# Evidence of a dynamically evolving Galactic warp

## Abstract

In a cosmological setting, the disk of a galaxy is expected to continuously experience gravitational torques and perturbations from a variety of sources, which can cause the disk to wobble, flare and warp1,2. Specifically, the study of galactic warps and their dynamic nature could reveal key information on the formation history of galaxies and the mass distribution of their haloes. Our Milky Way presents a unique case study for galactic warps, thanks to detailed knowledge of its stellar distribution and kinematics. Using a simple model of how the warp’s orientation is changing with time, here, we measure the precession rate of the Milky Way’s warp using 12 million giant stars from Gaia Data Release 23, finding that it is precessing at 10.86 ± 0.03 (statistical) ± 3.20 (systematic) km s−1 kpc−1 in the direction of Galactic rotation, about one-third the angular rotation 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 result of a recent or ongoing encounter with a satellite galaxy, rather than the relic of the ancient assembly history of the Galaxy.

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## Data availability

The dataset can be downloaded at https://figshare.com/articles/Giants_P18_csv/11382705.

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## Acknowledgements

E.P. and R.D. thank S. Casertano, V. Debattista and K. V. Johnston for discussions. E.P. thanks B. Bucciarelli for providing the software for the propagation of astrometric covariances. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This work was supported by ASI (Italian Space Agency) under contract 2018-24-HH.0. This work was funded in part by the DLR (German space agency) via grant 50 QG 1403.

## Author information

Authors

### Contributions

E.P. contributed to the sample preparation, modelling and data analysis, and wrote the manuscript together with R.D.; R.D. contributed to the model construction and interpretation of the results; R.A., C.B.J. and M.F. helped with the statistical inference and revised the text; M.G.L., R.L.S. and A.S. contributed to the project planning and revised the text.

### Corresponding author

Correspondence to E. Poggio.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Extended data

### Extended Data Fig. 1 The cumulative luminosity function for $${{\mathcal{M}}}_{G}={M}_{G}+{A}_{G}$$.

For different values of extinction, the cumulative luminosity function was obtained by applying to our modelled colour-magnitude diagram (see text) the colour-colour cuts performed to select our sample.

### Extended Data Fig. 2 The completeness of the dataset as a function of apparent magnitude G.

The completeness is the fraction of stars found in both Gaia DR2 and 2MASS, derived using the cross-match table of Gaia DR2 and 2MASS provided by the Gaia Archive (https://gea.esac.esa.int/archive/).

### Extended Data Fig. 3 Shape parametrizations of the geometrical warp models adopted in this work, following Equation (2) and (17), and the obtained precession rates.

The radius Rw was scaled to account for different assumptions about the Sun - Galactic center distance in this work and in the considered papers.

### Extended Data Fig. 4 Spatial and kinematic parameters adopted in this work.

The values and their corresponding uncertainties are taken directly, as stated, from the cited references (last column).

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Poggio, E., Drimmel, R., Andrae, R. et al. Evidence of a dynamically evolving Galactic warp. Nat Astron 4, 590–596 (2020). https://doi.org/10.1038/s41550-020-1017-3

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