Lyα excess in high-redshift radio galaxies:: a signature of star formation

About 54 per cent of radio galaxies at z >= 3 and 8 per cent of radio galaxies at 2 less than or similar to z < 3 show unusually strong Ly alpha emission, compared with the general population of high-redshift ( z greater than or similar to 2) radio galaxies. These Ly alpha-excess objects (LAEs) show Ly alpha/He II values consistent with or above standard photoionization model predictions. We reject with confidence several scenarios to explain the unusual strength of Ly alpha in these objects: shocks, low nebular metallicities, high gas densities and absorption/ scattering effects. We show that the most successful explanation is the presence of a young stellar population which provides the extra supply of ionizing photons required to explain the Ly alpha excess in at least the most extreme LAEs ( probably in all of them). This interpretation is strongly supported by the tentative trend found by other authors for z >= 3 radio galaxies to show lower ultraviolet rest-frame polarization levels, or the dramatic increase in the detection rate at submm wavelengths of z > 2.5 radio galaxies. The enhanced star formation activity in LAEs could be a consequence of a recent merger which has triggered both the star formation and the active galactic nucleus/radio activities. The measurement of unusually high Ly alpha ratios in the extended gas of some high-redshift radio galaxies suggests that star formation activity occurs in spatial scales of tens of kpc. We argue that, although the fraction of LAEs may be incompletely determined, both at 2 less than or similar to z < 3 and at z >= 3, the much larger fraction of LAEs found at z >= 3 is a genuine redshift evolution and not due to selection effects. Therefore, our results suggest that the radio galaxy phenomenon is more often associated with a massive starburst at z > 3 than at z < 3.