Modelling Starbursts in HII Galaxies: What do we need to fit the observations?

We have computed a series of realistic and self-consistent models that have been shown to be able to reproduce the emitted spectra of HII galaxies in a star bursting scenario (Martin-Majon et al. 2008). Our models combine different codes of chemical evolution, evolutionary population synthesis and photoionization. The emitted spectrum of HII galaxies is reproduced by means of the photoionization code CLOUDY (Ferland, 1998), using as ionizing spectrum the spectral energy distribution (SED) of the modelled HII galaxy, calculated using the new and updated stellar population models PopStar (Molla & Garcia-Vargas 2009, in prep.).This, in turn, is calculated according to a star formation history and a metallicity evolution given by a chemical evolution model. Each model is characterized by three parameters which are going to determine the evolution of the modeled galaxy: an initial efficiency of star formation, the way in which burst take place, and the time of separation between these bursts. Some model results emerging from the combination of different values for these three parameters are shown here. Our technique reproduces observed abundances, diagnostic diagrams and equivalent width-colour relations for local HII galaxies.

💡 Research Summary

The paper presents a comprehensive, self‑consistent modeling framework designed to reproduce the observed spectra of H II galaxies under a star‑bursting scenario. The authors combine three distinct computational tools: (1) a chemical evolution code that tracks the time‑dependent metallicity and gas mass of a galaxy, (2) the PopStar stellar population synthesis model that generates the spectral energy distribution (SED) of the evolving stellar component, and (3) the CLOUDY photo‑ionization code that uses the PopStar SED as the ionizing source to compute the nebular emission lines.

Three key parameters control each model realization: the initial star‑formation efficiency (SFE), the mode of star‑burst occurrence (burst versus continuous‑burst), and the time interval between successive bursts. By varying SFE (0.1–0.5), burst mode, and inter‑burst spacing (0.1–1 Gyr), the authors explore a grid of evolutionary histories. Higher SFE leads to stronger early UV output, rapid metal enrichment, and a pronounced shift in diagnostic line ratios; longer inter‑burst intervals allow gas re‑accretion and dilution, moderating the metallicity growth.

The resulting SEDs are fed into CLOUDY, where gas density, geometry, and the metallicity supplied by the chemical evolution model are kept consistent. The simulated emission‑line ratios—particularly