The envelope of the red galaxies distribution in a z = 0.75 cluster field

Abstract

We report the observation of a group of 16 red (V -I > 2) galaxies, in the magnitude range 22 ≤ V ≤ 25.5, within a projected distance of 30″ (~0.2h^-1 Mpc for h = H₀/75km s^-1 Mpc^-1) of the ultra-steep spectrum radio source 1443-198 at z = 0.753. The colours of the galaxies range from the cut-off at V - I = 2.0 to V - I = 3.3. Eight galaxies are redder than V - I = 2.3 and three are redder than V - I = 3.0. Some objects with I ≈ 23 are undetected at the magnitude limit of V=25.5. This confirms previous results on the presence of a very red envelope in some distant clusters and suggests that a fraction of cluster ellipticals are already old at z ≈ 0.75. The radio galaxy has colours and a 4000 Å break amplitude typical of evolving E/S0 galaxies, and shows signs of interaction with other objects in its immediate vicinity.