Volume 379, Number 3, December I 2001
|Page(s)||1138 - 1152|
|Published online||15 December 2001|
Formation of a proto-quasar from accretion flows in a halo
Indian Institute of Astrophysics, Koramangala, Bangalore 560034, India
Accepted: 4 October 2001
We present a detailed model for the formation of massive objects at the centers of galaxies. The effects of supernovae heating and the conditions of gas loss are revisited. The escape time of the gas is compared with the cooling time, which provides an additional condition not previously considered. Its consequences for the allowed mass range of the halo is calculated and parameterized in terms of the spin parameter, , the redshift of collapse, zc, the fraction of baryons in stars, , and the efficiency of supernovae, ν. It is shown that sufficient gas is retained to form massive dark objects and quasars even for moderately massive halos but a decline is expected at low redshifts. Subsequently, a gaseous disk forms with a radial extent of a kpc, spun up by tidal torques and magnetized by supernovae fields with fields strengths of 10-100 . In a model of a self-similar accretion flow in an initially dominant halo, it is shown that for typical halo parameters, about accretes via small magnetic stresses (or alternatively by self-gravity induced instability or by alpha viscosity) in 108 years into a compact region. A model of a self-gravitating evolution of a compact magnetized disk ( pc), which is relevant when a significant fraction of the disk mass falls in, is presented, and it has a rapid collapse time scale of a million years. The two disk solutions, one for accretion in an imposed halo potential and the other for a self-gravitating disk, obtained here, have general utility and can be adapted to other contexts like protostellar disks as well. Implications of this work for dwarf galaxy formation, and a residual large scale seed field, are also breifly discussed.
Key words: accretion, accretion disks / magnetic fields / galaxies: formation / cosmology: theory
© ESO, 2001
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