The Curious Case of Lyman Alpha Emitters: Growing Younger from z ~ 3 to
  z ~ 2?
  Lyman Alpha Emitting (LAE) galaxies are thought to be progenitors of
present-day L* galaxies. Clustering analyses have suggested that LAEs at z ~ 3
might evolve into LAEs at z ~ 2, but it is unclear whether the physical nature
of these galaxies is compatible with this hypothesis. Several groups have
investigated the properties of LAEs using spectral energy distribution (SED)
fitting, but direct comparison of their results is complicated by
inconsistencies in the treatment of the data and in the assumptions made in
modeling the stellar populations, which are degenerate with the effects of
galaxy evolution. By using the same data analysis pipeline and SED fitting
software on two stacked samples of LAEs at z = 3.1 and z = 2.1, and by
eliminating several systematic uncertainties that might cause a discrepancy, we
determine that the physical properties of these two samples of galaxies are
dramatically different. LAEs at z = 3.1 are found to be old (age ~ 1 Gyr) and
metal-poor (Z < 0.2 Z_Sun), while LAEs at z = 2.1 appear to be young (age ~ 50
Myr) and metal-rich (Z > Z_Sun). The difference in the observed stellar ages
makes it very unlikely that z = 3.1 LAEs evolve directly into z = 2.1 LAEs.
Larger samples of galaxies, studies of individual objects and spectroscopic
measurements of metallicity at these redshifts are needed to confirm this
picture, which is difficult to reconcile with the effects of 1 Gyr of
cosmological evolution.
