4.4. FeII/MgII and Cosmology
The observed ratios of MgII 2800 to FeII (UV) emission lines might provide the Fe / age discriminator discussed in Section 2.3. The FeII emission spectrum is rich and blended, and the emission characteristics are not well understood because the Fe+ atom is complex. Nonetheless, it is worth investigating because of the potential importance of Fe / abundances. FeII / MgII appears to be the best available emission-line diagnostic of Fe / because these lines have similar wavelengths and they should form in similar regions. They are also measurable in significant numbers of QSOs. Previous work on FeII (Wills et al. 1985) suggested that the Fe/Mg abundance ratio is typically several times larger than solar in QSOs (at moderate redshifts), indicating that the star formation began at least ~ 1 Gyr prior to the look-back times of those objects. More recent observations indicate that the FeII / MgII line ratios are large, consistent with large Fe/Mg abundances, even in the highest redshift QSOs (at z > 4, Thompson, Elston & Hill 1999, Taniguchi et al. 1997). New theoretical calculations (Verner et al. 1999, Sigut & Pradhan 1998) of the FeII emission from BELRs will further test and quantify the FeII / MgII emission lines for abundance purposes.