The spatial extent of the Z = 2.04 absorber in the spectrum of PKS 0458-020

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Abstract

The z = 2.29 QSO PKS 0458-020 is especially interesting because (1) it chances to lie behind a high column density absorber at z = 2.04 that is responsible for a strong 21 cm absorption line and (2) it has radio structure on a wide range of scales so that it offers the opportunity to use high-resolution radio interferometry to probe a number of different paths through the absorbing medium. Preliminary results from VLA and VLBI observations serve to define the structure of the background radio source. The line depth predicted by assuming that the gas only covers the compact core component differs from the observed line depth at the 6 σ level of significance. Rather, the gas must cover a large fraction of the extended components. We find that the absorber is a disk-like structure that extends across galactic dimensions of at least 8h^-1^ kpc. The invariance of velocity structure across the illuminated disk further indicates that its radius of curvature exceeds 40h^-1^ kpc, in the case of a flat rotation curve with speed equal to 200 km s^-1^. A VLA L band observation of the redshifted λ = 6 cm l_10_-l_11_ transition of H_2_CO finds no trace of the molecular line in a spectrum having velocity resolution 2.3 km s^-1^ and rms noise level σ_tau_ = 1.75 x 10^-3^.

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