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Narrow He II emission in star-forming galaxies at low metallicity. Stellar wind emission from a population of Very Massive Stars

In a recent study star-forming galaxies with HeII emission between redshifts 2 and 4.6 have been found to occur in two modes, distinguished by the width of their HeII emission lines. Broad HeII emission has been attributed to stellar emission from a population of evolved Wolf-Rayet (WR) stars while narrow HeII emission has been attributed to nebular emission excited by a population of very hot PopIII stars formed in pockets of pristine gas at moderate redshifts. In this work we propose an alternative scenario for the origin of the narrow HeII emission, namely very massive stars (VMS) at low metallicity (Z) which form strong but slow WR-type stellar winds due to their proximity to the Eddington limit. We estimate the expected HeII line fluxes and equivalent widths based on wind models for VMS and population synthesis models, and compare the results with recent observations of star-forming galaxies at moderate redshifts. The observed HeII line strengths and equivalent widths are in line with what is expected for a population of VMS in one or more young super-clusters located within these galaxies. In our scenario the two observed modes of HeII emission originate from massive stellar populations in distinct evolutionary stages at low Z. If this interpretation is correct there is no need to postulate the existence of PopIII stars at moderate redshifts to explain the observed narrow HeII emission. An interesting possibility is the existence of self-enriched VMS with similar WR-type spectra at extremely low Z. Stellar HeII emission from such very early generations of VMS may be detectable in future studies of star-forming galaxies at high redshifts with the James Webb Space Telescope. The fact that the HeII emission of VMS is largely neglected in current population synthesis models will generally affect the interpretation of the integrated spectra of young stellar populations.