Star Formation in Accretion Disks Around Massive Black Holes and Pregalactic Enrichment
Abstract
High redshift quasars are known to harbour in their very center (~one parsec) high velocity outflows of gas with a metallicity larger than solar (observed as the so-called "Broad Absorption Lines"). We propose that this enrichment takes place in the accretion disk fueling the black holes. In the gravitationally unstable part of the disk, between 0.01 and 1 pc, we show that stars can form and grow by accretion, then evolve to the stage of supernovae and explode in the disk. The gaseous fraction of the disk is maintained in a state of marginal instability by the heating of the stars, and supernovae provide the transfer of momentum required for a large accretion rate. This mechanism is particularly efficient for disks made of primordial gas, and it works as well for small (growing) black holes as for powerful quasars. In the course of the process, the overabundant gas flows out of the disk, enriching in heavy elements the forming host galaxy and the intergalactic medium. This mechanism explains the enrichment and the outflows in quasars, but we propose that it could also be the starting point of heavy element formation in the Universe, provided that massive black holes now present in the centres of galaxies formed early in the past. It can explain without the need to appeal to a pregalactic "Population III", the abundances observed in the oldest stars of the Galactic halo, the abundances in the intergalactic medium (i.e. the L-α forest clouds), and the abundances in the damped L-α and metal systems which are supposed to be tracers of massive galaxies.
- Publication:
-
Galaxy Evolution: Connecting the Distant Universe with the Local Fossil Record
- Pub Date:
- 1999
- Bibcode:
- 1999gecd.conf..501C