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We present high-resolution imaging in the rest-frame optical of the weakradio source LBDS53W091. Previous optical spectroscopy has shown that thisobject has an evolved stellar population of age >3Gyr at z=1.55, determinedfrom the amplitude of rest-frame UV spectral breaks. We have obtained deepHubble Space Telescope imaging over 10 orbits with NICMOS camera 2, using theF160W H-band filter (1.6microns) which is a good approximation to therest-frame R-band. Our observations reveal a radial light profile which is wellfit by a de Vaucouleurs' r^{1/4} law, with a scale length of r_e=0.3"(3kpc/h_{50} for Omega_M=0.3 and Omega_{Lambda}=0). The elliptical morphologyof the radio galaxy indicates a dynamically-evolved old system, consistent withthe spectroscopic results. Some surrounding objects lie on the color:magnituderelation for a cluster at z=1.55, and are likely to be associated. The group ofgalaxies are somewhat more luminous than the fundamental plane of ellipticalsat z=0 subject to (1+z)^4 surface brightness dimming, but are consistent withestimates of the fundamental plane at high redshift subject to passiveluminosity evolution from a formation epoch of z>3.Comment: To appear in proceedings of ESO/UCM meeting on "The Mass of Galaxies  at Low and High Redshift" held in Venice, 24-26 October 2001. Springer-Verlag  "ESO Astrophysics Symposia", eds. R. Bender and A. Renzini, 6 pages and 5 EPS  figure

HST Imaging of a z=1.55 Old Galaxy Group