The dynamical distinction between elliptical and lenticular galaxies in distant clusters: further evidence for the recent origin of S0 galaxies

Abstract

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 - e ) versus σ and v/σ versus 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.

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