2007ApJ...665.1067M

We examine resolved spectroscopic data obtained with the Keck II telescope for 44 spheroidal galaxies in the fields of two rich clusters, Cl 0024+16 (z=0.40) and MS 0451-03 (z=0.54) and contrast this with similar data for 23 galaxies within the redshift interval 0.3<z<0.65 in the GOODS northern field. For each galaxy we examine the case for systemic rotation, derive central stellar velocity dispersions σ and photometric ellipticities ε. Using morphological classifications obtained via Hubble Space Telescope imaging as the basis, we explore the utility of our kinematic quantities in distinguishing between pressure-supported ellipticals and rotationally supported lenticulars (S0s). We demonstrate the reliability of using the v/(1-ε) versus σ and v/σ versus ε distributions as discriminators, finding that the two criteria correctly identify 63%±3% and 80%±2% of S0s at z∼0.5, respectively, along with 76⁺⁸_–3% and 79%±2% of ellipticals. We test these diagnostics using equivalent local data in the Coma Cluster, and find that the diagnostics are similarly accurate at z=0. Our measured accuracies are comparable to the accuracy of visual classification of morphologies, but avoid the band-shifting and surface brightness effects that hinder visual classification at high redshifts. As an example application of our kinematic discriminators, we then examine the morphology-density relation for elliptical and S0 galaxies separately at z∼0.5. We confirm, from kinematic data alone, the recent growth of rotationally supported spheroidals. We discuss the feasibility of extending the method to a more comprehensive study of cluster and field galaxies to z≃1, in order to verify in detail the recent density-dependent growth of S0 galaxies.