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4.2. Abundance Ratios

In elliptical galaxies, alpha-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 alpha-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] appeq 0.5 for the nuclear disks in cluster ellipticals [64, 65]. High alpha-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 ltapprox 1 Gyr (eg. [75]).

High abundances of alpha-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 alpha/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.