VLT Spectroscopy of the z=4.11 Radio Galaxy TN J1338-1942

Abstract

We present optical, infrared and radio data of the z=4.11 radio galaxy TN J1338-1942, including an intermediate resolution spectrum obtained with FORS1on 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 Ly-alpha objects in its class. The Ly-alpha 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 VLT observations are sufficiently sensitive to detect the continuum flux both blue-ward and red-ward of the Ly-alpha emission, allowing us to measure the Ly-alpha forest continuum break (Ly-alpha '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. [abridged]