The colour gradient in M31: evidence for disc formation by biased infall?

Extract

A photochemical evolution model has been used to investigate the chemical and photometric evolution of galactic discs which form by prolonged infall of halo material on a timescale that increases with radius, termed biased infall, (Josey & Arimoto, 1991). We find that the decline in the mean age and metallicity of the stellar population with radius generates significant colour gradients in the disc and suggest that biased infall may be responsible for the colour variations which have been observed in a number of spirals. Age variations are found to be the primary factor responsible for gradient production in the U, B and V bands while metallicity effects become increasingly important at longer wavelengths. Our model has been applied to M31, in which strong radial colour gradients have been observed, and we find that its chemical and photometric properties can be largely accounted for if its disc formed on a timescale that increased from 0.7 Gyr at the centre to 5 Gyr at a radius of 10 kpc (see Fig.1).