|Annu. Rev. Astron. Astrophys. 1999. 37:
Copyright © 1999 by . All rights reserved
7.1. High-Redshift Star Formation
We can conclude from the previous sections that QSOs are associated with vigorous star formation, consistent with the early-epoch evolution of massive galactic nuclei or dense protogalactic clumps (Section 5). However, QSO abundances provide new constraints. For example, the general result for Z Z suggests that most of the enrichment and local star formation occur before QSOs turn on or become observable. The enrichment times can be so short in principle (Figure 13; Hamann & Ferland 1993b) that the star formation might also be coeval with QSO formation. In any event, the enrichment times cannot be much longer that ~ 1 Gyr for at least the highest redshift objects [depending on the cosmology (Figure 1)].
If QSO metallicities are representative of a well-mixed interstellar medium, we can conclude further that the star formation was extensive. That is, a significant fraction of the initial gas must be converted into stars and stellar remnants to achieve Zgas Z. The exact fraction depends on the IMF. A solar neighborhood IMF (Scalo 1990, as in the Solar Neighborhood model of Section 6) would lead to mass fractions in gas of only 15% at Z ~ Z and would not be able to produce Zgas above a few Z at all. Flatter IMFs (favoring massive stars) could reach Zgas Z while consuming less of the gas. For example, the gas fraction when Zgas ~ Z in the Giant Elliptical model of Section 6.2 is nearly 70%.
Figure 7 in Section 2.6.3 illustrates the main star formation characteristics required by the QSO data. The solid curves on the right-hand side of that figure show theoretical BEL ratios from photoionization simulations that use nominal BELR parameters plus abundances from the two chemical-evolution models in Figure 13 (see Hamann & Ferland 1993b for more details). The evolution is assumed to begin with the Big Bang, and the conversion of time into redshift assumes a cosmology with Ho = 65 km s-1 Mpc-1, M = 1, and = 0. (Lower values of Ho or M would push the theoretical curves slightly toward the right in that figure, for example by ~ 20% to ~ 50% in z if M = 0.3, see also Figure 1.) The main results are that the Solar Neighborhood evolution is too slow and, in any case, does not reach high enough metallicities or nitrogen enhancements to match most of the high-redshift QSOs. Much shorter time scales and usually higher metallicities, as in the Giant Elliptical simulation, are needed.
A trend in the NV BELs suggests further that the metallicities are typically higher in more luminous QSOs (Section 2.6.3). That result needs confirmation, but it could result naturally from a mass-metallicity relationship among QSO host galaxies that is similar (or identical) to the well-known relation in low-redshift galaxies (Section 6.1; Hamann & Ferland 1993b). By analogy with the galactic relation, the most luminous and metal-rich QSOs might reside in the most dense or massive host environments. This situation would be consistent with studies showing that QSO luminosities, QSO masses, and central black-hole masses in galactic nuclei all appear to correlate with the mass of the surrounding galaxies (McLeod et al. 1999, McLeod & Rieke 1995, Bahcall et al. 1997, Magorrian et al. 1998, Laor 1998; see also Haehnelt & Rees 1993). Direct application of the galactic mass-metallicity relation suggests that metal-rich QSOs reside in galaxies (or protogalaxies) that are minimally as massive (or as tightly bound) as our own Milky Way.
7.2. Fe / : Timescales and Cosmology
One of the most interesting predictions from galactic studies (Section 6.3) is that Fe / ratios in QSOs might constrain the epoch of their first star formation and perhaps the cosmology. In particular, large Fe/ ratios (solar or higher) would suggest that the local stellar populations are at least ~ 1 Gyr old. At the highest QSO redshifts (z ~ 5), this age constraint would push the epoch of first star formation beyond the limits of current direct observation, to z > 6 (Figure 1). The 1 Gyr constraint would also be difficult to reconcile with M 1 in Big Bang cosmologies (because the age of the universe at z 5 in this cosmology is less than 1 Gyr). Conversely, measurements of low Fe / would suggest that the local stellar populations are younger than ~ 1 Gyr (although we could not rule out the possibility that only SN IIs contributed to the enrichment for some other reason). Some BEL studies have already suggested that Fe / is above solar in z > 4 QSOs (Section 2.6.4), implying (albeit tentatively) that these systems are already 1 Gyr old.
7.3. Comparisons to Other Results
Quasar abundances should be viewed in the context of other measures of the metallicity and star formation at high redshifts. Damped-Ly absorbers in QSO spectra, which probe lines of sight through large intervening galaxies [probably spiral disks (Prochaska & Wolfe 1998)] have mean (gas-phase) metallicities of ~ 0.05 Z at z 2 (Lu et al. 1996, Pettini et al. 1997, Lu et al. 1998, Prochaska & Wolfe 1999). The Ly forest absorbers, which presumably probe much more extended and tenuous intergalactic structures (Rauch 1998), typically have metallicities < 0.01 Z at high redshifts (Rauch et al. 1997, Songalia & Cowie 1996, Tytler et al. 1995). The much higher metal abundances near QSOs are consistent with the rapid and more extensive evolution expected in dense environments (Gnedin & Ostriker 1997). Perhaps this evolution is similar to that occurring in the many star-forming objects that are now measured directly at redshifts comparable to and greater than the QSOs (see references in Section 5.1).
The detections of strong dust and molecular gas emissions from QSOs support the evidence from their high abundances that considerable local star formation preceded the QSO epoch. The dust and molecules, presumably manufactured by stars, appear even in QSOs at z 4 (Isaac et al. 1994, Omont et al. 1996, Guilloteau et al. 1997).