Cepheids as tracers of star formation in M 31. II. NGC 206: evidence for spiral arm interactions.

NGC 206 in the Andromeda Galaxy (M 31) is the largest and perhaps the most massive region in the Local Group with coherent star formation taking place on a timescale of =~30Myr. We have incorporated observations of the H I emission and blue stars with our new identifications of Cepheid variables and those from observations by Baade and collaborators to study the star formation history in this region in comparison with the rest of M 31. We find that NGC 206 represents the latest phase of an enhanced level of star formation in the southern region of M 31. NGC 206 is located at the intersection of two spiral arms, suggesting that the interaction between the spiral arms is responsible for the enhanced levels of star formation. The locations of the Cepheid variables demonstrate the motion of the interaction point. We find a relative velocity of 32km/s between the velocity of the stars in M 31 and the velocity of the spiral arm pattern.