Spiral Structure in the Outer Galactic Disk. I. The Third Galactic Quadrant

We combine optical and radio observations to trace the spiral structure in the third quadrant of the Milky Way. The optical observations consist of a large sample of young open clusters and associations, whereas the radio observations consist of a survey of nearby and distant clouds observed in CO. Both the optical and radio samples are the largest ones thus far presented in the literature. We use this unique material to analyze the behavior of interstellar extinction and to trace the detailed structure of the third Galactic quadrant (TGQ).We find that the outer (Cygnus) grand design spiral arm is traced by stellar and CO components, while the Perseus arm is traced solely by CO and is possibly being disrupted by the crossing of the Local (Orion) arm. The Local arm is traced by CO and young stars toward l = 240° and extends for over 8 kpc along the line of sight reaching the outer arm. Finally, we characterize the Galactic warp and compare the geometries implied by the young stellar and CO components.