Lyman-α emitters as tracers of the transitioning Universe

Of all the many ways of detecting high-redshift galaxies, the selection of objects by their redshifted Lyα emission has become one of the most successful. But what types of galaxies are selected in this way? Until recently, Lyα emitters were understood to be small star-forming galaxies, the possible building-blocks of larger galaxies. But with increased number of observations of Lyα emitters at lower redshifts, a new picture emerges. Lyα 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 Lyα emitters increases dramatically between redshifts three and two. Here, the fraction of AGN among the LAEs is shown to follow a similar evolutionary path. We argue that Lyα 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. Lyα emitters should thus be excellent tracers of galaxy evolution in future simulations and modelling.