Evolution of the Lyman-alpha Halos around High-Redshift Radio Galaxies

TL;DR

This study provides the first constraints on extended Ly-alpha emission around z ~ 1 radio galaxies, revealing a strong evolution in luminosity compared to higher redshift counterparts, likely due to gas depletion and temperature changes.

Contribution

First observational constraints on Ly-alpha halos at z ~ 1, demonstrating significant luminosity evolution compared to higher redshift radio galaxies.

Findings

Ly-alpha luminosities at z ~ 1 are much smaller than at higher redshifts.

Ly-alpha luminosity evolves as (1+z)^5 from z > 2 to z ~ 1.

No strong correlation between Ly-alpha luminosity and radio power.

Abstract

We have obtained the first constraints on extended Ly-alpha emission at z ~ 1 in a sample of five radio galaxies. We detect Ly-alpha emission from four of the five galaxies. The Ly-alpha luminosities range from 0.1 - 4 times 10^43 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 the z >~ 2 radio galaxies, then their Ly-alpha 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.