Lyman Alpha Galaxies: Primitive, Dusty, or Evolved?

Abstract

We present stellar population modeling results for ten newly discovered Lyman alpha emitting galaxies (LAEs), as well as four previously known LAEs at z ~ 4.5 in the Chandra Deep Field-South. We fit stellar population models to these objects in order to specifically learn if there exists more than one class of LAE. Past observational and theoretical evidence has shown that while many LAEs appear to be young, they may be much older, with Lyα equivalent widths enhanced due to resonant scattering of Lyα photons in a clumpy interstellar medium (ISM). Our results show a large range of stellar population age (3-500 Myr), stellar mass (1.6 × 10⁸ to 5.0 × 10¹⁰ M _sun), and dust extinction (A ₁₂₀₀ = 0.3-4.5 mag), broadly consistent with previous studies. With such a large number of individually analyzed objects, we have looked at the distribution of stellar population ages in LAEs for the first time, and we find a very interesting bimodality, in that our objects are either very young (< 15 Myr) or old (> 450 Myr). This bimodality may be caused by dust, and it could explain the Lyα duty cycle which has been proposed in the literature. We find that eight of the young objects are best fit with a clumpy ISM. We find that dust geometry appears to play a large role in shaping the spectral energy distributions that we observe, and that it may be a major factor in the observed Lyα equivalent width distribution in high redshift Lyα galaxies, although other factors (i.e., outflows) may be in play. We conclude that 12 out of our 14 LAEs are dusty star-forming galaxies, with the other two LAEs being evolved galaxies.

Based in part on observations taken at the Cerro Tololo Inter-American Observatory.