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

TL;DR

This paper introduces a new catalogue of distances and velocities for 103 HII regions in the outer Milky Way, improving the accuracy of spiral structure mapping and Galactic dynamics analysis.

Contribution

It provides a novel method combining spectrophotometric and stellar data to refine distances and velocities of HII regions, reducing scatter and aligning with VLBI measurements.

Findings

50% reduction in distance scatter for Perseus arm objects

Improved agreement with VLBI parallax distances

Enhanced understanding of Galactic spiral structure

Abstract

We present a new catalogue of spectrophotometric distances and line-of-sight systemic velocities to 103 HII regions between 90$^{\circ}\leq \ell \leq~$195$^{\circ}$ (longitude quadrants II and part of III). Two new velocities for each region are independently measured using 1-arcminute resolution 21~cm HI and 2.6~mm $^{12}$CO line maps (from the Canadian Galactic Plane Survey and FCRAO Outer Galaxy Surveys) that show where gaseous shells are observed around the periphery of the ionized gas. Known and neighbouring OB-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 HII emission (and whose distance is not more than 3 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 HII region lessens the impact of anomalous reddening for certain individuals. Final mean distances of 9 common objects with VLBI parallax distances show a 1:1 correspondence. Further, comparison with previous catalogues of HII 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 $\sim$1/$\sqrt{2}$ in scatter around a common angular velocity relative to the Sun $\Omega-\Omega_0$(km~s$^{-1}$~kpc$^{-1}$). The purpose of the catalogue is to provide a foundation for more detailed large-scale Galactic spiral structure and dynamics (rotation curve, density wave streaming) studies in the 2$^{\textrm{nd}}$ and 3$^{\textrm{rd}}$ quadrants, which from the Sun's location is the most favourably viewed section of the Galaxy.