THE AGES OF HIGH-REDSHIFT RADIO GALAXIES

We compare the observed spectral energy distributions (SEDs) of high-redshift radio galaxies with stellar population synthesis models. We show that the "flat ultraviolet plus red bump" SED observed in these galaxies can be produced with a normal initial mass function (IMF) in ≤3 × 108 yr and persists for more than 6 × 108 yr if most of the stars form on time scales ≤1 × 108 yr. This implies that many of the observed powerful radio galaxies with 1 < z < 4 are young and that they are being observed soon after the event that produced the bulk of their stars. If such an event were organized by the powerful radio sources associated with these objects, this could account for the alignment of the optical/infrared emission with the radio axes observed in these sources, while maintaining the low dispersion and continuity of the infrared Hubble diagram.