SPECTROSCOPY OF ULTRA-STEEP SPECTRUM RADIO-SOURCES - A SAMPLE OF Z-GREATER-THAN-2 RADIO GALAXIES

We present spectroscopic observations for 64 radio galaxies having ult ra steep radio spectra. Twenty-nine objects have redshifts z > 2, the largest redshifts being almost 4. Our ultra steep spectrum (USS) crite rion (alpha < -1) has proven to be the most efficient way of finding d istant radio galaxies. We find that even among the USS sources, there is a strong statistical correlation between the spectral index and red shift. 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 al low us to decide whether there is a decrease in co-moving source densi ty at the highest redshifts. We have analyzed the spectra of the 30 ob jects with the highest redshifts (z > 1.9). For these high redshift ra dio galaxies, Ly alpha is almost always the dominant emission line, wi th a rest frame equivalent width ranging from similar to 100 Angstrom to more than 1000 Angstrom. The equivalent widths of the most importan t emission lines (Ly alpha, C IV, He II, C III]) are found to correlat e 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 lik ely the dominant ionization mechanism. There are significant velocity differences between the various emission lines of our high redshift ra dio galaxies; in particular the Ly alpha line is shifted with respect to the higher ionization lines. Velocity shifts range from 100 to almo st 1000 km s(-1) in some cases. Simulations show that the effects of a ssociated H I absorption on the Ly alpha emission line may be responsi ble for most of these velocity shifts. However, other mechanisms such as organized kinematics of the Ly alpha 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.