3.5 Black Hole Formation and AGNs at High z?
The observations of high-redshift galaxies tell us that some structures (albeit perhaps only exceptional ones) must have attained galactic scales by the epoch z = 5. Massive black holes (manifested as quasars) accumulate in the deep potential wells of these larger systems. Quasars may dominate the UV background at z < 3: if their spectra follow a power-law, rather than the typical thermal spectrum of OB stars, then quasars are probably crucial for the second ionization of He, even if H was ionized primarily by starlight. (One interesting point that somewhat blurs this issue has recently been made by Tumlinson and Shull (1999). They note that, if the metallicity were zero, there would be no CNO cycle; high-mass stars therefore need to contract further before reaching the main sequence, and so have hotter atmospheres, emitting more photons above the He ionization edge.)
At redshifts z = 10, no large galaxies may yet have assembled, but CDM-type models suggest that `subgalaxies' would exist. Would these have massive holes (perhaps `mini-AGNs') in their centres? This is interesting for at least two reasons: first, the answer would determine how many high-energy photons, capable of doubly-ionizing He, were produced at very high redshifts (Haiman and Loeb 1998); second, the coalescence of these holes, when their host `subgalaxies' merge to form large galaxies, would be signalled by pulse-trains of low-frequency gravitational waves that could be detected by space-based detectors such as LISA (Haehnelt 1994).
The accumulation of a central black hole may require virialised systems with large masses and deep potential wells (cf Haehnelt and Rees 1993, Haehnelt and Kauffmann (1999)); if so, we would naturally expect the UV background at the highest redshifts to be contributed mainly by stars in `subgalaxies'. However, this is merely an expectation; it could be, contrariwise, that black holes readily form even in the first 108 M CDM condensations (this would be an extreme version of a `flattened' IMF), Were this the case, the early UV production could be dominated by black holes. This would imply that the most promising high-z sources to seek at near-IR wavelengths would be miniquasars, rather than `subgalaxies'. It would also, of course, weaken the connection between the ionizing background and the origin of the first heavy elements.