The Bar and Spiral Structure Legacy (BeSSeL; Brunthaler et al. 2011) survey is a Very Long Baseline Array (VLBA) key science project which provided a catalog of accurate distances to more than 100 Galactic high-mass star-forming regions, visible from the Northern hemisphere (Reid et al. 2019).
The Scutum (also called Scutum-Centaurus or Scutum-Crux) spiral arm is located in the inner Galaxy (from the Sun’s point of view), between the Sagittarius and the Norma arms. The Scutum arm was identified as strong condensations in neutral hydrogen and CO observations in longitude-velocity plots.
Anomalous peculiar motions in the Scutum spiral arm
Parallax and proper motion observations of maser sources
As part of the BeSSeL survey, we published new distances and proper motions to methanol and water masers in 16 star-forming regions in the Scutum spiral arm. Together with sources from the literature and another study led by Li et al. (in prep.), we now have parallax and proper motion information for 44 targets in the arm. Applying a universal rotation curve, the proper motions of the sources can be translated into peculiar motions. We showed that 16 of the 44 sources exhibit very large peculiar motions, directed mostly toward the Galactic Center, which, to this extent, has only been seen in this spiral arm. We postulated that these motions are caused by the gravitational influence of the Galactic bar (Immer et al. 2019). We will test this hypothesis with Galactic scale simulations (Immer et al., in prep.).
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