Spatially extended absorption around the z= 2.63 radio galaxy MRC 2025−218: outflow or infall?

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 longslit 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 Lyα emission, along with bright continuum emission from the active nucleus. Lyα absorption is detected across ∼40 × 30 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 H I ,H II and H column densities for this absorber of 3 × 10 16 ,7 × 10 17 and 2 × 10 18 cm −2 , respectively. Moreover, the relatively bright emission from the active nucleus has allowed us to measure a number of metal absorption lines: C I ,C II ,C IV ,N V ,O I ,S iII ,S iIV ,A lII and Al III. 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 Si II ∗ and Si II 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. We find it plausible that the absorber around 2025−218 is in outflow. We also find good agreement between the observed properties of the H I absorber and the properties of the H I absorption expected from the cosmological infall model of Barkana & Loeb.