Star formation history in early-type galaxies. I. The line absorption   indices diagnostics
  The bottom line of this study is (i) to check whether normal (quiescent) and interacting EGs have a different behavior in the popular diagnostic planes such as Hb vs. [MgFe] (and others); (ii) to seek whether the signature of mergers and/or interactions may mirror in some specific changes of the indices that could un-equivocally hint for burts of stellar activity; (iii) to evaluate the intensity of those bursts or secondary episodes of star formation; (iv) to explore whether other alternatives can exist, i.e. distinct from obvious ones resting on large age range. The data show that both normal, field and interacting galaxies have the same scattered but smooth distribution in the Hb vs. [MgFe] diagnostic plane. There are two interpretations for this: (1) EGs span larges ranges of ages and metallicities. (2) The bulk population of stars is old but subsequent episodes of star formation scatter the object in the diagnostic planes. Both these interpretation are not able to reproduce the observed distribution. The above schemes are too demanding because of the many ad hoc ingredients that have to be introduced. Furthermore they neglect the observationally grounded hint that the stellar content of EGs is likely enhanced in alfa-elements. We propose here a new scheme, in which the bulk dispersion of galaxies in the index-index plane is caused by a different mean degree of enhancement. The possibility that EGs span large ranges of [alpha/Fe] and metallicities but narrow ranges of ages for the bulk population favors the monolithic scheme. 
R. Tantalo & C. Chiosi