Young stars as tracers of a barred-spiral Milky Way

Abstract

Identifying the structure of our Galaxy has always been fraught with difficulties, and while modern surveys continue to make progress building a map of the Milky Way, there is still much to understand. The arm and bar features are important drivers in shaping the interstellar medium, but their exact nature and influence still require attention. We present results of smoothed particle hydrodynamic simulations of gas in the Milky Way including star formation, stellar feedback, and ISM cooling, when exposed to different arm and bar features, with the aim of better understanding how well newly formed stars trace out the underlying structure of the Galaxy. The bar is given a faster pattern speed than the arms, resulting in a complex, time-dependent morphology and star formation. Inter-arm branches and spurs are easily influenced by the bar, especially in the two-armed spiral models where there is a wide region of resonance overlap in the disc. As the bar over-takes the spiral arms it induces small boosts in star formation and enhances spiral features, which occur at regularly spaced beat-like intervals. The locations of star formation events are similar to those seen in observational data, and do not show a perfect 1:1 correspondence with the underlying spiral potential, though arm tangencies are generally well traced by young stars. Stellar velocity fields from the newly formed stars are compared to data from Gaia DR2, showing that the spiral and bar features can reproduce many of the nonaxisymmetric features seen in the data. A simple analytical model is used to show many of these feature are a natural response of gas to rigidly rotating spiral and bar potentials.

The power of coordinate transformations in dynamical interpretations of Galactic structure

“…It is nontrivial to disentangle the signatures of the Galactic bar and spiral structure, especially when the number of spiral arms and nature of the spiral structure itself remain uncertain. Similarly, Pettitt, Ragan & Smith (2020) showed that global features in velocity space unveiled by Gaia could be equally well reproduced by spiral or bar features.…”

“…It is entirely possible (and likely) that some of this kinematic substructure arises from spiral structure (e.g. Sellwood et al 2019;Pettitt et al 2020) but it should not be assumed that they mark the current location of spiral arms. Following this transformation, we now have a map of the kinematic response to the potential, not the potential itself.…”

How do different spiral arm models impact the ISM and GMC population?

“…It has been shown in numerous previous works that bars can influence the dynamics of gas out to the OLR (e.g. Koda & Wada 2002;Pettitt et al 2020), which could change the trends in streaming motions in their analysis (performed between 6-8 kpc) compared to what would be seen in a spiral-only model. Secondly, their arms are quite weak compared to the static spiral they use for comparison, with numerous interarm branches and wide density peaks.…”

“…Tidal forces exerted in such interactions can readily create two-armed spiral features (Toomre & Toomre 1972;Struck, Dobbs & Hwang 2011;Pettitt & Wadsley 2018) and such a mechanism is likely to have played a role in some of the more well-known spiral galaxies such as M51 and M81 (Yun 1999). Bars can also drive spiral features in discs, particularly in the gas (Wada & Koda 2001;Pettitt, Ragan & Smith 2020), though for this study we limit ourselves to bar-free systems where the origin of spirality is more ambiguous.…”

The SEDIGISM survey: First Data Release and overview of the Galactic structure

“…-era understanding of the Galactic bar also suggests slower pattern speeds than assumed in earlier works, which place corotation as far out as 6 kpc (Sanders et al 2019;Bovy et al 2019). The ISM responds strongly to the motion of the bar out to corotation, and even as far as the more distant Outer Lindblad Resonance for certain models of bars (Sormani et al 2015;Pettitt et al 2020). Any spiral arm-like features are thus inherently coupled to the bar within at least corotation and more sophisticated modelling is required to fully understand the kinematics of the gas.…”