The Anticenter Shell and the Anticenter Chain

Wide-bandwidth and high-sensitivity H I observations toward the Galactic anticenter cast a new light on the nature of the anticenter shell (ACS). The isolation of the anomalous-velocity H I from the intermediate-velocity gas permitted by differential rotation dramatically changes the interpretation of the supershell. Instead of appearing as an almost complete shell, the ACS decomposes into two separate filaments elongated parallel to the plane. The H I features are explained by a global model involving the continued impact of anticenter chain high-velocity clouds (ACHVCs) with the disk of the Galaxy.

The ``northern filament'' is hooked at its high-longitude end and turns to run nearly perpendicular to the plane. This ``V''-shaped remnant is expected for an oblique impact of a high-velocity cloud (HVC) with the disk. HVCs are not expected to be corotating with the Galaxy, therefore any impact with the rotating disk will be oblique. An H I cloud with a slightly more negative velocity is found embedded in the northern filament. It is termed AC 0 and is considered the almost completely merged forerunner of AC I and the rest of the ACHVCs.

Additional H I filaments previously found at intermediate velocities are similarly elongated parallel to the Galactic plane. All the filaments posses nearly linear velocity gradients with respect to Galactic longitude. The standard kinematic distances computed from these velocity gradients are in general large and dissimilar. They can be explained by differential rotation, but the assumption of a flat rotation curve must be removed. Remnants formed from the oblique impact of HVCs are expected to have smaller circular velocities than the quiescent disk gas. This results in higher observed velocity gradients and an overestimate of the kinematic distances.

The model presented here is able to explain the three fundamental observables of the H I toward the anticenter: (1) the presence of the ACHVCs and their termination at the Galactic plane, (2) the dominance of negative velocities, and (3) the tendency for most features to be elongated parallel to the Galactic plane.