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

Abstract

The neutral hydrogen structure of high-velocity cloud A0 (at about −180 km s −1 ) has been mapped with a 9. 1 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 −1 . Many bright features with narrow line widths of order 6 km s −1 , 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 colocated 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 Alfvén 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 µG while for the clouds they are about 4 µG. The dependence of the derived field strength on distance is discussed.

Exploring Hydrodynamic Instabilities along the Infalling High-Velocity Cloud Complex A

“…The real-time duration of the movie is 1 minute and 21 seconds. core A0 (Verschuur 2013). The calibration and continuum level removal techniques in for the GBT07A-104 (PI Martin) observations are described in Boothroyd et al (2011) and Martin et al (2015).…”

“…), these fields may have been inherited from their parent galaxy (Milky Way: Haverkorn 2015; Small Magellanic Cloud: LoboGomes et al 2015; Large Magellanic Cloud: Mao et al 2012). It is unknown if the HVCs that originate from an intergalactic medium filament or though halo gas condensations will have a magnetic field.No magnetic field has been directly measured for Complex A. Nonetheless,Verschuur (2013) placed indirect constraints on the strength of the Complex A's toroidal magnetic field by assuming that its broad H i lines are a causes by a combination of thermal broadening and magnetic turbulence that results from Alfvén waves. In that study, they surmised that the lack of Hα emission detected in the Wisconsin Hα Mapper (WHAM) Northern Sky Survey (NSS) of the Milky Way(Haffner et al 2003) was an indication that this complex is colder than T H i < 7 × 10 3 K and therefore the H i emission should only have narrow emission-line profiles of FWHM < 25 km s −1 .…”