The Collisions of HVCs with a Magnetized Gaseous Disk

Summary

We present two-dimensional MHD numerical simulations for the interaction of high-velocity clouds (HVC) with a magnetized gaseous disk. The initial magnetic field is oriented parallel to the disk. The impinging clouds move in oblique trajectories and fall toward the disk with different initial velocities. The B-field lines are distorted and compressed during the collision, increasing the field tension and preventing the cloud material from penetrating into the disk. The perturbation, however, creates a complex, turbulent, pattern of MHD waves that are able to traverse the galactic disk and, for unstable disks, can trigger the Parker instability.

Introduction

High velocity clouds (HVC) are atomic H I clouds located at high latitudes in our Galaxy, and moving at velocities ∣V_LSR∣≥ 90 km/s (see Bajaja et al. 1985, and Wakker & van Woerden 1997). Their distance is unknown, but limits to the locations of some particular clouds indicate z-heigths of a few kiloparsecs, setting a possible mass range of 10⁵-10⁶ M_⊙. Thus, a HVC complex moving with a speed of 100 km/s has a kinetic energy of about 10⁵²⁻⁵³ erg. These values indicate that the bulk motion of the HVC system could represent a rich source of energy and momentum for the interstellar medium (equivalent to that from several generations of superbubbles).

There is evidence for possible collisions between HVCs and gaseous disks, both in our Galaxy and in external galaxies.