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Astron. Astrophys. 331, L1-L4 (1998)

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3. Implications for galaxy formation and suppression

Near the hole, the ionizing radiation certainly suppresses star formation in normal molecular clouds, which, if of density n, survive only at a distance greater than
[FORMULA] from the central quasar. The ultraviolet flux is effective at destroying [FORMULA] molecules produced via [FORMULA] formation to much larger distances, e.g. the [FORMULA] photodissociation rate is [FORMULA] at [FORMULA] from the quasar whereas the rate of the compet ing [FORMULA] formation process is [FORMULA] (Tegmark et al. 1997). This would tend to suppress formation of dwarf galaxies out to [FORMULA], If the radiation extends to the x-ray band, there is however a narrow regime where the hard ionizing flux, by maintaining a fraction of free electrons deep inside the cloud, may actually enhance star formation by stimulating [FORMULA] formation (Haiman, Rees and Loeb 1996).

There are several further noteworthy consequences of the hypothesis that supermassive black holes form within the first subgalactic structures that virialise at high redshift, and are in place before most galactic stars have formed. AGN activity generates outflows that can interact dynamically with the surrounding protogalactic gas as well as provide a possible early flux of hard ionizing photons. Star formation in the accretion disk surrounding the broad emission line region of the AGN is likely to proceed under conditions very different from those encountered even in starbursts. Star-forming clouds at distance [FORMULA] from the supermassive black hole must be dense enough to avoid tidal disruption, or n 'gsim; [FORMULA]. One infers that the Jeans mass ([FORMULA]) is reduced to stellar scale, and also that the specific angular momentum of such a clump is reduced (as [FORMULA]), relative to values in conventional star-forming clouds.

Suppose that the circum-quasar accretion disk is dense enough to be self-shielding and to be predominantly molecular. If enriched to near-solar abundance level, CO molecules and other species provide important cooling and should result in a temperature of order [FORMULA] One then infers that two of the classical barriers to star formation are likely to be overcome. Hence any gas which falls close enough to the centre to be part of an accretion disc should convert efficiently into stars. Outflow from these stars that form under the gravitational influence of the hole provides a source of metal enrichment, and may thereby stimulate more widespread star formation in a protogalaxy. Magnetic flux will be ejected, the accretion disk providing the conditions for an efficient dynamo. The subsequent turbulent mixing by supernova remnants and stretching by differential rotation could provide a possible origin for the interstellar magnetic field.

There are also interesting consequences for the intergalactic medium (IGM), since quasar-driven winds could readily eject enriched material from the shallow potential wells that characterize the earlier stages of hierarchical clustering. This could account for the apparent presence of up to 1 percent of the solar abundance of heavy elements even at high z. If this is the case, a time delay is likely between the epoch of most quasar activity and the epoch of the bulk of star formation in the universe. One observes a time delay corresponding to [FORMULA]: studies of quasars have confirmed a peak in the quasar number density between [FORMULA] and 3, and the star formation history of the universe is found to peak between [FORMULA] and 2.

The cores of present-day galaxies may carry traces of the black hole merging history. Megers and accretion will allow the black hole to grow to the critical value as determined by Eq. (1). Our key prediction is the relation between central black hole mass and spheroid mass.

Luminous elliptical cores are best explained by heating associated with binary black hole decay following a major merger (Magorrian et al. 1997). The present model envisages that this process operates at all stages of the hierarchy but only towards

the end would the gas be mostly exhausted. Perhaps this would help explain the transition with increasing luminosity from coreless spheroids to hot galaxies with cores.

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© European Southern Observatory (ESO) 1998

Online publication: February 16, 1998
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