Old elliptical galaxies at z ≃ 1.5 and the Kormendy relation

Deep spectroscopy of the two Leiden–Berkeley Deep Survey mJy radio galaxies LBDS 53W069 and 53W091 has previously shown them to have old (≳3 Gyr) stellar populations at z≃ 1.5. Here we present the results of Hubble Space Telescope (HST) observations with the Wide Field and Planetary Camera 2 (WFPC2) in F814W and with the Near-Infrared Camera and Multi-Object Spectrograph (NICMOS) in F110W. We find that 53W069 has a de Vaucouleurs r1/4 profile in both the F814W and F110W data, with a mean effective radius of 0.30 ± 0.06 arcsec (2.7 ± 0.5 kpc). The rest-frame U−B colour gradient is consistent with that of present-day ellipticals, requiring a stellar population of supersolar (3 Z⊙) metallicity that formed on a very short time-scale at high redshift (z > 5). 53W091 has a regular r1/4 profile in F110W with an effective radius of 0.32 ± 0.08 arcsec (2.9 ± 0.7 kpc). The F814W profile is more extended and is consistent with the presence of a blue exponential disc that contributes 20 ± 10 per cent of the flux within re. We find a rest-frame U−B colour gradient that is significantly larger than that observed in field ellipticals at z≤ 1, implying a stellar population of mixed metallicity (1–3 Z⊙) that formed in a high-redshift rapid burst. We have compared these two LBDS radio galaxies with the Kormendy relations of ten 3CR radio galaxies at z≃ 0.8 and a sample of cluster ellipticals at z∼ 0.4. The LBDS galaxies follow the Kormendy relation for the more radio-luminous 3CR galaxies, assuming passive evolution of their stellar populations, although they are smaller than the 3CR galaxies whose mean effective radius is 12 kpc. Their sizes and radio luminosities are consistent with scaling relations applied to the 3CR galaxies, in which both radio power and effective radius scale with galaxy mass. Compared with the sample of cluster ellipticals, 53W069 and 53W091 lie well within the scatter of the Kormendy relation. We conclude that the hosts of these millijansky radio sources at z≃ 1.5 are passively evolving elliptical galaxies that will evolve into ordinary L* ellipticals by the present day.