The Great Wave: Evidence of a large-scale vertical corrugation propagating outwards in the Galactic disc

Abstract: We analyse the three-dimensional structure and kinematics of two samples of young stars in the Galactic disc, containing respectively young giants ($\sim$17$\, $000 stars out to heliocentric distances of $\sim$7 kpc) and classical Cepheids ($\sim$3400 stars out to heliocentric distances of $\sim$15 kpc). The vertical structure of the two samples exhibit a consistent shape of the Milky Way's warp, whose amplitude reaches $\sim$700 pc at a Galactocentric radius R $\sim$ 14 kpc. Moreover, both samples show evidence of a large-scale vertical corrugation on top of the warp with a vertical height of $\sim$150-200 pc, extending over a large portion of the Galactic disc between Galactocentric radii $R \sim$ 10-12 kpc in the third Galactic quadrant (galactic longitudes $180^\circ < l < 270^\circ$) and $\sim$12-14 kpc in the second Galactic quadrant ($90^\circ < l < 180^\circ$). Its total length is at least 10 kpc and can possibly reach $\sim$ 20 kpc with the Cepheid sample. The stars in the corrugation exhibit both radial and vertical systematic motions, with Galactocentric radial velocities towards the outer disc of about 10-15 km/s. In the vertical motions, once the warp signature is subtracted, the residuals show a large-scale feature of systematically positive vertical velocities, which is shifted to slightly larger Galactocentric radii with respect to the spatial vertical corrugation (with a phase difference of roughly $\pi/2$), indicating an oscillatory behaviour. A comparison of the observed shift with a simple toy model suggests that the corrugation can be interpreted as a wave propagating towards the outer disc. The wave mapped in this work is located at larger heliocentric distances compared to the Radcliffe wave, a $\sim$2.7 kpc filament of dense gas clouds close to the Sun, and exhibits a larger coverage of the Galactic disc.

• The paper presents evidence for a large-scale vertical corrugation in the Milky Way's Galactic disc, found using young giants and classical Cepheids data from Gaia DR3.
• This corrugation exhibits wave-like characteristics, propagating outwards with observed systematic radial and vertical velocity patterns.
• Stars within the corrugation show outward bulk radial motions (10-15 km/s), suggesting a coupling of radial and vertical dynamics in this disc disturbance.

Analyzing Vertical Corrugations in the Galactic Disc from Young Stellar Populations

This analysis presents a detailed examination of the vertical structure and kinematics of the Galactic disc using two distinct samples of young stellar populations, specifically young giants and classical Cepheids. Both samples reveal the existence of a large-scale vertical corrugation superimposed on the well-documented warp of the Milky Way. This discovery is significant, as it contributes to the current understanding of the Milky Way’s structure and dynamics by showcasing complex motion patterns beyond classical concepts.

Data and Methodology

The study utilizes data from the Gaia Data Release 3 (DR3) and a classical Cepheids catalog to trace the young populations in the Galactic disc. The young giant sample comprises approximately 16,723 stars, limited to heliocentric distances of approximately 7 kpc, while the Cepheid sample encompasses 3,400 stars extending to distances of roughly 15 kpc. These distinct samples allow for complementary analyses with the young giants offering dense coverage of closer regions and the Cepheids extending further across the disc.

The analysis involves fitting a classical m=1 warp model to the data, where deviations from this model indicate additional vertical disturbances in the Galactic disc. The study employs both parametric global models and non-parametric tilted rings methods to fit the warp and analyze residuals. The assessment extends to kinematic analyses, examining radial and vertical velocity fields to understand the dynamics of the observed corrugation.

Key Findings

1. Three-Dimensional Warp Structure: The inferred warp structure reveals an amplitude growing with Galactocentric radius, peaking at approximately 0.7 kpc at 14 kpc. The line-of-nodes suggests a twisting phase starting at around 12 kpc, consistent with previous findings using similar stellar populations.
2. Vertical Corrugation: A large-scale corrugation is discovered in both samples, showing vertical displacements of about 150-200 pc. This corrugation spans at least 10 kpc within the young giants sample and possibly up to 20 kpc when considering Cepheids.
3. Wave-like Nature: The analysis of the vertical motion residuals identifies a systematic pattern of radial and vertical velocities supporting the interpretation of the corrugation as a wave propagating outward. This manifests as systematic positive vertical velocities adjacent to the corrugation’s positive crest in the Galactic disc.
4. Radial Motion Association: Notably, the stars within the corrugation exhibit bulk radial motions of about 10-15 km/s directed outwards, suggesting a coupling of radial and vertical motion in the wave dynamics.

Implications and Future Work

These observations imply that the Galactic disc is experiencing complex perturbative processes, possibly influenced by past or ongoing satellite interactions or intrinsic dynamical responses. The radial-coupled vertical wave challenges previous assumptions about isolated vertical motions in Galactic dynamics, suggesting a more intricate interaction model where radial and vertical motions are linked.

The findings pave the way for further exploration into the origins and implications of such corrugations. Comparisons with theoretical models and N-body simulations could enhance the understanding of the dynamic history of the Milky Way, including interactions with satellite galaxies like the Sagittarius dwarf galaxy.

Future research should aim to incorporate additional datasets and refine kinematic models to quantify the propagation characteristics and potential driving forces behind these vertical structures. Understanding the source and evolution of such features will crucially inform models of Galactic formation and evolution.