Secondary episodes of star formation in elliptical galaxies

We put upper limits on the secondary burst of star formation in elliptical galaxies of the González sample, based on the colour dispersion around the U - V versus central velocity dispersion relation, and the equivalent width of Hβ absorption. Note that most of these galaxies locate in small groups. There is a significant number of Hβ strong galaxies that have EW(Hβ) > 2 Å, but they do not always have bluer colours in U - V. To be consistent with the small colour dispersion of U - V, the mass fraction of the secondary burst to the total mass should be less than 10 per cent at the maximum within the most recent 2 Gyr. This result suggests that even if recent galaxy merging has produced some ellipticals, it should not have been accompanied by an intensive starburst, and hence it could not involve large gas-rich systems. The capture of a dwarf galaxy is more likely to explain the 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 per cent of mass is present in a 2-Gyr-old starburst to cover the full range of the observed Hβ (de Jong & Davies). 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.