  Of the many ways of detecting high redshift galaxies, the selection of
objects due to their redshifted Ly-alpha emission has become one of the most
successful. But what types of galaxies are selected in this way? Until
recently, Ly-alpha emitters were understood to be small star-forming galaxies,
possible building-blocks of larger galaxies. But with increased number of
observations of Ly-alpha emitters at lower redshifts, a new picture emerges.
Ly-alpha emitters display strong evolution in their properties from higher to
lower redshift. It has previously been shown that the fraction of
ultra-luminous infrared galaxies (ULIRGs) among the Ly-alpha emitters increases
dramatically between redshift three and two. Here, the fraction of AGN among
the LAEs is shown to follow a similar evolutionary path. We argue that Ly-alpha
emitters are not a homogeneous class of objects, and that the objects selected
with this method reflect the general star forming and active galaxy populations
at that redshift. Ly-alpha emitters should hence be excellent tracers of galaxy
evolution in future simulations and modeling.
