Near infra-red and optical colour gradients in E-type galaxies - Inferences on dust content

Colour gradients are considered for a sample of circa 50 E-type galaxies in the Local Supercluster. The new data includes isophotal colour profiles in J - H, J - K, V - J and V - K, measured using 2MASS frames mostly from the Large Galaxies Atlas, V frames from previous work and V profiles from the literature. This is supplemented by U - B, B - V, B - R, V - I colour gradients obtained anew from published photometric data. Colour gradients in E galaxies show remarkably large variations from object to object and do not correlate with other properties. Metallicity gradients are the primary cause as shown before. Age gradients with opposite effects are possibly needed to explain objects with small colour gradients. Some empirical evidence of such age effects has been found for a subset of objects with morphological peculiarities and younger stars mixed. Dust has only modest effects on colour gradients, as shown by the fact that objects with zero IRAS 100 mu flux have the same average values of the gradients, except in V - J and V - K, as those with non zero flux (cf. Table 7). This last subsample however exhibits poor but definite correlations between IRAS flux and gradients, which might be caused by the presence of a few relatively dusty galaxies in the sample. Given the absence of a correlation between any gradients and galaxy velocity dispersion ( and hence mass), the observations do not agree with the predictions of the monolithic scenario for the formation of E galaxies. Simulated datasets of dummy objects mimicking the hierarchical scenario have been obtained, and used to test a technique for estimating the dust content of E-galaxies from the comparison of the V - K ( or V - J) colour gradients with the U - B ( or B - V) ones: the contents of diffuse dust, gauged in terms of published models, are obtained for a dozen objects.