4.2. Abundance Ratios

In elliptical galaxies, -process elements are more abundant with respect to Fe than they are in the disk of the Milky Way [74]. This may constrain the timescale for star formation in ellipticals, since -process elements are produced in SN II, which explode on a short timescale, while Fe is also produced in SN Ia, which explode after ~ 1 Gyr. Indeed, [Mg/Fe] 0.5 for the nuclear disks in cluster ellipticals [64, 65]. High -process abundances indicate that SN Ia played little role in enriching these galaxies; on the face of it, they also imply that cluster ellipticals formed on timescales 1 Gyr (eg. [75]).

High abundances of -process elements with respect to Fe are also seen in X-ray observations of the diffuse gas in galaxy clusters (eg. [76], but see [77]). The large amounts of metals in cluster gas require remarkably high SN rates which may not be possible with a Salpeter IMF [78]. These results undermine the argument that high /Fe ratios imply short enrichment timescales, since abundances in the cluster gas presumably represent integrated metal production over ~ 10 Gyr. The abundance patterns of cluster ellipticals are clearly inconsistent with mergers of present-day spirals, but do not preclude mergers of moderately gas-rich galaxies containing substantial stellar disks.