The accretion disks around super-massive black holes : do they really exist ?

Suzy Collin and Jean-Marc Huré (DAEC, Observatoire de Paris), put back into question, in a recent publication, the "standard" model of accretion disk around super-massive black holes : this "standard" disk cannot explain all observations on active galactic nuclei and quasars : continuum radiation and broad emission lines. Another interpretation is proposed. Active Galactic Nuclei (AGN) and quasars (QSO) are the most luminous objects in the sky. These are powered by accretion of interstellar gas onto a supermassive black hole (BH) of about 100 millions solar masses which stands at the center of galaxies. For Seyfert galaxies, a few tenths of solar masses of gas per year is required to fuel the black hole ; in the case of the brightest QSOs, this is as much as a hundred stars like our Sun. Because the gas has initially some rotation energy, it does not free falls onto the black hole but spirals forming a disc. The properties of this disc --- and specially the most central regions where the UV and X-ray spectrum is emitted--- are not elucidated yet, mainly due to the lack of spatial resolution (a resolution of one millionth of arc second would be necessary to resolve this region in nearby galaxies). Observations tend to show that the very outer regions of the disc (at one parsec from the central object) probably rotate with the keplerian velocity, like planets do about the Sun. According to the theory, the global spectrum of an optically thick, keplerian disc (a "standard disc") should resemble a big bump peaking in the visible. The presence of this continuum radiation feature in the spectrum of AGN and QSOs has been for the last three decades one of the best evidence that accretion involves a standard disc. AGN also emit radiation in the radio, X and gamma ranges due to a relativistic jet of particles, and in the far-infrared range as well due to the presence of dust surrounding the central region (see figure) . Superimposed to this wide, continuum emission, AGN spectra display broad emission lines emitted by an ionized gas. Optical and UV lines are broadened by Doppler effect. Their width is compatible with motions of 10 000 km/s which correspond to gas rotating at about 1000 - 10000 gravitational radii RG from the BH (RG 1.5 10 13 cm for a 10 8 solar mass BH). This zone is called Broad Line Region (or BLR). In the X-ray range, a 7 keV iron line with a width of 50 000 -100 000 km/s is probably emitted very close to the black hole horizon (10 RG). astro-ph/0103303. Contact : Suzy Collin et Jean-Marc Hur&eacute (Département DAEC, Observatoire de Paris)