NGC 2655: From inner polar ring to outer shells and tails

摘要

NGC 2655, a nearby bright S0/a galaxy in a loose group, has strongly asymmetric central dust lanes and an extended disk of neutral hydrogen gas. Here we present deep optical images showing tidal arms and regions of diffuse starlight well beyond the main galaxy, suggestive of a recent merger. Our maps in the 21 cm line show a layer of neutral hydrogen extending to at least 7.5' or 40 kpc from the center, with a broken streamer of gas trailing off toward the small neighbor galaxy UGC 4714. The global profile has sloping "shoulders" of extreme-velocity gas, usually a sign of interaction. The diffuse stellar light corresponds generally but not in detail with the distribution of H I. In particular, the stellar light of the main tidal arm extends beyond the H I layer, which is limb-brightened as if it had been compressed from outside. The pattern of gas velocities is generally bisymmetric, but the H I layer is strongly warped; it does not share the kinematics of the stellar disk. The gas orbits twist by at least 90 degrees between 1' and 3' of the center, as the H I layer appears to warp through edge-on. The complex optical tails and H I dynamics suggest that the galaxy has undergone multiple mergers. Both at small and at large radii, the gas layer twists in a trailing sense relative to its orbital motion. If the twist results from differential precession in the galaxy's gravitational potential, this would indicate that the dark halo is prolate, elongated along the rotation axis of the disk.