HST imaging of an old galaxy group at z=1.55

Abstract

We present high-resolution imaging in the rest-frame optical of the weak radio source LBDS53W091 (Windhorst, van Heerde and Katgert: 1984, AA Spinrad et al.: 1997, ApJ 484, 581). We have obtained deep Hubble Space Telescope imaging over 10 orbits with NICMOS (Thompson et al.: 1998, ApJLett 492, 95), using camera 2 with the F160W H-band filter (l.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 1/4 law, with a scale length of r e = 0.3″ (2.3h 70 −1 kpc for Ω M = 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 (Djorgovski and Davis: 1987, ApJ 313, 59; Dressler et al.: 1987, ApJ 313, 42) 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 (e.g., van Dokkum et al.: 1998, ApJLett 504, 17).