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

Abstract

About 54 per cent of radio galaxies at z≥ 3 and 8 per cent of radio galaxies at 2 ≲z < 3 show unusually strong Lyα emission, compared with the general population of high-redshift (z≳ 2) radio galaxies. These Lyα-excess objects (LAEs) show Lyα/He ii values consistent with or above standard photoionization model predictions. We reject with confidence several scenarios to explain the unusual strength of Lyα 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 Lyα 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 Lyα 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 ≲z 3 than at z < 3.