X-ray Properties of Young Early Type Galaxies: II. Abundance Ratio in
  the Hot ISM
  Using Chandra X-ray observations of young, post-merger elliptical galaxies,
we present X-ray characteristics of age-related observational results, by
comparing with typical old elliptical galaxies in terms of metal abundances in
the hot interstellar matter (ISM). While the absolute element abundances may be
uncertain because of unknown systematic errors and partly because of the
smaller amount of hot gas in young ellipticals, the relative abundance ratios
(e.g., the alpha-element to Fe ratio, most importantly Si/Fe ratio) can be
relatively well constrained. We find that in two young elliptical galaxies (NGC
720 and NGC 3923) the Si to Fe abundance ratio is super-solar (at a 99%
significance level), in contrast to typical old elliptical galaxies where the
Si to Fe abundance ratio is close to solar. Also the O/Mg ratio is close to
solar in the two young elliptical galaxies, as opposed to the sub-solar O/Mg
ratio reported in old elliptical galaxies. Both features appear to be less
significant outside the effective radius (roughly 30" for the galaxies under
study), consistent with the observations that confine to the centermost regions
the signatures of recent star formation in elliptical galaxies. Observed
differences between young and old elliptical galaxies can be explained by the
additional contribution from SNe II ejecta in the former. In young elliptical
galaxies, the later star formation associated with recent mergers would have a
dual effect, resulting both in galaxy scale winds - and therefore smaller
observed amounts of hot ISM - because of the additional SNII heating, and in
different metal abundances, because of the additional SNII yields.
