The Nature of Lyman Alpha Galaxies: A Tale of Two Populations
Lyman alpha emitting galaxies (LAEs) were originally predicted to be young galaxies bright in Lyman alpha due to copious star formation, with possible primordial metallicities. However, a two-phase interstellar medium (ISM) composed of neutral, dusty clumps in an ionized medium could enhance the equivalent width (EW) (Neufeld 1991; Hansen & Oh 2006). Because Lyman alpha photons are resonantly scattered, they can escape from this medium. However, the continuum photons interact with the dust and suffer extinction and reddening. In this scenario, an evolved stellar population could be observed with a large Lyman alpha EW, appearing like a much younger galaxy. We have investigated this scenario using a sample of narrow-band selected galaxies in the GOODS CDF-S (Finkelstein et al. in press). Out of four candidate LAEs, three were best-fit by young, albeit dusty, stellar populations with typical ages of 3 Myr, A_1200 ~ 1.3 and stellar masses of ~ 5x10^8 solar masses. These values are roughly consistent with our earlier study of 21 LAEs (Finkelstein et al. 2007). The fourth object was determined to be an 800 Myr, 7x10^9 solar mass stellar population, with an ISM geometry that enhanced the Lyman alpha EW by 50%. Identification of EW enhancement in this object shows that there may be two distinct populations of LAEs. One population is young with an intrinsically high EW. These usually contain some amount of dust, thus they are not primordial galaxies as previously thought. The other population harbors an evolved stellar population masquerading as a young galaxy due to dust enhancement of the Lyman alpha EW caused by the ISM geometry. Future studies with larger samples will determine how large a role this second population plays, but these types of objects could account for a large fraction of previously discovered Lyman alpha galaxies.