Large equivalent width Lyα line emission at z = 4.5: Young galaxies in a young universe?

The Large Area Lyman Alpha survey has found ≈ 150 Lyα emitters at z = 4.5. While stellar models predict a maximum Lyα equivalent width (EW) of 240 Å, 60% of the Lyα emitters have EWs exceeding this value. We attempt to model the observed EW distribution by combining stellar population models with an extrapolation of a 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 with an extreme slope of α = 0.5 (instead of 2.35), zero-metallicity stars, or narrow-line active galactic nuclei (AGNs). Not all the high-EW sources can be AGNs without exceeding the X-ray background or postulating a very steep redshift evolution. Only 7.5%-15% of galaxies need to show Lyα emission to explain the observed number counts. This raises the possibility either that star formation in high-redshift galaxies is episodic or that the Lyα galaxies we are seeing are the youngest 7.5%-15% and that Lyα is strongly quenched by dust at about 10⁷ yr of age.