High-Resolution Observations and the Physics of High-Velocity Cloud A0

TL;DR

This study maps the neutral hydrogen structure of high-velocity cloud A0 at high resolution, revealing filamentary and cloud features, and develops a magnetic stability model suggesting magnetic fields influence the cloud's structure and dynamics.

Contribution

It introduces a detailed HI mapping of high-velocity cloud A0 and proposes a magnetic stability model based on Alfven waves and magnetic turbulence.

Findings

70% of HI gas is in a narrow filament

Bright clouds with narrow line widths are located near the filament

Magnetic field strengths are estimated at about 4-6 microGauss

Abstract

The neutral hydrogen structure of high-velocity cloud A0 (at about -180 km/s) has been mapped with a 9.1 arcmin resolution. Gaussian decomposition of the profiles is used to separately map families of components defined by similarities in center velocities and line widths. About 70% of the HI gas is in the form of a narrow, twisted filament whose typical line widths are of order 24 km/s. Many bright features with narrow line widths of order 6 km/s, clouds, are located in and near the filament. A third category with properties between those of the filament and clouds appears in the data. The clouds are not always co-located with the broader line width filament emission as seen projected on the sky. Under the assumption that magnetic fields underlie the presence of the filament, a theorem is developed for its stability in terms of a toroidal magnetic field generated by the flow of gas along field lines. It is suggested that the axial magnetic field strength may be derived from the excess line width of the HI emission over and above that due to kinetic temperature by invoking the role of Alfven waves that create what is in essence a form of magnetic turbulence. At a distance of 200 pc the axial and the derived toroidal magnetic field strengths in the filament are then about 6 microGauss while for the clouds they are about 4 microGauss. The dependence of the derived field strength on distance is discussed.