8.4 Metal Production at High Redshift
The work of Steidel and collaborators (e.g. Steidel et al. 1996) has confirmed a substantial population of star-forming galaxies at z around 3, with a comoving number density of roughly 10 to 50% that of present day luminous (L L) galaxies. From these data it has been possible to sketch - although with large uncertainties and questions - the star-forming history of the Universe at high redshifts (Madau et al. 1996). From this work, and other similar samples, e.g. from the HDF, the overall SFR of the galaxy population seems to increase from z = 0 to z = 2, during which time a significant fraction of the heavy elements in the Universe are formed, while it tails off again towards higher redshift. Recently, Steidel et al. (1999) questioned the decrease in the number of star forming galaxies at high redshift, and suggest that the cosmic star formation rate as inferred from Lyman break galaxies has been constant from z = 1 to z = 4. Recent results from sub-mm observations of redshifted dust emission with the SCUBA bolometer array (e.g. Hughes et al. 1998) and from the ISO satellite (Aussel et al. 1999; Puget and Guiderdoni 1999) suggest that a significant amount of high redshift star formation may be dust obscured, meaning that estimates of the cosmic metal production rate from optical observations might be too low. The cosmic metal production history may also be constrained by observations of the extragalactic background at different wavelengths (see Pagel 1997 for a discussion).
Numerical simulations by Cen and Ostriker (1999) predict the evolution of the metal content of the Universe as a function of density, by incorporating star formation and its feedback on the IGM. At a given density (corresponding e.g. to a rich cluster, a disc galaxy, dwarf etc.) their models predict an evolution with redshift, but more importantly, the metallicity was found to be a strong function of density. At low redshift, low density environments will still be very metal-poor as suggested by the presence of very metal-poor gas-rich dwarf galaxies, and Ly absorption systems (Shull et al. 1998). Ferrara and Tolstoy (1999) find in their simulations of the feedback from star formation on the ISM in dwarf galaxies, that low mass galaxies could be the main contributor to metals in the IGM.
The idea of so-called population III objects has been discussed for quite a while. These hypothetical objects would be (or host) the first generation of stars, appearing before the main epoch of galaxy formation, and thus be a source of pre enrichment in the Universe. The subject remains speculative, but the existence of Pop III stars cannot be ruled out (Cayrel 1996).