2009ApJ...694L..31Z

We have obtained the first constraints on extended Lyα emission at z ∼ 1 in a sample of five radio galaxies. We detect Lyα emission from four of the five galaxies. The Lyα luminosities range from 0.1-4 x10⁴³ erg/s and are much smaller than those observed for halos around higher redshift radio galaxies. If the z ~ 1 radio galaxies are the descendents of the z ≲ 2 radio galaxies, then their Lyα luminosities evolve strongly with redshift as ~(1 + z)⁵. There do not appear to be strong correlations between other parameters, such as radio power, suggesting that this observed evolution is real and not an observational artifact or secondary correlation. We speculate that this evolution of luminous halos may be due to gas depletion (as gas cools, settles, and forms stars) accompanied by an overall rise in the mean gas temperature and a decrease in specific star formation rate in and around these massive galaxies.