Comment on 'a slightly oblate dark matter halo revealed by a retrograde precessing Galactic disk warp' by Huang et al

TL;DR

This paper critiques a recent study on the Galactic warp, arguing that its interpretation of warp precession based on Cepheid data is flawed due to overlooked variables and correlations.

Contribution

It clarifies the proper interpretation of warp measurements and highlights the importance of considering warp twist and metallicity-related correlations in such analyses.

Findings

The observed positive derivative of warp phase with age is explained by an omitted-variable bias.

Neglecting warp twist and metallicity-age correlations leads to incorrect precession rate estimates.

The interpretation of Cepheid data as warp precession is unfounded.

Abstract

Huang et al. (2024) measured the derivative of the phase $\phi_{\mathrm{w}}$ of the Galactic warp traced by classical Cepheids with respect to their age $\tau$ and interpreted it as the warp precession rate $\omega\equiv\mathrm{d}\phi_{\mathrm{w}}/\mathrm{d}t=-\mathrm{d}\phi_{\mathrm{w}}/\mathrm{d}\tau$. This interpretation is unfounded: young stars follow trajectories close to those of their parental gas and trace the instantaneous gas warp, not its shape at their time of birth: $\phi_{\mathrm{w}}$ should hardly depend on Cepheid age. We show that the measured $\mathrm{d}\phi_{\mathrm{w}}/\mathrm{d}\tau>0$ is consistent with an omitted-variable bias from neglecting the natural twist $\mathrm{d}\phi_{\mathrm{w}}/\mathrm{d}R$ of the warp and the $R$-$\tau$ correlation for Cepheids (originating from the Galactic metallicity gradient and the Cepheid metallicity-age correlation).