Colour distributions in E-S0 galaxies VI. A discussion of colour gradientsin ellipticals

The data from the surveys by Franx et al. (1989), Peletier et al. (1990), G oudfrooij et al. (1994a), completed by our own measurements, have been used to build a sample of ellipticals with the full array of U-B, B-V, B-K and V-I colours, and corresponding gradients. A number of objects with data for only 3 colours are also considered. Photometric profiles were re-calibrate d in a coherent colours system, and a seemingly successful attempt was made to improve gradients by minor corrections to adopted sky background values . "External" probable errors are obtained for the revised data by comparisons between the various sources. These have poor statistical weight in U-B and V-I. An ad hoc "standard" gradient Delta(s) is defined as a weighted average of the four gradients in the above listed colours (or only three). The correla tions of the various gradients to a, are considered, and their slopes Delta (U-B)/Delta(s),... are compared to the corresponding calculated ratios for current hypothesis regarding the origin of colour gradients in ellipticals, i.e. population gradients and large scale dust distributions. Both the rat ios for the U-B and V-I colours are not compatible with the explanation in terms of diffuse dust but tend to agree with the explanation in terms of me tallicity variations. The correlation between the gradient Delta(U-B) and the standard one Delta( s) might be worse than could be explained by observational errors. Both pop ulation variations and possibly complex dust distributions should be invoke d to explain the complications of colour gradients.