The formation epoch of massive ellipticals, as traced by radio galaxies

Abstract

I review the current status of an on-going investigation into the stellar and dynamical ages of the oldest known galaxies at redshifts greater than 1. The spectroscopic data, when compared with the predictions of a range of recent evolutionary synthesis models, continue to indicate that the oldest galaxies at z = 1.5 are already at least 3 Gyr old. Moreover, comparison with new models which incorporate chemical evolution, suggests that this result may be less sensitive to the assumed metallicity than was previously suspected. Such ages should therefore be taken seriously as constraints both on theories of galaxy formation and on cosmological models. Dynamically these objects also appear well evolved; they display de Vaucouleurs surface brightness profiles, and I demonstrate that their HST-derived morphological parameters place them on the same Kormendy relation as the z = 1 3CR galaxies. Finally I consider the ages of these intermediate redshift objects in the context of our recently-completed SCUBA sub-mm survey of radio galaxies spanning the redshift range z = 1 - 4. These data indicate that the main epoch of star-formation in radio galaxies lies at z = 4, a result which fits naturally with the ages of the oldest galaxies at z = 1.5 within an open Universe.