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As a by-product of our continuum subtraction process the R-band images were also used to study another important constituent of the ISM, namely the dust. Edge-on galaxies are ideally suited candidates for this kind of investigation, since the dust lane is clearly visible in projection, although it should be noted that not all edge-on galaxies bear a bright and filamentary dust lane. In recent studies of the edge-on spiral NGC891, using high spatial resolution broad band imaging in various filterbands, high latitude dust features have been discovered (Howk & Savage, 1997; Rossa, 2001). Hundreds of dust features are recognized from which over a dozen individual dust features have already been studied in more detail (Howk & Savage, 1997). They are located at distances of 300 pc leq z leq 1500pc above the galactic plane. Although other investigations dealing with dust in edge-on galaxies have been carried out in the past (e.g., Sofue et al., 1994; Sofue, 1987), the recent works by Howk & Savage (1999, 1997) are one of the first linking the extraplanar dust features to the interaction between the disk and the halo of the galaxy, although Sofue (1987) already argued that the dust features might be connected to a magnetic process termed magnetic fountain.

4.1. Analysis

In order to study the individual dust features in more detail the R-band images have been processed to enhance the visibility of the dust structures against the galaxy background. As performed in the studies by Howk & Savage (1999, 1997), we have similarly applied an unsharp-masking method to our R-band images. We have gaussian-filtered the images with a FWHM of 10-15pixels for the different data sets. Then the original R-band frames have been divided by the gaussian-filtered images to produce the final unsharp-masked images. By applying this procedure to the whole frame, point sources with high count-rates (i.e. bright stars) appear as artifacts.

The detection of individual dust features is a straightforward method, since in the positive grey scale images the white structures (i.e. the dust) clearly separated from other morphological features which are recognizable in the galaxy background (stars, remaining cosmics, gas, background galaxies). This method was only applied to enhance the visibility. Possible artifacts, which arise due to the smoothing process, can easily be identified on the original R-band frames. However, the detection of extraplanar dust features rests on several galaxy parameters. The inclination is a very important factor. Galaxies with inclinations i leq 80° do not separate the prominent dust lane clearly enough in projection. The distribution of dust filaments along the dust lane can be quite different for each galaxy, which is partly due to a projection effect, and partially attributed to the intrinsic morphology and distribution of the dust within each galaxy.

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