A first approach to study the stellar populations which build up galaxies can be done from the colours of galaxies. From that we can gain an understanding of the history of the star formation. For the detailed analysis of the colour profiles of galaxies one needs to adopt a priori assumptions concerning the evolutionary stellar population synthesis, the initial mass function, the metallicity and the star formation history, as well as about the dust geometry and its characteristics. This makes the conversion of broad-band colour gradients to abundance and population gradients in external galaxies controversial and conclusions are not easy to derive.
Because of their sensitivity, colours, which are the difference in magnitudes between two wavelengths, and colour gradients have originally been used to study the metal abundances and ages of stellar populations in the disks of external spiral galaxies. In contrast to the large number of studies of radial colour gradients in moderately inclined and face-on spiral galaxies, the colour behaviour of highly inclined and edge-on galaxies has not received much attention.
In edge-on disk galaxies, the interpretation of intrinsic colours and colour gradients is severely hampered by the presence of dust in the galaxy planes [De Grijs and Peletier2000]. In general, the dust lanes appear as red peaks in the vertical colour profiles. However, from a comparison with published colours of moderately inclined Sc galaxies, Kuchinski & Terndrup kuchinski1996 have shown that for these late-type galaxies there is little or no reddening away from the dust lane. Since statistical studies have shown that the dust content of Sc galaxies is large compared with other disk-dominated galaxy types (e.g. De Grijs et al. 1997), we may assume that the effects of reddening on the intrinsic galaxy colours away from the dust lane are largest for these galaxy types. Thus, colours and colour gradients measured at those distances from the galactic places where the influence of the dust lane is negligible are likely to reflect the intrinsic galaxy properties.
For example, Van der Kruit & Searle (1981b) observed that, at various galactocentric distances, the vertical colours of NGC 891 are getting systematically bluer with greater height above the plane. On the other hand, Jensen & Thuan jensen1982 did not find any evidence for a similar vertical colour gradient in NGC 4565 in the region where the old thin disc dominates. However, as soon as the light of the thick disk starts to dominate, the disk colours become redder with increasing distance from the galactic plane. A similar result was obtained for NGC 5907, which was interpreted as an extended stellar halo redder than the galactic disc or a very thick-disk component (see for references De Grijs & Peletier 2000).
To summarise, the colour gradients of the colour profiles can be explained by (1) changes in the stellar population, which we are interested in, and (2) variations in reddening due to dust extinction, which we try to avoid [De Jong1996].