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A. Bunker (UC, Berkeley & Institute of Astronomy, Cambridge, U.K.), H. Spinrad (UC, Berkeley), R. McLure (Dept. of Astrophysics, Oxford, U.K.), A. Dey (KPNO), J. Dunlop, J. Peacock (Institute for Astronomy, Edinburgh, U.K.), D. Stern (JPL), R. Thompson (Steward Obs., U. Arizona), I. Waddington (Arizona State U. & Bristol U., U.K.), R. Windhorst (Arizona State U.)
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 (1.6\mum) 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' r1/4 law, with a scale length of re=0.3'' (2.3\,h70-1\,kpc for \OmegaM=0.3 and \Omega\Lambda=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)4 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.
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