Very massive stars in high-redshift galaxies

A significant fraction of Ly alpha-emitting galaxies (LAEs) at z >= 5.7 have rest-frame equivalent widths (EWs) greater than similar to 100 angstrom. However, only a small fraction of the Ly alpha flux produced by a galaxy is transmitted through the intergalactic medium, which implies intrinsic Ly alpha EWs that are in excess of the maximum allowed for a Population II stellar population having a Salpeter mass function. In this paper, we study characteristics of the sources powering Ly alpha emission in high-redshift galaxies. We propose a simple model for Ly alpha emitters in which galaxies undergo a burst of very massive star formation that results in a large intrinsic EW, followed by a phase of Population II star formation with a lower EW. We confront this model with a range of high-redshift observations and find that the model is able to simultaneously describe the following eight properties of the high-redshift galaxy population with plausible values for parameters like the efficiency and duration of star formation: (i)-(iv) the ultraviolet and Ly alpha luminosity functions of LAEs at z = 5.7 and 6.5, (v)-(vi) the mean and variance of the EW distribution of Ly alpha-selected galaxies at z = 5.7, (vii) the EW distribution of i-drop galaxies at z similar to 6, and (viii) the observed correlation of stellar age with EW. Our modelling suggests that the observed anomalously large intrinsic EWs require a burst of very massive star formation lasting no more than a few to 10 per cent of the galaxy's star-forming lifetime. This very massive star formation may indicate the presence of Population III star formation in a few per cent of i-drop galaxies, and in about half of the Ly alpha-selected galaxies.