Stellar Kinematics and Structural Properties of Virgo Cluster Dwarf Early-Type Galaxies from the SMAKCED Project. I. Kinematically Decoupled Cores and Implications for Infallen Groups in Clusters

摘要

We present evidence for kinematically decoupled cores (KDCs) in two dwarfearly-type (dE) galaxies in the Virgo cluster, VCC 1183 and VCC 1453, studiedas part of the SMAKCED stellar absorption-line spectroscopy and imaging survey.These KDCs have radii of 1.8'' (0.14 kpc) and 4.2'' (0.33 kpc), respectively.Each of these KDCs is distinct from the main body of its host galaxy in twoways: (1) inverted sense of rotation; and (2) younger (and possibly moremetal-rich) stellar population. The observed stellar population differences areprobably associated with the KDC, although we cannot rule out the possibilityof intrinsic radial gradients in the host galaxy. We describe a statisticalanalysis method to detect, quantify the significance of, and characterize KDCsin long-slit rotation curve data. We apply this method to the two dE galaxiespresented in this paper and to five other dEs for which KDCs have been reportedin the literature. Among these seven dEs, there are four significant KDCdetections, two marginal KDC detections, and one dE with an unusual centralkinematic anomaly that may be an asymmetric KDC.The frequency of occurence ofKDCs and their properties provide important constraints on the formationhistory of their host galaxies. We discuss different formation scenarios forthese KDCs in cluster environments and find that dwarf-dwarf wet mergers or gasaccretion can explain the properties of these KDCs. Both of these mechanismsrequire that the progenitor had a close companion with a low relative velocity.This suggests that KDCs were formed in galaxy pairs residing in a poor groupenvironment or in isolation whose subsequent infall into the cluster quenchedstar formation.