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Session 109 - Galaxies: Collisions, Counter-Rotating Disks.
Display session, Thursday, January 18
North Banquet Hall, Convention Center
As part of a detailed survey of the kinematics and dynamics in a sample of non-interacting Sa galaxies, we present HI synthesis maps, optical images and optical emission line and absorption line spectra indicating the presence of an extended counter--rotating disk in NGC 4138. Despite these peculiar kinematics, NGC 4138 appears morphologically smooth and undisturbed, although a ring of HII regions surrounds the nucleus. The primary stellar disk contains \sim80% of the stars. The remaining stars, along with the HI and emission line gas, are rotating counter to the primary stellar disk with comparable observed rotation velocity. The counter-rotating stars are present throughout the disk of the galaxy, but the HI gas extends to 2.2 times the radius of the stellar disk. The velocity field shows that the HI rotation curve exterior to the optical edge could decline by more than 100 km\, s^-1, although the presence of gas at large projected distances along the minor axis suggest the presence of a significant warp. Dynamical models require little (or no) dark matter. The counter--rotating disk may either be the result of a merger of a spiral with a gas-rich dwarf, or the continual infall of material of opposite spin vector onto the galaxy.
