SIMULATIONS OF STARBURST SPECTRAL EVOLUTION SUPERIMPOSED ON OLD POPULATIONS - EXTENSION TO DWARF ELLIPTICALS

In order to gain a better understanding of starburst and merger events, we have extended to dwarf galaxies our previous spectral simulations of burst evolution over 3 Gyr, superimposed on an old underlying stellar population. We have used a library of star cluster and galaxy integrated spectra in the 3700-9700 Angstrom wavelength range. Two kinds of galaxy spectra representing the underlying red stellar populations have been considered: (i) an average spectrum of the central regions of NGC 3056 and 4476, for which previous population syntheses have shown a maximum metallicity of [Z/Z(circle dot) ]=-0.5, and (ii) a combination of globular cluster spectra in the range -2 less than or equal to[Z/Z(circle dot) ]less than or equal to -1, which could represent the old stellar population in the transition between dwarf ellipticals and dwarf spheroidals. These spectra were combined with those of Large Magellanic Cloud (LMC) and Galactic disc star clusters in the 3-Myr to 3-Gyr age range, in order to simulate starburst events in dwarf galaxies and model the subsequent spectral evolution of the composite system. The star cluster spectra have been properly scaled to represent starburst-to-galaxy mass fractions of 10, 1 and 0.1 per cent. The resulting models form a spectral atlas for comparisons with observations. Rest-frame BVRI magnitudes and the related colours have been derived from these models, as well as the equivalent widths for a set of metallic and Balmer features. The behaviour of these indices is analysed as a function of the burst age and strength. The results can be used to study the properties of star-forming events in galaxies.