The Stellar Populations of HII galaxies: A tale of three bursts

TL;DR

This study models the star formation history of HII galaxies using spectral energy distribution fitting across UV to mid-IR wavelengths, revealing they experience three major starburst episodes with current bursts being maximal but contributing minimally to total stellar mass.

Contribution

Introduces a three-burst star formation history model for HII galaxies and refines burst age estimates by accounting for old stellar populations.

Findings

Star formation in HII galaxies occurs in three major episodes.

Current starbursts contribute less than a few percent of total stellar mass.

Hβ luminosity correlates with young star mass, supporting a universal IMF.

Abstract

We present a UV to mid-IR spectral energy distribution study of a large sample of SDSS DR13 HII galaxies. These are selected as starburst (EW(H$\alpha) > 50$\AA) and for their high excitation locus in the upper-left region of the BPT diagram. Their photometry was derived from the cross-matched GALEX, SDSS, UKDISS and WISE catalogues. We have used CIGALE modelling and SED fitting routine with the parametrization of a three burst star formation history, and a comprehensive analysis of all other model parameters. We have been able to estimate the contribution of the underlying old stellar population to the observed equivalent width of H$\beta$ and allow for more accurate burst age determination. We found that the star formation histories of HII Galaxies can be reproduced remarkably well by three major eras of star formation. In addition, the SED fitting results indicate that: i) in all cases the current burst produces less than a few percent of the total stellar mass: the bulk of stellar mass in HII galaxies have been produced by the past episodes of star formation; ii) at a given age the H$\beta$ luminosity depends only on the mass of young stars favouring a universal IMF for massive stars; iii) the current star formation episodes are {\it maximal} starbursts, producing stars at the highest possible rate.