SIMBAD references

We present intermediate resolution (∼3AA) 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 HI absorption with column densities in the range 10¹⁸-10^19.5cm^–2. Since in most cases the Lyα emission is absorbed over its entire spatial extent (up to 50kpc), 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 ∼0.03pc with a total mass of ∼10⁸M_☉. Our observations show that strong HI 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 HI 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 (<50kpc) radio galaxies 9 out of 10 have strong associated HI 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 dispersion than smaller radio sources. The sizes of the Lyα regions range from ∼15 to ∼130kpc and radio sizes from ∼20 to ∼180kpc. The Lyα velocity dispersions range from ∼700km/s(FWHM) in the largest radio sources to ∼1600km/s(FWHM) in the smallest. In the smaller radio sources Lyα is often observed to be more extended than the radio emission. We have also defined several parameters to describe the spatial distortion of the Lyα velocity field and of the radio structure. We find a strong correlation between the amount of distortion present in the Lyα spectra and the distortion of the radio structure. These strong correlations show that the radio sources have a profound influence on their environments. The correlations of radio size with the gas velocity dispersion and the Lyα size, are evidence for interaction of the radio jet with the ionized gas. Three different scenarios to explain these correlations are discussed: (o) The first is based on the properties of the environment. The smaller radio sources are situated in denser (cluster) environments than the larger sources. The relatively dense environment is responsible for the strong extended HI absorptions. Kinetic energy is transferred from the radio plasma to the gaseous medium, resulting in larger line widths and reduced propagation velocities for the radio lobes. The turbulence associated with the radio plasma may expose more gas from dense clouds to the ionizing radiation, causing the Lyα emission region to have an extent similar to the radio source. (o) The second scenario linking radio size and Lyα is based on evolution of the radio source. When the radio source is small it interacts strongly with the dense central gas, while as it tunnels through the medium to larger sizes the interaction and thus the gas velocity dispersion decreases. The radio size - Lyα size relation has the same origin as in scenario 1. (o) The third scenario is based on orientation effects. If radio galaxies are quasars whose nuclei and broad line regions are obscured towards our line of sight, larger line widths may be expected if we observe closer to the edge of the ionization cone. In this scenario the radio size - Lyα size relation would be simply due to projection effects. The largest radio sources have the largest Lyα sizes with the smallest velocity dispersions because they are oriented closest to the plane of the sky and the broad line regions are most strongly obscured. This would not explain, however, why the largest sources show almost no associated HI absorption.