Large Equivalent Width Lyα line Emission at z=4.5: Young Galaxies in a Young Universe?
Abstract
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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- February 2002
- DOI:
- 10.1086/338980
- arXiv:
- arXiv:astro-ph/0111126
- Bibcode:
- 2002ApJ...565L..71M
- Keywords:
-
- Cosmology: Observations;
- Galaxies: Evolution;
- Galaxies: Formation;
- Galaxies: General;
- Galaxies: Statistics;
- Astrophysics
- E-Print:
- submitted to ApJ letters