The colour gradient in M 31 : evidence for disc formation by biased infall ?
Abstract
The chemical and photometric evolution of galactic discs which form by prolonged infall of halo material on a timescale that increases with radius (biased infall) is investigated using an evolutionary population synthesis model. 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 is applied to M 31, 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. In our synthesis of the colour profile we have corrected for the bulge contribution and reddening by dust both in our own Galaxy and in M 3l. The bulge dominates the observed colours at small radii but its influence becomes negligible beyond ~5 kpc. The colour variation in the NE half of the disc is in good agreement with the model predictions while that in the SW half is significantly redder which suggests that recent star formation in this part of the disc has been suppressed.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- February 1992
- Bibcode:
- 1992A&A...255..105J
- Keywords:
-
- Galactic Bulge;
- Galactic Evolution;
- Galactic Structure;
- Interstellar Chemistry;
- Metallicity;
- Astronomical Models;
- Astronomical Photometry;
- Star Distribution;
- Star Formation;
- Stellar Color;
- Astrophysics