2003MNRAS.340...29C

In systems undergoing starbursts the evolution of the young stellar population is expected to drive changes in the emission-line properties. This evolution is usually studied theoretically, with a combination of evolutionary synthesis models for the spectral energy distribution of starbursts and photoionization calculations. In this paper we present a more empirical approach to this issue. We apply empirical population synthesis techniques to samples of starburst and HII galaxies in order to measure their evolutionary state and correlate the results with their emission-line properties. A couple of useful tools are introduced that greatly facilitate the interpretation of the synthesis: (1) an evolutionary diagram, the axes of which are the strengths of the young, intermediate age and old components of the stellar population mix; and (2) the mean age of stars associated with the starburst, {formmu0}. These tools are tested with grids of theoretical galaxy spectra and found to work very well even when only a small number of observed properties (absorption-line equivalent widths and continuum colours) is used in the synthesis. Starburst nuclei and HII galaxies are found to lie on a well-defined sequence in the evolutionary diagram. Using the empirically defined mean starburst age in conjunction with emission-line data, we have verified that the equivalent widths of Hβ and [OIII] decrease for increasing {formmu1}. The same evolutionary trend was identified for line ratios indicative of the gas excitation, although no clear trend was identified for metal-rich systems. All these results are in excellent agreement with long-known, but little tested, theoretical expectations.