Evidence of a dynamically evolving Galactic warp (1912.10471v3)

Abstract: In a cosmological setting, the disc of a galaxy is expected to continuously experience gravitational torques and perturbations from a variety of sources, which can cause the disc to wobble, flare and warp. Specifically, the study of galactic warps and their dynamical nature can potentially reveal key information on the formation history of galaxies and the mass distribution of their halos. Our Milky Way presents a unique case study for galactic warps, thanks to the detailed knowledge of its stellar distribution and kinematics. Using a simple model of how the warp's orientation is changing with time, we here measure the precession rate of the Milky Way's warp using 12 million giant stars from Gaia Data Release 2, finding that it is precessing at $10.86 \pm 0.03_{stat} \pm 3.20_{sys}$ km/s/kpc in the direction of Galactic rotation, about one third the angular velocity at the Sun's position in the Galaxy. The direction and magnitude of the warp's precession rate favour the scenario that the warp is the results of a recent or ongoing encounter with a satellite galaxy, rather than the relic of the ancient assembly history of the Galaxy.

• The paper reveals that the Milky Way warp precesses at 10.86 km s⁻¹ kpc⁻¹, indicating active gravitational influences.
• It employs a kinematic model with data from over 12 million giant stars in Gaia DR2 to track the warp’s orientation changes.
• The findings challenge static warp models, suggesting that recent satellite interactions are dynamically shaping the Galactic disc.

Evidence of a Dynamically Evolving Galactic Warp

The paper "Evidence of a dynamically evolving Galactic warp" by Poggio et al. presents a comprehensive paper of the evolutionary dynamics of the galactic warp in the Milky Way. Utilizing data from the Gaia Data Release 2, the research investigates the precession rate of the Milky Way's warp, revealing important insights into the Galactic disc's interaction with external gravitational forces.

Key Findings and Methodology

Using a sample of over 12 million giant stars, the authors measure the warp's precession rate to be $10.86 \pm 0.03_{stat} \pm 3.20_{syst}$ km s⁻¹ kpc⁻¹. This precession rate is approximately one-third of the angular rotation velocity at the Sun's location in the Galaxy. The findings suggest that the warp's precession is consistent with recent or ongoing gravitational interactions with a satellite galaxy, rather than being a relic from the Galaxy's ancient assembly history.

The paper employs a kinematic model to describe how the orientation of the warp changes over time. Two primary models are considered:

1. Warp rotating about the inner disc's vertical axis.
2. The inner disc precessing about an axis not aligned with its normal axis due to an external torque.

The latter model, despite its theoretical backing, is found to produce negligible precession rates within current observational precision. Thus, the observed precession and the lack of significant time-variable amplitude effects lead the authors to favor the first model.

Implications and Theoretical Considerations

The measured precession rate provides crucial insights into the dynamics of the Galactic disc. It challenges models of static warps and supports the theory that external forces from satellite galaxies, such as ongoing accretion or interaction, are actively influencing the disc. This points to a more dynamic and interactive galaxy morphology shaped significantly by recent environmental interactions.

Comparison with predictions from various theoretical models of halo interaction reveals that the observed precession rate surpasses expectations from models involving gravitational torques from misaligned halos or accumulated external masses. This discrepancy implies that more complex models accounting for transient and dynamic interactions, possibly with known satellites, need exploration.

Conclusion and Future Work

This paper makes a significant contribution by providing evidence of the dynamic nature of the Milky Way's warp. It emphasizes the importance of utilizing high-precision astrometric data from Gaia, which has permitted such detailed dynamical analyses. Future research should focus on more sophisticated models incorporating possible differential precession and evolving warp amplitudes. Continued observations may further elucidate the complex gravitational interplay shaping our galaxy's structure. This work lays a foundation for exploring similar phenomena in other galactic systems, offering broader insights into galactic formation and evolution in the context of the cosmos.