Simulations of starburst spectral evolution superimposed on old populations : extension to dwarf ellipticals

Abstract

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 Å 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/Z̛0 ]= -0.5, and (ii) a combination of globular cluster spectra in the range - 2≤s;[Z/Z̛] ≤ -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 1O, 1 and 0.1 per cent. The resulting models forma spectral atlas for comparisons with observa­ tions. 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 índices 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.