A CGPS Look at the Spiral Structure of the Outer Milky Way. I. Distances and Velocities to Star-Forming Regions

Abstract

We present a new catalog of spectrophotometric distances and line of sight systemic velocities to 103 H II regions between 90≤ℓ≤195 (longitude quadrants II and part of III). Two new velocities for each region are independently measured using 1 arcmin resolution 21 cm H I and 2.6 mm 12CO line maps (from the Canadian Galactic Plane Survey and Five College Radio Astronomy Observatory Outer Galaxy Surveys) that show where gaseous shells are observed around the periphery of the ionized gas. Known and neighboring O- and B-type stars with published UBV photometry and MK classifications are overlaid onto 21 cm continuum maps, and those stars observed within the boundary of the H II emission (and whose distance is not more than three times the standard deviation of the others) are used to calculate new mean stellar distances to each of the 103 nebulae. Using this approach of excluding distance outliers from the mean distance to a group of many stars in each H II region lessens the impact of anomalous reddening for certain individuals. The standard deviation of individual stellar distances in a cluster is typically 20% per stellar distance, and the error in the mean distance to the cluster is typically ±10%. Final mean distances of nine common objects with very long baseline interferometry parallax distances show a 1:1 correspondence. Further, comparison with previous catalogs of H II regions in these quadrants shows a 50% reduction in scatter for the distance to Perseus spiral arm objects in the same region, and a reduction by ∼1/ 2 in scatter around a common angular velocity relative to the Sun W-W0(km s−1 kpc−1 ). The purpose of the catalog is to provide a foundation for more detailed large-scale Galactic spiral structure and dynamics (rotation curve, density wave streaming) studies in the 2nd and 3rd quadrants, which from the Sun’s location is the most favorably viewed section of the Galaxy