Secondary Episode of Star Formation in Elliptical Galaxies

We put upper limits of secondary burst of star formation in elliptical galaxies of the Gonzalez’s (1993) sample, most of which locates in small groups, based on the colour dispersion around the U − V versus central velocity dispersion relation, and the equivalent width of Hβ absorption. There are significant number of Hβ strong galaxies which have EW(Hβ)> 2 Å, however they do not always have bluer colours in U − V . To be consistent with small colour dispersion of U − V , the mass fraction of secondary burst to the total mass should be less than only 10 % at the maximum within recent 2 Gyr. This result suggests that even if recent galaxy merging produce some ellipticals, it should not have been accompanied by an intensive star burst, and hence it could not involve large gas-rich systems. A capture of dwarf galaxy is more likely to explain dynamical disturbances observed in some elliptical galaxies. The above analysis based on the U − V is not compatible with the one based on the line indices, which requires that more than 10 % of mass is present in a 2 Gyr old star burst to cover the full range of the observed Hβ (de Jong & Davies 1997). The discrepancy might be partly explained by the internal extinction localized at the region where young stars form. However, considering that the Hβ index might have great uncertainties both in models and in observational data, we basically rely on U − V analysis.