|Photograph by Dorothy Davis Locanthi, courtesy AIP Emilio Segrè Visual Archives|
The study of galactic nuclei has advanced rapidly during the past few years, not least due to major advances in high angular resolution instrumentation at a variety of wavelength domains. Observations carried out with space-borne telescopes, such as the Hubble Space Telescope (HST), or from the ground, using adaptive optics, have allowed us to study the kinematics of stars or gas in regions reaching down to sub-parsec scales for external galaxies and to the milliparsec range for the Milky Way. An outstanding conclusion is that dark compact objects, very probably massive black holes (MBHs), with masses ranging between one million and a thousand million solar masses, are present at the centre of most galaxies for which such observations can be made. A deep link exists between the central MBH and its host galaxy.
This linkage has been illuminated by the discovery of correlations between the mass of the MBH and global properties of the spheroid, most tightly in its velocity dispersion, the so-called Mass - relation. The centre-most part of a galaxy, its nucleus, consists of a cluster of a few ten millions to a few hundred million stars surrounding the MBH, with a size of a few pc.
The nucleus is naturally expected to play a major role in the interaction between the MBH and the host galaxy. In the nucleus stellar densities in excess of a million per cubic pc and relative velocities of the order a few 100 to a few 1000 km/s are reached. In these exceptional conditions, and unlike anywhere else in the bulk of the galaxy, collisional effects come into play. These include 2-body relaxation, that is, mutual gravitational deflections, and genuine contact collisions.
The modes of interactions between stars and the MBH which are of particular importance are firstly, that stars can produce gas to be accreted on to the MBH through normal stellar evolution, collisions or disruptions of stars by the strong central tidal field. These processes may contribute significantly to the mass of the MBH . Tidal disruptions trigger phases of bright accretion that may reveal the presence of a MBH in an otherwise quiescent, possibly very distant, galaxy. Secondly, stars can be swallowed whole if they are kicked directly through the horizon (so-called direct plunges) or gradually inspiral due to the emission of Gravitational Waves (GWs). The later process, known as an "Extreme Mass Ratio Inspiral" (EMRI) is one of the main targets of the future space-borne GW antenna, LISA (Laser Interferometer Space Antenna). Mergers of supermassive or intermediate-mass black holes are also crucial targets for the detector and the data it will deliver will give answer to some of the most interesting, though also most difficult, questions of Astrophysics nowadays.