2. VERY EARLY STAR FORMATION

Giant elliptical galaxies are found to have experienced the bulk of their star formation at early times and to have undergone enrichment very rapidly. A similar evolution appears to have taken place in the bulges of spiral galaxies. But how early is "early"? Age dating resolved old stellar field populations in nearby galaxies remains difficult due to crowding, extinction, the superposition of stellar populations of different ages, and the general difficulty of associating individual red giants with a specific age. These problems are exacerbated in galaxies where only integrated-light properties can be analyzed, which makes it very difficult to date stellar populations older than a few Gyr.

2.1. Quasars at redshift 6

An alternative approach, limited to very luminous and hence presumably very massive, actively star-forming objects, is the analysis of galaxies observed at high redshift. Surprisingly, even the very young quasars discovered at a redshift of z ~ 6 (age of the Universe: ~ 900 Myr) reveal metal absorption lines that translate into supersolar metallicities, indicating very rapid, early enrichment. These objects may be the precursors of giant ellipticals (Fan et al. 2001). The masses of the central black holes in z ~ 6 QSOs are estimated to range from several 10⁸ to several 10⁹ M. This suggests formation redshifts of more than 10 for putative 100 M seed black holes if continuous Eddington accretion is assumed. Other, more rapid black hole formation mechanisms may also play a role (e.g., Fan 2007).

For one of the highest-redshift quasars known, SDSS J114816.16+525150.3 at z = 6.42, sub-millimeter and radio observations suggest a molecular gas mass (CO) of ~ 5 × 10¹⁰ M within a radius of 2.5 kpc around the central black hole if the gas is bound. The star formation rate in this object is ~ 1000 M yr^-1, akin to what has been derived for star-bursting ultraluminous infrared galaxies. Even within 20 kpc, the inferred mass estimate for the QSO's host is still comparatively low, ~ 10¹¹ M. A central massive 10¹² M bulge has evidently not yet formed (Walter et al. 2004; Fan 2007).

In several z 6 QSOs dust has been detected. One of the primary sources for dust in the present-day Universe are low- and intermediate-mass asymptotic giant branch stars, but these need at least some 500 Myr to 1 Gyr to begin generating dust in their envelopes. In quasars in the early Universe, other mechanisms are believed to be responsible, including dust production in supernovae of type II (Maiolino et al. 2004) and in quasar winds (Elvis et al. 2002). Recently two z ~ 6 QSOs without dust were detected, which indicates that these objects are probably first-generation QSOs forming in an essentially dust-free environment (Jiang et al. 2010). This discovery illustrates that even at these very early times of less than 1 Gyr after the Big Bang, galactic environments differ in the onset of massive star formation and in the degree of heavy-element pollution, with some regions already having experienced substantial enrichment. One may speculate that the densest regions are the first ones to start massive star formation, and that the dusty high-redshift QSOs trace these regions.

Magnesium, an element, is produced in supernovae of type II and hence expected to be generated soon after massive star formation commences. Iron is predominantly produced in supernovae of type Ia, requiring a minimum delay time of ~ 300 Myr when instantaneous starbursts are considered (e.g., Matteucci & Recchi 2001). Fe is detected in z ~ 6 QSOs despite their young age. The Fe II / Mg II ratio (which is essentially a proxy for Fe/) is found to be comparable to that of lower-redshift QSOs (e.g., Barth et al. 2003; Freudling et al. 2003; Kurk et al. 2007) and to be around solar or super-solar metallicity. The near-constancy of the Fe II/Mg II ratio as a function of redshift implies a lack of chemical evolution in QSOs since z ~ 6 and suggests a formation redshift of the SN Ia progenitors of z 10. Extremely rapid enrichment on a time scale of just a few hundred Myr must have occurred in these QSOs, much faster than the slow enrichment time scales of spiral galaxies. Nonetheless, also here evolutionary differences are becoming apparent: Some of the z ~ 6 QSOs are less evolved, not showing strong Fe II emission lines (Iwamuro et al. 2004) and hence no significant SN Ia contributions yet.

2.2. Galaxies at redshift 7

Moving to even higher redshifts, the analysis of spectral energy distributions obtained from near-infrared imaging data of galaxies at redshifts of 7 (age of the Universe ~ 770 Myr) and 8 (~ 640 Myr) revealed median ages of ~ 200 Myr for their stellar populations, but even younger ages are not excluded (Finkelstein et al. 2010). The typical stellar masses of the galaxies at z ~ 7 are < 10⁹ M; they may be as low as only 10⁷ M at z ~ 8. While some galaxies are consistent with having no internal dust extinction, the median value is A_V ~ 0.3 mag. Finkelstein et al. (2010) estimate that some 10⁶ M of dust may have been produced in these galaxies through massive (12 - 35 M) stars that turned into SNe II, a process expected to take less than 20 Myr (Todini & Ferrara 2001).

Finkelstein et al. (2010) derive metallicities of 0.005 Z for the majority of their galaxies, while some may have somewhat higher values (0.02 Z). Inferring maximal stellar masses of a few times 10⁹ M for the z ~ 7 and 8 galaxies, the authors emphasize that these masses are still considerably lower than suggested for L_* counterparts at z < 6. These distant, luminous galaxies whose colors resemble those of local, metal-poor star-bursting dwarf galaxies trace the earliest times of high-redshift galaxy formation accessible to us with the current instrumentation.