The Morphology -- Density Relation in z ~1 Galaxy Clusters

We measure the morphology--density relation (MDR) and morphology-radius relation (MRR) for galaxies in seven z ~1 clusters that have been observed with the Advanced Camera for Surveys on board the Hubble Space Telescope. Simulations and independent comparisons of our visually derived morphologies indicate that ACS allows one to distinguish between E, S0, and spiral morphologies down to F850LP = 24 mag, corresponding to L/L^* = 0.21 and 0.30 at z = 0.83 and z=1.24, respectively. We adopt density and radius estimation methods that match those used at lower redshift in order to study the evolution of the MDR and MRR. We detect a change in the MDR between 0.8 < z < 1.2 and that observed at z ~0, consistent with recent work -- specifically, the growth in the bulge-dominated galaxy fraction, f_E+S0, with increasing density proceeds less rapidly at z ~1 than it does at z ~0. At z ~ 1 and \Sigma \ge 500 galaxies Mpc^-2, we find f_E+S0 = 0.72±.10. At z ~0, an E+S0 population fraction of this magnitude occurs at densities about 5 times smaller. The evolution in the MDR is confined to densities \Sigma \gtrsim 40 galaxies Mpc^-2 and appears to be primarily due to a deficit of S0 galaxies and an excess of Spiral+Irr galaxies relative to the local galaxy population. The f_E -- density relation exhibits no significant evolution between z = 1 and z = 0. Implications for the evolution of the disk galaxy population in dense regions are discussed in the context of these observations. \

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