Abstract
The spectra of active galactic nuclei (AGNs) are often characterized by a wealth of emission lines with different profiles and intensity ratios that lead to a complicated classification. Their electromagnetic radiation spans more than 10 orders of magnitude in frequency. In spite of the differences between various classes, the origin of their activity is attributed to a combination of emitting components, surrounding an accreting supermassive black hole (SMBH), in the unified model. Currently, the execution of sky surveys, with instruments operating at various frequencies, provides the possibility to detect and to investigate the properties of AGNs on very large statistical samples. As a result of the spectroscopic surveys that allow the investigation of many objects, we have the opportunity to place new constraints on the nature and evolution of AGNs. In this contribution, we present the results obtained by working on multi-frequency data, and we discuss their relations with the available optical spectra. We compare our findings with the AGN unified model predictions, and we present a revised technique to select AGNs of different types from other line-emitting objects. We discuss the multi-frequency properties in terms of the innermost structures of the sources.
Highlights
When discussing active galactic nuclei (AGNs), we generally refer to the nuclei of galaxies for which a supermassive black hole (SMBH), with a mass ranging between 106 and 1010 M, is fed by a continuous flow of matter from the surrounding environment
Because lines excited by non-thermal ionizing continua differ in intensity and distribution from lines excited by the continuum of hot thermal sources [8,9], we can apply a set of diagnostic diagrams, based on the intensity ratios of specific lines, in order to recognize the footprint of ionization from thermal and non-thermal sources in external galaxies
The processes that occur in the unresolved central regions of AGNs leave characteristic signatures in the emission and absorption components of the observed radiation, and they control which parts of the source are visible
Summary
When discussing active galactic nuclei (AGNs), we generally refer to the nuclei of galaxies for which a supermassive black hole (SMBH), with a mass ranging between 106 and 1010 M , is fed by a continuous flow of matter from the surrounding environment This process, denoted as accretion, leads to the conversion of the gravitational binding energy of the accreted material into heat and radiative energy, through the effects of the viscous interactions that arise in the accreted matter as it is accelerated up to several thousand kilometers per second by the strong gravitational pull of the black hole [1]. While the first to be clearly identified were located in galaxies with exceptionally bright optical nuclei [2], nearly 10% of the total population were subsequently found to radiate large fractions of their power in the radio and the high-energy domains [3] They are distributed from the local universe, where the low-luminosity objects are more common, all the way up to very high redshifts (z ≥ 7), where powerful activity becomes more frequent
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