Giant Black Holes and Disks on the Balance

Quasars and active galactic nuclei (AGN) are likely powered by matter accretion onto a super-massive black hole located at their center. Before being swallowed by the black hole, matter spirals towards the center, while forming an accretion disc. Unfortunately, such a disc is too small so that one can in general solve it with present day telescopes. But the technique of radio interferometry with very long base (VLBI, with base length of the size of the earth’s radius) make it possible somehow. In some objects, very intense maser emission from small molecular clouds containing water vapor and probably related to the disc have been detected. From the rotation curve of the masing disc, one can deduce some of its properties (the disc mass, its size).

Jean-Marc Huré, from the Laboratory Universe and Theories (LUTH) at Observatory of Paris-Meudon and University Paris VII, comes to show that in galaxy NGC 1068, the accretion disc would have a mass comparable with that of the black hole (with about 9 million solar masses), and a size reaching one parsec (3 light-years). Such informations bring an additional proof that the discs of quasars and AGN are indeed gigantic systems.

Quasars were discovered at the end of the Sixties. They are, with their low luminosity analogues called “Active Galactic Nuclei” (or AGN), among the most luminous objects in the Universe. Today still, all the mechanisms which could release such a power are far from being understood. However, it seems rather well established that the matter accretion on a super-massive black hole is the key-process.

The structure and the dynamics of the accretion disc remain quite mysterious. The disc is not directly observable because the resolution of current telescopes is still insufficient. It is primarily studied at short wavelengths (UV, X and gamma rays). But short wavelength spectra give information only on the internal regions of the disc (scale of the micro-parsec), very close to the black hole. The external parts of the disc (the milliparsec-scale) are made up of colder gas and radiate in the visible, infra-red, and mm bands. One suspects that at these distances, the mass of the disc (generally regarded as small) starts to play a role on its own dynamics, and thus on its evolution and its structure. At the parsec scale for example, models indicate that the mass of the disc could reach (even exceed) that of the black hole. One then expects very particular effects, like a non-keplerian rotation, and the generation of gravitational instabilities (spiral waves, etc.) who could lead to the formation of compact objects in the disc itself (like stars or planets) (Collin & Zahn, 1999, A & A, 344, 433). A point is that, the accretion disc is made of a certain amount of gas and dust, and thus it inevitably generates a certain gravity field. When this mass exceeds a fraction of the central mass (about 10% typically), then the departure to the keplerian rotation law is significant: the centrifugal force is no more compensated by the central attraction only but by the combined gravitational attraction of the black hole and of the disc.

From this point of view, an interesting case is that of active galaxy (of Seyfert-2 type) NGC 1068. One observed in this object an intense maser emission of water molecules at a distance ranging between 0.65 and 1.1 parsec of the black hole. These emissions would take place at the surface of the disc. The external rotation curve deduced from Doppler shifts does not resemble the kepler law. A recent calculation by Jean-Marc Huré, from the Observatory of Paris-Meudon and University PAris VII, (employing an inversion method of the Poisson’s equation) comes to support the assumption that the external disc could be well responsible for this non-keplerian behavior (Huré J.M., A & A Let, 2002, 395,21).

The results state indeed that one can reproduce this rotation curve provided that the outer disc has quite specific properties. Thus, parameters of the disc have been obtained. In particular, the disc would have a mass close to that of the black hole (approximately 9 million solar masses) and would be in a marginally stable state with respect to self-gravity. In addition to the constraints on the disc structure, the study also gives a value of the mass of the central black hole, inaccessible in such a galaxy by usual methods (briefly, because of a strong obscuration of this system by a torus of dust which interposes on the line of sight).

Another interesting galaxy is NGC 4258 : in this object, maser emission was also detected but here, the rotation of the disc seems in perfect agreement with Kepler’s law. Would the disc of NGC 4258 be thus not very massive, contrary to the case of NGC 1068? It is what everyone thinks… However a similar study (Huré J-M., astro-ph/0210421) shows that such a conclusion is far from being acquired. Indeed, it is possible to reproduce a keplerian rotation curve with a disc finally rather massive, reducing by 25% the mass of the black hole that one seemed to know quasi-perfectly.

The moral of the history is that the mass in the central parsec of the AGN and the Quasars is probably not concentrated into the black hole only. Other objects orbiting at these distances, to begin with the accretion disc, might contain a noticeable (even dominant) fraction of it. The inversion method used here enables to see indirectly how the mass is spatially distributed, to refine or to correct our estimations of black hole masses, and gradually to unveil the external part of the accretion discs.