1999A&A...352L..51D

We present optical, infrared and radio data of the z=4.11 radio galaxy TN J1338-1942 including an intermediate resolution spectrum obtained with FORS1 on the VLT Antu telescope. TN J1338-1942 was the first z>4 radio galaxy to be discovered in the southern hemisphere and is one of the most luminous Lyα objects in its class. The Lyα and rest-frame optical emission appear co-spatial with the brightest radio hotspot of this very asymmetric radio source, suggesting extremely strong interaction with dense ambient clouds. The Lyα is spatially extended by ∼4"(30kpc), has an enormous rest-frame equivalent width, W_λ^rest=210±50Å, and has a spectral profile that is very asymmetric with a deficit towards the blue. We interpret this blue-ward asymmetry as being due to absorption of the Lyα photons by cold gas in a turbulent halo surrounding the radio galaxy and show that the required neutral hydrogen column density must be in the range 3.5-13x10¹⁹cm^–2. The two-dimensional spectrum indicates that the extent of the absorbing gas is comparable (or even larger) than the 4"(30kpc) Lyα emitting region. The VLT observations are sufficiently sensitive to detect the continuum flux both blue-ward and red-ward of the Lyα emission, allowing us to measure the Lyα forest continuum break (Lyα `discontinuity', D_A) and the Lyman limit. We measure a D_A=0.37±0.1, which is ∼0.2 lower than the values found for quasars at this redshift. We interpret this difference as possibly due to a bias towards large D_A introduced in high-redshift quasar samples that are selected on the basis of specific optical colors. If such a bias would exist in optically selected quasars, - and even in samples of Lyman break galaxies -, then the space density of both classes of object will be underestimated. Furthermore, the average HI column density along cosmological lines of sight as determined using quasar absorption lines would be overestimated. Because of their radio-based selection, we argue that z>4 radio galaxies are excellent objects for investigating D_A statistics.