How old are HII galaxies

Using a novel approach we have reanalysed the question of whether the extreme star-forming galaxies known as H ii galaxies are truly young or rejuvenated old systems. We first present a method of inversion that applies to any monotonic function of time describing the evolution of independent events. We show that, apart from a normalization constant, the ‘true’ time dependence can be recovered from the inversion of its probability density function. We applied the inversion method to the observed equivalent width of the Hβ [EW(Hβ)] distribution for objects in the Spectrophotometric Catalogue of H ii Galaxies compiled by Terlevich and collaborators, and found that the global history of star formation behaves more like a continuous star-formation model than an instantaneous model. On the other hand, when the inversion method is applied to samples within a restricted metallicity range, we find that their history of star formation is much closer to what the instantaneous model predicts. Our main conclusion is that, globally, the evolution of H ii galaxies seems consistent with a succession of short starbursts separated by quiescent periods and that, while the emission lines trace the properties of the present burst, the underlying stellar continuum traces the whole star-formation history of the galaxy. Thus, observables like the EW(Hβ) that combine an emission line flux, i.e. a parameter pertaining to the present burst, with the continuum flux, i.e. a parameter that traces the whole history of star formation, should not be used alone to characterize the present burst.