1997A&A...326..505R

We present spectroscopic observations for 64 radio galaxies having ultra steep radio spectra. Twenty-nine objects have redshifts z>2, the largest redshifts being almost 4. Our ultra steep spectrum (USS) criterion (α←1) has proven to be the most efficient way of finding distant radio galaxies. We find that even among the USS sources, there is a strong statistical correlation between the spectral index and redshift. The most distant radio galaxies within the USS sample have the steepest radio spectra. In our sample there are 3 radio galaxies at z>3 compared with 26 at 2<z<3. However, the present data do not allow us to decide whether there is a decrease in co-moving source density at the highest redshifts. We have analyzed the spectra of the 30 objects with the highest redshifts (z>1.9). For these high redshift radio galaxies, Lyα is almost always the dominant emission line, with a rest frame equivalent width ranging from ∼100Å to more than 1000Å. The equivalent widths of the most important emission lines (Lyα, C IV, He II, C III]) are found to correlate strongly with each other. The large rest frame equivalent widths and the correlation between the equivalent widths of the emission lines, confirm that photoionization by a central continuum source is most likely the dominant ionization mechanism. There are significant velocity differences between the various emission lines of our high redshift radio galaxies; in particular the Lyα line is shifted with respect to the higher ionization lines. Velocity shifts range from 100 to almost 1000 km/s in some cases. Simulations show that the effects of associated H I absorption on the Lyα emission line may be responsible for most of these velocity shifts. However, other mechanisms such as organized kinematics of the Lyα emission line gas (e.g. inflow or outflow) and obscuration of the line emission from the far side of the radio galaxy may also play a role.