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A tilted dark halo origin of the Galactic disk warp and flare

Nature Astronomy volume 7, pages 1481–1485 (2023)Cite this article

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Abstract

The outer disk of the Milky Way Galaxy is warped and flared. Several mechanisms have been proposed to explain these phenomena, but none have quantitatively reproduced both features. Recent work has demonstrated that the Galactic stellar halo is tilted with respect to the disk plane, suggesting that at least some component of the dark matter halo may also be tilted. Here we show that a dark halo tilted in the same direction as the stellar halo can induce a warp and flare in the Galactic disk at the same amplitude and orientation as the data. In our model, the warp is visible in both the gas and stars of all ages, which is consistent with the breadth of observational tracers of the warp. These results, in combination with data in the stellar halo, provide compelling evidence that our Galaxy is embedded in a tilted dark matter halo. This misalignment of the dark halo and the disk holds clues to the formation history of the Galaxy and represents the next step in the dynamical modelling of the Galactic potential.

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Fig. 1: Present-day distribution of simulated stars in Galactocentric cylindrical coordinates.
Fig. 2: Comparison of the simulation to the observed warp and flare.
Fig. 3: The amplitude and orientation of the disk warp at R = 16 kpc.

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Data availability

The present-day star and gas particle data are provided under stars.npy and gas.npy in https://github.com/jiwonjessehan/MilkyWayWarp.

Code availability

All of the code and data files used to produce the results are shared in https://github.com/jiwonjessehan/MilkyWayWarp.

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Acknowledgements

L.H. acknowledges support from the Simons Collaboration on ‘Learning the Universe’.

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Authors and Affiliations

  1. Center for Astrophysics, Harvard & Smithsonian, Cambridge, MA, USA

    Jiwon Jesse Han, Charlie Conroy & Lars Hernquist

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Contributions

J.J.H. conceived of and led the project. C.C. contributed to the warp analysis and writing of the paper. L.H. contributed to the sticky particle simulation and the writing of the paper. All authors reviewed and commented on the manuscript.

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Correspondence to Jiwon Jesse Han.

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Supplementary Information

Supplementary Figs. 1–4: Supplementary Fig. 1: Present-day distribution of star particles in the simulation in Galactocentric cylindrical coordinates. Negative R indicates azimuthal angles that are within 90∘ of the northern warp, and positive R indicates azimuthal angles that are within 90∘ of the southern warp. The top panels show the warp, and the bottom panels show the vertical deviation from the average warp. The vertical deviation systematically increases towards the outer Galaxy, demonstrating the flare of the disk. Points that are in the outer Galaxy are plotted with larger circles. Particles coloured based on birth radius reveal that the magnitude of the warp correlates strongly with the birth radius. Supplementary Fig. 2: Time evolution of the warp amplitude at fixed radius R = 16 kpc. At t = 0, the disk is initialized to have no warp. Error bars indicate 1σ statistical uncertainty in the warp fit. Within a few hundred Myr, the warp reaches maximum amplitude. After a transient oscillatory phase from t = 500–1500 Myr, the warp reaches a steady-state amplitude. This plot demonstrates that the warp onsets quickly, within one rotation period of the disk (400 Myr for a star at 16 kpc). Supplementary Fig. 3: Variation of (1) the scale length of the tilted component of the dark halo and (2) the present-day scale length of the tracer particles. Apart from the two parameters being modulated, all other parameters are fixed to the simulation presented in Fig. 1. In each panel, we show the fraction of stars that are off the Z = 0 plane by more than 0.25 kpc, Nwarp/Nplane. If this fraction is greater than 0.5%, we fit a warp model (dotted lines) and show the warp amplitude at 20 kpc as Z20. The warp amplitude correlates positively with the scale length of the disk and anti-correlates with the scale length of the tilted dark halo. Supplementary Fig. 4: Variation of (1) the tilt angle of the dark halo (2) the mass fraction of the tilted component of the dark halo. Apart from the two parameters being modulated, all other parameters are fixed to the simulation presented in Fig. 1. Similar to Supplementary Fig. 3, we show the fraction of stars that are off the Z = 0 plane by more than 0.25 kpc, Nwarp/Nplane. If this fraction is greater than 0.5%, we fit a warp model (dotted lines) and show the warp amplitude at 20 kpc as Z20. The warp amplitude correlates positively with both the tilt angle and the mass fraction of the tilted halo.

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Han, J.J., Conroy, C. & Hernquist, L. A tilted dark halo origin of the Galactic disk warp and flare. Nat Astron 7, 1481–1485 (2023). https://doi.org/10.1038/s41550-023-02076-9

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