High redshift radio galaxies

Abstract

There is considerable evidence that powerful radio quasars and radio galaxies are orientation-dependent manifestations of the same parent population: massive spheroids containing correspondingly massive black holes. Following the recognition of this unification, research is directed to the task of elucidating the structure and composition of the active nuclei and their hosts to understand the formation and evolution of what we expect to become the most massive of galaxies. In contrast to the quasars, where the nucleus can outshine the galaxy at optical/near infrared wavelengths by a large factor, the radio galaxies contain a 'built-in coronograph' that obscures our direct view to the nucleus. These objects present our best opportunity to study the host galaxy in detail. Of particular interest are those sources with redshifts greater than about 2 that represent an epoch when nuclear activity was much more common that it is now and when we believe these objects were in the process of assembly. In combination with high resolution imaging from space (HST), optical spectropolarimetry with Keck II allows us to clearly separate the scattered nuclear radiation from the stellar and gaseous emission from the host galaxy. The rest-frame ultraviolet emission line spectra suggest that rapid chemical evolution is occurring at this epoch. Near infrared spectroscopy with the VLT is giving us access to both the lines and continuum in the rest-frame optical spectrum, allowing investigations of the evolved stellar population and extending the composition analysis with measurements of the familiar forbidden-line spectrum.