High-redshift radio galaxies and the role of radio sources in galaxy formation

Radio, optical, and infrared studies were made of a portion of ultra steep 4C radio sources. Most are at high redshift, including 4C40.36 at z = 2.267 and the most distant galaxy known, 4C41.17 at z = 3.8. Their optical/infrared continua were found to be generally aligned with their radio axes. Demonstration of this alignment effect in the near infrared, by means of an infrared image of 3C368, provides a powerful constraint on theories of the origin of the alignment. The first direct evidence that the extended optical and infrared continuum of high redshift radio galaxies is dominated by starlight is presented in the form of stellar absorption features discovered in combined spectra. The shape of the rest frame ultraviolet spectrum suggests that although the present star formation rate in these objects is high, it was systematically higher in the immediate past. A young model for the stellar populations in high redshift radio galaxies is discussed and suggests that some of these galaxies may be relatively young and that they are being observed near the time in which they formed the bulk of their stars. This model is attractive in its ability to account for the spectral energy distributions (SED's) , the optical and infrared alignments, and the low dispersion and continuity of the infrared Hubble diagram. Therefore the most plausible explanation for the alignment effect is that powerful radio sources were capable of stimulating vast amounts of star formation at early epochs. Investigations of the properties of radio cocoons indicates that weak radio sources may also trigger star formation during the formation of a galaxy. Given the ubiquity of active nuclei in galaxies, the upturn in the faint radio source counts, and the isotropic extragalactic non-thermal background, it is suggested that radio sources may have played a significant role in determining some of the characteristics of normal galaxies