Near-infrared imaging of gigahertz-peaked-spectrum radio galaxies: tracing the evolution of giant ellipticals?

Abstract

In this Letter we present K-band data for a sample of gigahertz-peaked-spectrum (GPS) radio galaxies. We show that the dispersion of the K-band Hubble diagram is small and comparable to that of the R-band Hubble diagram for these sources. The r - K colours of GPS galaxies evolve with redshift in agreement with models for both passively evolving and non-evolving elliptical galaxies. This behaviour indicates that the optical and infrared emission from these sources is dominated by starlight with little or no contamination by light from the active nuclei. At z ,.., 1 the absolute K­ band luminosities are about a magnitude fainter than expected for passively evolving ellipticals, just as in the case for radio-quiet giant ellipticals at similar redshifts. The lack of luminosity evolution may be caused by the dynamical evolution in the galaxies counteracting a change in the mass-to-light ratio of the systems: if the galaxies undergo mergers at z > 0.5, the high-redshift systems will be less massive than those at low redshift. In contrast to powerful radio sources in general, we believe that GPS radio galaxies are representative of early-type galaxies and may be excellent objects for studying the evolution of giant ellipticals at z > 1, given their relative ease of selection.