April 20, 2000 CfA Colloquium


Title: Kinematic Evidence for Minor Mergers in Isolated, Normal Sa Galaxies

Speaker: Martha Haynes

Abstract: The Sa galaxies are a heterogenous class, including gas-rich and gas-poor disks and large and small-bulged systems. In contrast to their later spiral counterparts, Sa's typically occupy higher density environments and require little or no dark matter. In order to uncover the reason for the diversity of the Sa class, we have conducted a comprehensive study of the dynamics and kinematics of a sample of nearby, morphologically normal, relatively isolated Sa galaxies, combining BVRI and H-alpha imaging, long-slit optical spectroscopy and HI synthesis mapping. In about half the sample, evidence of kinematic separation of ionized gas components is found in the long-slit spectroscopic data. The degree and circumstances of the distinct kinematics vary from complete counterrotation of all of the gas from the stars to nuclear gas disks decoupled from the stars to anomalous velocity central gas components that may be indicative of misaligned bars or oval distortions. In 8 of the 9 objects mapped in HI, the neutral gas extends far beyond the optical radius. In general, the HI surface density is very low, and the outer HI is patchy and asymmetric or found in a distinct ring, exterior to the optical edge. While the overall HI velocity fields are dominated by circular motions, strong warps are suggested in the outer regions by bending of the minor axis isovelocity contours and/or systematic shifts in position angle between inner and outer rings. The multiwavelength evidence can be interpreted in terms of the kinematic "memory" of past minor mergers in objects that otherwise exhibit no morphological signs of interaction.

Reference for students: Schweizer, Astro-ph/0002263