Swift/BAT 70 Month hard X-ray survey, which provides an accurate hard X-ray luminosities measurements in the 14−195 keV band. We have observed the selected sources using the NIR spectrographs ISAAC at VLT and LUCI at LBT, and also using the multiwavelenght spectrograph Xshooter at VLT. We found broad component in BLR emission lines (Paα ; Paβ and He I ) in ∼30% of our sources and, applying to them the NIR virial estimator, we have been able to measure in a direct way the AGN2 BH masses, finding that AGN2 have on average lower BH masses and higher Eddington ratio with respect AGN1 population of same luminosity.