The Gaseous Environments of Radio Galaxies in the Early Universe: Kinematics of the Lyman α Emission

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We present intermediate resolution (3 Å) spectra of the Lyα emission from 15 high redshift radio galaxies (z>2). Together with previously published spectra we analyze data for a sample of 18 objects. In 11 of the 18 radio galaxies we find deep troughs in the Lyα emission profile, which we interpret as H i absorption with column densities in the range 10¹⁸--10¹⁹.⁵ cm⁻². Since in most cases the Lyα emission is absorbed over its entire spatial extent (up to 50 kpc), the absorbers must have a covering fraction close to unity. Under plausible assumptions for the temperature and density of the absorbing gas this implies that the absorbing material must consist of 10¹² clouds of typical size 0.03 pc with a total mass of 10⁸ M. Our observations show that strong H i absorption occurs in >60% of the high redshift radio galaxies, while from the statistics of quasar absorption lines there is only a 2% probability of such a strong H i absorption line falling by chance in the small redshift interval of the Lyα emission line. These absorbers are therefore most likely to be physically associated with the galaxy hosting the radio source or its direct environment. There are strong correlations between the properties of the Lyα emission of the galaxies and the size of the associated radio source: (i) Of the smaller (<50 kpc) radio galaxies 9 out of 10 have strong associated H i absorption, whereas only 2 of the 8 larger (>50 kpc) radio galaxies show such strong absorption. (ii) Larger radio sources tend to have larger Lyα emission regions and (iii) a smaller Lyα velocity