2.3. Fe / as a Cosmological Clock
Another relevant result from galactic chemical evolution is that the ratio of Fe to -element (O, Ne, Mg, . . .) abundances can constrain the ages of star-forming systems (Wheeler et al. 1989). The Fe / age constraint follows from the different enrichment timescales; elements come from the supernova explosions of short-lived massive stars (primarily Type II - SN II's), while Fe has a large contribution from the longer-lived intermediate-mass binaries that produce Type Ia supernovae (SN Ia's). The subsequent delay in the Fe enrichment, roughly 1 Gyr, does not depend on the star formation rate or other uncertain parameters of the chemical evolution; it depends only on the lifetimes of the SN Ia precursors (see Section 3 below, also Yoshii et al. 1996). The ratio of Fe / abundances can therefore serve as an absolute ``clock'' for constraining the epoch of first star formation and, perhaps, the cosmology itself (see Hamann & Ferland 1993a - hereafter HF93a). For example, measurements of high Fe / (near solar or higher) in z > 4 QSOs would place the epoch of star formation beyond the limits of current direct observations (z > 6) and might be problematic for cosmologies with M = 1 (Fig. 1).