One of the best ways to map the Milky Way is to combine multiple data sets covering different parts of the galactic plane.
This image has masers (blue), estimated bar and ring positions, a new catalog of about 500 HII regions with Gaia-derived distances (red), star density from a new OB star catalog and part of figure 6 from the OB star paper showing individual stars with low error distances.
We present the third installment of the Alma Luminous Star catalogue, aimed at creating the most comprehensive and clean sample of Galactic massive stars. This update extends the sample by adding approximately 2000 OB stars, incorporating astrometric and photometric data from the Gaia Data Release 3 (DR3) alongside spectroscopic information from the Galactic O-Star Catalog based on recent ground-based spectroscopic surveys. Rigorous astrometric corrections are applied to Gaia DR3 parallaxes, proper motions, and photometry, ensuring accurate distance estimates through a Bayesian method suited to this stellar population's spatial distribution in the Milky Way. We perform some comparative analyses highlighting the improved distance accuracy over previous versions, underscore the importance of precise spectral classifications with competing catalogues, and identify areas for improvement in Gaia DR3 effective temperature and extinction estimates for massive stars. We also address the challenges of having robust definitions for these objects. In addition, we explore the catalogue's ability to trace Galactic features such as spiral arms, spurs and OB associations (with some insights on the nature of Gould's Belt). Finally, we discuss the potential for further expanding the sample with upcoming surveys. This effort marks a significant advancement in the creation of a reliable census of Galactic massive stars, contributing to our understanding of the Milky Way's structure and star formation history. This catalogue should serve as a valuable reference for the massive star community, supporting research on stellar interiors, winds, stellar feedback, and other processes that make OB stars key to the evolution of galaxies.
Context. The spiral structure of the Milky Way is not conclusive, even for the disc regions in the solar neighbourhood. In particular, the arm-like structures uncovered from the overdensity maps of evolved stars are inconsistent with the commonly adopted spiral arm models based on young objects. Aims. We aim to re-examine the arm segments traced by young objects and better understand the nearby spiral structure. Methods. We identified the exciting stars of 459 H II regions and calculated their parallax-based distances according to the Gaia DR3. Together with other H II regions with spectrophotometric or parallax-based distances in the literature, we used the largest ever sample of 572 H II regions with accurate distances to reveal the features shown in their distributions projected onto the Galactic disc. We then compared the results to the features traced by other young objects (high-mass star-forming region masers, O-type stars, and young open clusters) and evolved stars. Results. The structures outlined by different kinds of young objects do not exhibit a significant deviation from each other. The distributions of young objects are in agreement with three arm-like features emerging in the overdensity map of evolved stars. In particular, the Local Arm outlined by young objects follows an arm-like feature delineated by evolved stars and probably spirals outwards towards the direction of ℓ ~ 240° in the third Galactic quadrant. Conclusions. We conclude that the arm segments traced by young objects and evolved stars are consistent with each other, at least in the solar neighbourhood. In particular, the Local Arm delineated by young objects is reinterpreted as an arm segment with a large pitch angle of 25.2° ± 2.0°, whose inner edge is in good agreement with the recently discovered Radcliffe Wave.
Just for fun, here is the above image with my two-armed model of the Milky Way: Carina arm (green) and Norma arm (blue).
Of course there is no consensus on the structure of the Milky Way so this is just my model.
Galaxy Map