Milky Way versus Andromeda: a tale of two disks

Aims. We study the chemical evolution of the disks of the Milky Way (MW) and of Andromeda (M31), to identify the common properties and differences between the two major galaxies of the Local Group. Methods. We use a large set of observational data for M31, including observations of the star formation rate (SFR) and gas profiles, as well as stellar metallicity distributions along its disk. When expressed in terms of the corresponding disk scale lengths, we show that the observed radial profiles of MW and M31 exhibit interesting similarities, suggesting the possibility of a description within a common framework. Results. We find that the profiles of stars, gas fraction, and metallicity of the two galaxies, as well as most of their global properties, are well described by our model, provided that the star formation efficiency in M31 disk is twice as high as in the MW. We show that the star formation rate profile of M31 cannot be described by any form of the Kennicutt-Schmidt law (KS Law) for star formation. We propose that these discrepancies are caused by the fact that M31 has an active star formation history in the recent past, consistent with the hypotheses of a head-on collision with the neighboring galaxy (most probably M32) about 200 Myr ago. Conclusions. The MW has most probably experienced quiescent secular evolution, making possible a fairly successful description with a simple model. If M31 is more typical of spiral galaxies, more complex models, involving galaxy interactions, will be required for the description of spirals.