2 Observations, data reduction and construction of the colour maps

The galaxies were observed through broad band B, V, R and I filters, as a part of our study of the integrated properties of the host galaxies of Seyfert 2 nuclei (Kotilainen 1998). We used the BroCam 1024² px CCD camera with pixel scale 0.176'' px^-1 at the Cassegrain focus of the 2.5 m Nordic Optical Telescope (NOT) on La Palma, in August 1995 and July 1996. Conditions were photometric throughout with generally subarcsec seeing. The images were reduced with standard procedures, i.e.subtraction, flatfielding, residual sky subtraction, and removal of cosmic rays and bad pixels. Multiple exposures were coadded after accurate alignment to produce the final images. Photometric calibration was obtained against faint standard stars selected from Landolt (1992). Airmass correction was applied using extinction coefficients appropriate for La Palma. We estimate the internal statistical errors from the standard star observations to be $\pm$0.03 mag. For further information and for photometry of the galaxies, see Kotilainen (1998).

Colour maps were produced from the B and I band images smoothed to the same (lower) resolution, after careful alignment using the centroids of the light distribution of field stars as reference points. Misalignments of only a small fraction of a pixel may remain, and they do not affect the results for structures covering a large number of pixels. The images were smoothed with a Gaussian filter with a width corresponding to the seeing, and the I band image was divided by the B band image. In the resulting logarithmic colour maps, dark shades indicate blue and light shades red emission. At the edge of the colour maps noise dominates, but closer to the nucleus the colours are reliable. To check that the detected structures in the colour maps are not artifacts caused by e.g. variable seeing or alignment errors, we have inspected the colour residuals of field stars. Reassuringly, they do not show any artifacts but cancel out well in the colour maps. Another problem is the contamination from emission lines within the broad bands. As discussed in KW97, while the V and R filters are severely affected by emission lines, the B and I filters are practically devoid of strong emission lines, and are dominated by continuum emission. This conclusion is supported by a literature search of equivalent widths (EW) of major optical emission lines in the nuclear spectra of the sample galaxies. The average contribution of emission lines to the B and I filters in this sample are 6% and 4%, respectively. In the next section, we present and discuss for each galaxy a direct B band image for morphological information and B-I colour maps (Figs. 1-26).