Massive stars in subparsec rings around galactic centres

Department of Physics & Astronomy, University of Leicester, Leicester LE1 7RH
Monthly Notices of the Royal Astronomical Society (Impact Factor: 5.52). 10/2006; 372(1):143 - 150. DOI: 10.1111/j.1365-2966.2006.10772.x
Source: arXiv

ABSTRACT We consider the structure of self-gravitating marginally stable accretion discs in galactic centres in which a small fraction of the disc mass has been converted into protostars. We find that protostars accrete gaseous disc matter at prodigious rates. Mainly due to the stellar accretion luminosity, the disc heats up and thickens geometrically, shutting off further disc fragmentation. The existing protostars, however, continue to gain mass by gas accretion. As a result, the initial mass function for disc-born stars at distances R∼ 0.03–3 pc from the supermassive black hole should be top-heavy. The effect is most pronounced at around R∼ 0.1 pc. We suggest that this result explains observations of rings of young massive stars in our Galaxy and in M31, and we predict that more such rings will be discovered.

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    ABSTRACT: I review the recent progresses that have been obtained, especially through the use of high-resolution numerical simulations, on the dynamics of self-gravitating accretion discs. A coherent picture is emerging, where the disc dynamics is controlled by a small number of parameters that determine whether the disc is stable or unstable, whether the instability saturates in a self-regulated state or runs away into fragmentation, and whether the dynamics is local or global. I then apply these concepts to the case of AGN discs, discussing the implications of such evolution on the feeding of supermassive black holes. Nonfragmenting, self-gravitating discs appear to play a fundamental role in the process of formation of massive black hole seeds at high redshift ( z ~ 10–15) through direct gas collapse. On the other hand, the different cooling properties of the interstellar gas at low redshifts determine a radically different behaviour for the outskirts of the accretion discs feeding typical AGNs. Here the situation is much less clear from a theoretical point of view, and while several observational clues point to the important role of massive discs at a distance of roughly a parsec from their central black hole, their dynamics is still under debate.
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