4.3. Discussion

The presented results of this investigation of the extraplanar dust distribution in edge-on galaxies are in very good agreement with those by Howk & Savage (1999, 2000). They investigated a small sample of 12 nearby edge-on spirals using multicolor broad band (B and V) observations (Howk & Savage, 1999). Four of their studied objects are also covered in our H survey, namely NGC891, NGC3628, NGC4302, and NGC4634. Based on their small sample they derive physical properties for a few individual dust features. They derive large column densities of 3 × 10²⁰ cm^-3 N_H 2 × 10²¹ cm^-3, and derive dust masses of ~ a few 10⁵ M. Although their observations have been carried out in the B, and V bands, and our observations were performed in the somewhat more transparent R-band, we find a good general agreement between their and our data for the structure of the high-| z| dust features.

The coincidence of high latitude gas and dust structures suggests that these both phases of the ISM are most likely tied to the same driving force, the star formation processes in the underlying galaxy disk. However, the mechanism for the dust ejection might be different from the ones responsible for expelling the gas to high galactic latitudes. While models for the gas transport include magneto-hydrodynamic (MHD) flows such as the galactic fountains (Shapiro & Field, 1976), chimneys (Norman & Ikeuchi, 1989), favored expulsion models for the dust may be considered via radiation pressure on dust grains, termed photolevitation (e.g., Ferrara et al., 1991; Franco et al., 1991), and flows initiated by magnet field instabilities (Parker instabilities), caused by SNe explosions (e.g., Shchekinov et al., 2001; Parker, 1992).

The reason to assume that the two distinct phases of the ISM are expelled by different processes is justified by the basic notion, that dust grains in the harsh environments of the star formation processes will be completely destroyed, and should not be observed at high galactic latitudes.