The Milky Way as seen by classical Cepheids

As a relatively young and bright population and the archetype of standard candles, classical Cepheids are an ideal population on which to trace the non-axisymmetric structure in the young stellar disk to large distances. We used the new distances derived in Paper I based on mid-IR WISE photometry for a selected sample of 2857 dynamically young Cepheids to trace the spiral arms of the Milky Way. The Perseus and Sagittarius-Carina arms are clearly evident in the third and fourth Galactic quadrants, while the Local and Scutum arms are much weaker, and extinction severely limits our view of the latter innermost spiral arm. Pitch angles were derived for each arm over various ranges of Galactic azimuth, each covering at least 90° in azimuth. Our method of detecting spiral arms and deriving pitch angles does not rely on pre-assigning sources to specific arms. While the spiral structure in the first and second quadrant is not obvious in part because of extinction effects, it is not inconsistent with the structure seen in the third and fourth quadrants. In summary, the Cepheids allow us to map spiral structure in the third and fourth Galactic quadrants where currently few masers have astrometric parallaxes, significantly extending our understanding of the Milky Way at large scales.