ar X iv : a st ro - p h / 02 05 23 9 v 1 1 6 M ay 2 00 2 HST Imaging of a z = 1 . 55 Old Galaxy Group

We present high-resolution imaging in the rest-frame optical of the weak radio source LBDS53W091. Previous optical spectroscopy has shown that this object has an evolved stellar population of age > 3Gyr at z = 1.55, determined from the amplitude of rest-frame UV spectral breaks. We have obtained deep Hubble Space Telescope imaging over 10 orbits with NICMOS camera 2, using the F160W H-band filter (1.6μm) which is a good approximation to the rest-frame R-band. Our observations reveal a radial light profile which is well fit by a de Vaucouleurs’ r law, with a scale length of re = 0.3 ′′ (3h 50 kpc for ΩM = 0.3 and ΩΛ = 0). The elliptical morphology of the radio galaxy indicates a dynamically-evolved old system, consistent with the spectroscopic results. Some surrounding objects lie on the color:magnitude relation for a cluster at z = 1.55, and are likely to be associated. The group of galaxies are somewhat more luminous than the fundamental plane of ellipticals at z = 0 subject to (1 + z) surface brightness dimming, but are consistent with estimates of the fundamental plane at high redshift subject to passive luminosity evolution from a formation epoch of z > 3.