Colour Gradients in the Optical and Near-IR

For many years broadband colours have been used to obtain insight into the contents of galaxies, in particular to estimate stellar and dust content. Broadband colours are easy to obtain for large samples of objects, making them ideal for statistical studies. In this paper I use the radial distribution of the colours in galaxies, which gives more insight into the local processes driving the global colour differences than integrated colours. Almost all galaxies in my sample of 86 face-on galaxies become systematically bluer with increasing radius. The radial photometry is compared to new dust extinction models and stellar population synthesis models. This comparison shows that the colour gradients in face-on galaxies are best explained by age and metallicity gradients in the stellar populations and that dust reddening plays a minor role. The colour gradients imply M/L gradients, making the ‘missing light’ problem as derived from rotation curve fitting even worse. 1 The colour gradients A sample of 86 spiral galaxies was imaged in the B, V , R, I, H and K passbands to study light and colour distributions as a function of radius. Full details of sample selection and data reduction are described in de Jong & van der Kruit (1994). The galaxies were selected to be face-on and to have a diameter of at least 2. The sample is statistically complete and can be corrected for selection effects. It can therefore be used to analyze the nature of the Freeman law (Freeman 1970) and this analysis has been reported elsewhere (de Jong 1995a, 1995b). The luminosity profiles were determined in the usual way by measuring the average surface brightness on annuli of increasing radius. Radial colour profiles were created by combining profiles in different passbands. The run of colour as function of radius is put on a common scale for all galaxies in Fig. 1, where the average B–K colour at each radius is plotted as function of the average R surface brightness at this radius.