Old stellar populations in distant radio galaxies

Abstract

I describe the current status of our attempts to determine the age of the oldest known stellar populations at high redshift, in the red mJy radio galaxies 53W091 ($z = 1.55$) and 53W069 ($z = 1.43$). During the past year the original conclusion of Dunlop et al. (1996) - that 53W091 is $>3$ Gyr old - has been questioned from two, basically orthogonal directions. First, reports that the near-infrared light from 53W091 is highly polarized have cast some doubt on whether its red colour is genuinely due to an old population of stars. Second, assuming that all the light is indeed due to stars, it has been claimed that 53W091 is in fact only 1-2 Gyr old. Here I present a preliminary analysis of new infrared polarimetric observations of 53W091 which show that the first of these criticisms can be rejected with very high confidence. I then explore why different modellers have derived different ages for 53W091, and present new model fits to the spectrum of 53W069 which demonstrate that different spectral synthesis codes are certainly in good agreement that this galaxy is 3-4 Gyr old. Finally I present a preliminary analysis of the morphologies and scale-lengths of 53W091 and 53W069 as derived from new I-band WFPC2 HST images, and compare the results with those for 3CR galaxies at comparable redshifts. I conclude that the scalelengths and luminosities of radio galaxies at $z \simeq 1.5$ appear to scale together as would be predicted from the Kormendy relation for low-redshift elliptical galaxies.