| mf.description.abstract-ext |
Active galactic nuclei Nowadays it is widely accepted that every massive galaxy harbors a
supermassive black hole (SMBH) at its center. A number of apparent correlations between
SMBH mass and host galaxy structural and dynamical properties have been observed. The
correlation between the masses of SMBHs and their host galactic bulges suggest a link between
their growth (Kormendy & Richstone, 1995; Kormendy & Gebhardt, 2001). Active galactic
nucleus (AGN) represents a phase in the life of a galaxy, during which the SMBH growth is
directly observable. The term AGN encompasses a variety of energetic phenomena in galactic
centers triggered by the matter spiralling into a SMBH at a relatively high rate.
The radiation coming from AGNs originates in the conversion of gravitational potential energy
into thermal energy as matter spirals towards the SMBH through an accretion disk (Lynden-
Bell, 1969). Their luminosity can be up to 10000 greater then the total luminosity of a normal
galaxy. The radiated AGN continuum covers a broad range of spectrum, from the X to radio
domain, it is partially polarized and variable in time. Radiation from the central engine is
ionizing the surrounding medium, creating conditions for the strong emission line spectrum, superimposed
on the continuum. Sometimes, highly collimated and fast outflows (“jets”) emerge
perpendicular to the accretion disk. Since the discovery of Keel (1980) that the orientation of Seyfert 1 galaxies is not random, it
xxx
has been recognized that the appearance of an AGN varies with the viewing angle. This has
led to the picture of “orientation unification” (see Antonucci, 1993; Urry & Padovani, 1995)
where the structure of AGNs is believed to be basically similar but what we see is a strong
function of orientation (see Fig. 9). In this unified model, the central black hole is surrounded
by a geometrically-thin accretion disk which is the source of the strong X-ray emission and
UV/optical continuum (see Jovanovi´c, 2012, and references therein). Above and below the disk
is the broad-line region (BLR), turbulent, rapidly-moving, dense, emission-line gas orbiting
the black hole (see Gaskell, 2009, for a review). Both the accretion disk and the BLR are
surrounded by a geometrically- and optically-thick, roughly toroidal structure of dust and gas
(the “dusty torus”), which is absorbing the incoming radiation and re-emitting it in the infrared
(IR). In addition to these components there is lower density, more slowly moving gas present
on a scale similar to or significantly larger than that of the torus. This gas can be seen when it is
illuminated by the cone of ionizing radiation emanating from inside the torus. It is a source of
narrow emission lines and thus is know as the “narrow-line region” (NLR). The broad emission
lines and the thermal continuum emission can only be seen when the torus is close to faceon
and thus, such an object appears as a type 1 active galaxy. Close to edge-on orientations,
the dusty torus blocks the radiation coming from the accretion disk and BLR. In this case an
UV/optical bump and broad emission lines are absent and an object appears as a type 2 active
galaxy. If jet of matter, ejected perpendicular to the accretion disk is present, then viewing
such an object along the jet would exhibit strong non-thermal, polarized and rapidly variable
continuum (see ? for an in-depth discussion on di erent types of AGN).
The masses of SMBHs can be readily estimated in some types of AGN, (Dibai, 1977) and AGNs
are currently our only way of studying the evolution of SMBHs over cosmic time. Furthermore,
the brightest AGNs are the most luminous quasi-steady compact sources of radiation in the
universe and hence they are valuable probes of cosmic evolution up to very high redshifts. In
order to understand black hole growth across cosmic time and the connection between galaxies
and black holes, we need to understand how AGNs work. We need to test the basic picture
outlined above and, in particular, to be able to explain observations which presently challenge
this picture and might force modifications of it. |
en_US |