HI in the proto - cluster environment at z > 2 : absorbing haloes and the Ly α forest ⋆

We present VLT-UVES echelle spectra of the Lyα emission and absorption in five radio galaxies at redshifts z=2.55–4.1. Together with data from our pilot study, we have a sample of 7 such systems with radio source sizes ∼ 1 − 90 kpc with which to address the origin of the absorbing gas. Echelle resolution again reveals that some systems with NHI > 10 cm in lower resolution data in fact consist of several weaker absorbers with NHI < 10 cm . We identify two groups of HI absorbers: strong absorbers with NHI ≃ 10 − 10 cm and weaker systems with NHI ≃ 10 − 10 cm . There are none at intermediate NHI. The strong absorbers may be a by-product of massive galaxy formation or could instead represent material cooling behind the expanding bow-shock of the radio jet, as simulated by Krause. New observations are required to discriminate between these possibilities. We argue that the weaker absorbers with NHI ≃ 10 − 10 cm are part of the Lyα forest, as their rate of incidence is within a factor of 2–4 of that in the IGM at large. Such column densities are consistent with models of a multi-phase proto-intracluster medium at z > 2.