Spectroscopy of Ultra--Steep Spectrum Radio Sources: A Sample of z>2 Radio Galaxies

Abstract

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, Ly$α$ is almost always the dominant emission line, with a rest frame equivalent width ranging from $\sim100$ Å to more than 1000 Å. The equivalent widths of the most important emission lines (Ly$α$, C\,{\small IV}, He\,{\small II}, C\,{\small 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 Ly$α$ line is shifted with respect to the higher ionization lines. Velocity shifts range from 100 to almost 1000 km s$^{-1}$ in some cases. Simulations show that the effects of associated H\,{\small I} absorption on the Ly$α$ emission line may be