Journal
ASTROPHYSICAL JOURNAL
Volume 565, Issue 2, Pages L71-L74Publisher
IOP PUBLISHING LTD
DOI: 10.1086/338980
Keywords
cosmology : observations; galaxies : evolution; galaxies : formation; galaxies : general; galaxies : statistics
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The Large Area Lyman Alpha survey has found approximate to150 Lyalpha emitters at z = 4.5. While stellar models predict a maximum Lyalpha equivalent width (EW) of 240 Angstrom, 60% of the Lyalpha 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 alpha = 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 Lyalpha emission to explain the observed number counts. This raises the possibility either that star formation in high-redshift galaxies is episodic or that the Lyalpha galaxies we are seeing are the youngest 7.5%-15% and that Lyalpha is strongly quenched by dust at about 10(7) yr of age.
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