Large Equivalent width Lyman-alpha line emission at z=4.5: young galaxies in a young universe

Abstract

The Large Area Lyman Alpha survey has found ~ 150 Lyman-alpha emitters at z=4.5. While stellar models predict a maximum Lyman-alpha equivalent width (EW) of 240 angstrom, 60% of the Lyman-alpha emitters have EWs exceeding this value. We attempt to model the observed EW distribution by combining stellar population models with an extrapolation of Lyman break galaxy luminosity function at z=4, incorporating observational selection effects and Malmquist bias. To reproduce the high EWs seen in the sample we need to postulate a stellar initial mass function (IMF) with extreme slope alpha = 0.5 (instead of 2.35); zero metallicity stars; or narrow-lined active galactic nuclei. The models also reveal that only 7.5-15% of galaxies need show Lyman-alpha emission to explain the observed number counts. This raises the possibility that either star-formation in high redshift galaxies is episodic or the Lyman-alpha galaxies we are seeing are the youngest 7.5-15% and that Lyman-alpha is strongly quenched by dust at about 10 Mega-years of age.