Impact of very massive stars on the chemical evolution of extremely metal-poor galaxies

Abstract

Context. In recent observations of extremely metal-poor, low-mass, starburst galaxies, almost solar Fe/O ratios are reported, despite N/O ratios consistent with the low metallicity. Aims. We aim to investigate if the peculiar Fe/O ratios can be a distinctive signature of an early enrichment produced by very massive objects dying as pair-instability supernova (PISN). Methods. We ran chemical evolution models with yields that account for the contribution by PISN. We used both the non-rotating stellar yields from a recent study and new yields from rotating very massive stars calculated specifically for this work. We also searched for the best initial mass function (IMF) that is able to reproduce the observations. Results. We can reproduce the observations by adopting a bi-modal IMF and by including an initial burst of rotating very massive stars. Only with a burst of very massive stars can we reproduce the almost solar Fe/O ratios at the estimated young ages. We also confirm that rotation is absolutely needed to concomitantly reproduce the observed N/O ratios. Conclusions. These results stress the importance of very massive stars in galactic chemical evolution studies and strongly support a top-heavy initial mass function in the very early evolutionary stages of metal-poor starburst galaxies.