High-resolution observations of the neutral hydrogen gas in the edge-on galaxy NGC 4631 reveal an expanding, shell-like structure of diameter 2.8 kpc and mass 1-2×108 Script M sign⊙ in the disk. The "supershell" is the most energetic and one of the largest known in a galactic disk. The formation of such a shell as a result of gas swept up by stellar winds and supernova from an OB association would require energy input from ≳104 OB stars. Instead, in this paper we use three-dimensional hydrodynamical simulations to examine the possibility that the shell is formed by the oblique impact of a high velocity cloud (HVC) with the galactic disk. Our results indicate that an HVC with initial radius 500 pc and mass of 1.2×107 Script M sign⊙ which impacts the disk at 200 km s-1 evacuates a cavity of diameter 2.2 kpc and sweeps about 1.3×108 Script M sign⊙ of ambient material into a thin shell which is structurally and kinematically similar to the observed shell. Clouds with different combinations of parameters but similar kinetic energies lead to similar results. Thus, the properties of the impactor cannot be inferred uniquely from the observed properties of the shell. Moreover, three-dimensional magnetohydrodynamic models which include the effect of an equipartition strength magnetic field in the galactic disk do not change the structure of the shell substantially. We find that in order to produce a structure like the observed shell, an oblique impact is necessary. In addition, the synthetic position-velocity diagrams and channel maps are sensitive to the assumed viewing angle, allowing us to constrain the geometry of the impact. We find an excellent fit to the observational data can be made for an impact trajectory inclined at an angle of 45° to the galactic plane and viewed at an angle 20° away from the in-plane component of the velocity vector of the impactor. Our results strengthen the interpretation of the supershell as being the result of an HVC impact.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science