Structure of spiral arm S6 in the Andromeda Nebula

The star distribution across spiral arm S6 in Baade's M31 field IV is investigated. The brightest stars congregate toward the arm midline, along which maximum star density also occurs. The symmetric structure of S6 may result from its being located near the corotation radius, as indicated by the spiral-pattern rotational velocity Ω/sub p/roughly-equal10 km sec-1 kpc-1 derived from the Cepheid age gradient in S4. Arm S6 has features similar to the galactic spiral arms in the solar neighborhood