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Fig. 2.

image

Milky Way-type galaxy simulations and kinematic properties of the overdensities in the Milky Way disc. Panels a–c: Milky Way-type spiral galaxy (Model I). The stellar surface density in (a) shows only marginal evidence of density waves because they are blurred by epicyclic motions. After the transformation to guiding coordinate space (b), the model spiral arms are much clearer and show a morphology similar to the gas density (see Appendix A). These maps are shown in log-scale. Panel c: stellar radial velocity distributions for the four spiral arms highlighted in panel b where the mean radial velocity within the arms is negative (inwards), exactly as expected from density wave theory (Lin et al. 1969). Panels d–f: Milky Way-type barred galaxy (Model II). Transforming stellar surface density (e) to guiding coordinate space (f) shows prominent overdensities at the bar’s corotation (CR) and outer Lindblad (OLR) resonances, as determined from spectral analysis in Appendix A. Panel g shows a broad, nearly symmetric radial velocity distribution for stars at CR (blue), as viewed from the solar position at (x, y) = (0, −8.2) kpc, and an outward-skewed distribution at OLR (red). Panels g and h: kinematics of stars in Gaia GRV2 associated with the large-scale stellar overdensities (SDS) detected in guiding coordinate space (see Fig. 1). Panel g: radial velocity distribution for four structures (SDS 1, 3, 4, 6) reveals a mean radial motion towards the galactic centre similar to that seen in the spiral arms of Model I. Panel h: radial velocity distributions in SDS 2, 5 resemble the distributions at CR and OLR in the in the barred galaxy Model II. Error bars on all profiles are determined as described in Appendix A.

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