The ages of quasar host galaxies

We present the results of fitting deep off-nuclear optical spectra of radio -quiet quasars, radio-loud quasars and radio galaxies at z similar or equal to 0.2 with evolutionary synthesis models of galaxy evolution. Our aim was to determine the age of the dynamically dominant stellar populations in th e host galaxies of these three classes of powerful active galactic nuclei ( AGN). Some of our spectra display residual nuclear contamination at the sho rtest wavelengths, but the detailed quality of the fits longward of the 400 0-Angstrom break provides unequivocal proof, if further proof were needed, that quasars lie in massive galaxies with (at least at z similar or equal t o 0.2) evolved stellar populations. By fitting a two-component model we hav e separated the very blue (starburst and/or AGN contamination) from the red der underlying spectral energy distribution, and find that the hosts of all three classes of AGN are dominated by old stars of age 8-14 Gyr. If the bl ue component is attributed to young stars, we find that, at most, 1 per cen t of the visible baryonic mass of these galaxies is involved in star format ion activity at the epoch of observation, at least over the region sampled by our spectroscopic observations. These results strongly support the concl usion reached by McLure et al. that the host galaxies of luminous quasars a re massive ellipticals which have formed by the epoch of peak quasar activi ty at z similar or equal to 2.5.