Narrow He?II emission in star-forming galaxies at low metallicity - Stellar wind emission from a population of very massive stars

摘要

Context. In a recent study, star-forming galaxies with He?ii?λ1640 emission at moderate redshifts between 2 and 4.6 have been found to occur in two modes that are distinguished by the width of their He?ii emission lines. Broad He?ii emission has been attributed to stellar emission from a population of evolved Wolf-Rayet (WR) stars. The origin of narrow He?ii emission is less clear but has been attributed to nebular emission excited by a population of very hot Pop?III stars formed in pockets of pristine gas at moderate redshifts. Aims. We propose an alternative scenario for the origin of the narrow He?ii 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. Methods. We estimated the expected He?ii line fluxes and equivalent widths based on wind models for VMS and Starburst99 population synthesis models and compared the results with recent observations of star-forming galaxies at moderate redshifts. Results. The observed He?ii 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. Conclusions. In our scenario the two observed modes of He?ii emission originate from massive stellar populations in distinct evolutionary stages at low Z (~0.01 Z⊙). If this interpretation is correct, there is no need to postulate the existence of Pop?III stars at moderate redshifts to explain the observed narrow He?ii emission. An interesting possibility is the existence of self-enriched VMS with similar WR-type spectra at extremely low Z. Stellar He?ii 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 (JWST). The fact that the He?ii emission of VMS is largely neglected in current population synthesis models will generally affect the interpretation of the integrated spectra of young stellar populations.