  We present an investigation into the absorber in front of the z=2.63 radio
galaxy MRC 2025-218, using integral field spectroscopy obtained at the Very
Large Telescope, and long slit spectroscopy obtained at the Keck II telescope.
The properties of MRC 2025-218 are particularly conducive to study the nature
of the absorbing gas, i.e., this galaxy shows bright and spatially extended
Ly-alpha emission, along with bright continuum emission from the active
nucleus. Ly-alpha absorption is detected across ~40x30 kpc^2, has a covering
factor of ~1, and shows remarkably little variation in its properties across
its entire spatial extent. This absorber is kinematically detached from the
extended emission line region (EELR). Its properties suggest that the absorber
is outside of the EELR. We derive lower limits to the HI, HII and H column
densities for this absorber of 3x10^16, 7x10^17 and 2x10^18 cm^-2,
respectively. Moreover, the relatively bright emission from the active nucleus
has allowed us to measure a number of metal absorption lines: CI, CII, CIV, NV,
OI, SiII, SiIV, AlII and AlIII. The column density ratios are most naturally
explained using photoionization by a hard continuum, with an ionization
parameter U~0.0005-0.005. Shocks or photoionization by young stars cannot
reproduce satisfactorily the measured column ratios. Using the ratio between
the SiII* and SiII column densities, we derive a lower limit of >10 cm^-3 for
the electron density of the absorber. The data do not allow useful constraints
to be placed on the metallicity of the absorber. We consider two possibilities
for the nature of this absorber: the cosmological infall of gas, and an outflow
driven by supernovae or the radio-jets.
