The stellar populations of spiral galaxies

We have used a large sample of low-inclination spiral galaxies with radiall y resolved optical and near-infrared photometry to investigate trends in st ar formation history with radius as a function of galaxy structural paramet ers. A maximum-likelihood method was used to match all the available photom etry of our sample to the colours predicted by stellar population synthesis models. The use of simplistic star formation histories, uncertainties in t he stellar population models and considering the importance of dust all com promise the absolute ages and metallicities derived in this work; however, our conclusions are robust in a relative sense. We find that most spiral ga laxies have stellar population gradients, in the sense that their inner reg ions are older and more metal rich than their outer regions. Our main concl usion is that the surface density of a galaxy drives its star formation his tory, perhaps through a local density dependence in the star formation law. The mass of a galaxy is a less important parameter; the age of a galaxy is relatively unaffected by its mass; however, the metallicity of galaxies de pends on both surface density and mass. This suggests that galaxy-mass-depe ndent feedback is an important process in the chemical evolution of galaxie s. In addition, there is significant cosmic scatter suggesting that mass an d density may not be the only parameters affecting the star formation histo ry of a galaxy.