Mapping Milky Way disk perturbations in stellar number density and vertical velocity using Gaia DR3

Abstract

We have mapped the number density and mean vertical velocity of the Milky Way's stellar disk out to roughly two kiloparsecs from the Sun using Gaia Data Release 3 (DR3) and complementary photo-astrometric distance information from StarHorse. For the number counts, we carefully masked spatial regions that are compromised by open clusters, great distances, or dust extinction and used Gaussian processes to arrive at a smooth, non-parametric estimate for the underlying number density field. We find that the number density and velocity fields depart significantly from an axisymmetric and mirror-symmetric model. These departures, which include projections of the Gaia phase-space spiral, signal the presence of local disturbances in the disk. We identify two features that are present in both stellar number density and mean vertical velocity. One of these features appears to be associated with the Local Spiral Arm. It is most prominent at small heights and is largely symmetric across the mid-plane of the disk. The density and velocity field perturbations are phase-shifted by roughly a quarter wavelength, suggesting a breathing mode that is propagating in the direction of Galactic longitude l ∼ 270 deg. The second feature is a gradient in the stellar number density and mean vertical velocity with respect to galactocentric radius. This feature, which extends across the entire region of our analysis, may be associated with the extension of the Galactic warp into the solar neighbourhood in combination with more localised bending waves.

The missing radial velocities of Gaia: Blind predictions for DR3

“…Here, there is evidence of vertical bulk motions in the disc. The positive slope of 𝑊 + 𝑊 ⊙ with Galactocentric radius suggests a large-scale bending mode and is in agreement with the results of various recent studies mapping the vertical velocity field (Widmark et al 2022b;Nelson & Widrow 2022;Wang et al 2023).…”

Two-point Correlation Function Studies for the Milky Way: Discovery of Spatial Clustering from Disk Excitations and Substructure

“…Similarly, Widmark et al (2022) find evidence for breathing modes in the solar neighborhood and suggest that the pattern speed of the local spiral arm is slower than the rotation of stars in the solar neighborhood. Widmark et al also claim that their Gaussian process fit method indicates that the Galactic warp affects only the stars in the thick disk, while the thin disk is largely unaffected.…”

“…That is, the gradual warping introduces a small z-f coupling only for the thick disk, and thus only the high-|z| peaks separated by some azimuthal distance exhibit a measurable breaking of axial symmetry in their vertical structure. Nonetheless, the Gaia data used by Widmark et al (2022) may be more difficult to interpret because of dust and stellar crowding effects in the midplane. Suggesting that the thick and thin disks respond in observably different ways to perturbations may also suggest that they acted in the relatively recent past, which is intriguing.…”

A Wave-corrected Assessment of the Local Midplane

“…There is, however, a small numerical change in the values of 〈z〉 with our choice of z ℓ , evident from the vertical axis scaling in panels (a) and (c) in Figure 4-yielding a slight downwards shift with larger values of z ℓ . This small shift may be due to the warping of the disk, which may disproportionately affect the thick disk stars (Widmark et al 2022).…”