HVCS Interacting With Their Environment

Abstract

We present observational evidence and review theoretical considerations for an interaction of high-velocity clouds (HVCs) with their ambient medium. HVCs showing either a head-tail structure or a horseshoe shape are likely to be interacting. Such HVCs are observed in complexes in the gaseous Galactic Halo (e.g. complex A), in HVC complexes in the outer Galactic Halo (e.g. the Magellanic Stream), and in a sample of compact HVCs that might be located in the intergalactic medium of the Local Group. The overall morphology of the head-tail HVCs is the same for clouds located in dierent environments: cold, dense condensations are found near the sharp (leading) edge of the HVC, embedded in a warm, neutral medium forming a head-tail structure. The observed morphology — decreasing column density, increasing velocity dispersion and decreasing radial velocity in the direction of the tail — is consistent with gas having been stripped off the main body of the HVC. The detections of Hα emission, O vi absorption, and possibly X-ray emission associated with HVCs provide further evidence for a ram-pressure interaction. Moreover, recent numerical simulations modeling the ram-pressure interaction of HVCs with their ambient medium predict the existence of head-tail structures.