A twisted and precessing Cepheid warp in the outer Milky Way disc

Abstract

ABSTRACT We examine the Galactic warp in a sample of all classical Cepheids with Gaia Data Release 3 radial velocity. In each radial bin, we determine (1) the inclined plane normal to the mean orbital angular momentum of the stars and (2) that best fitting their positions. We find no warping inside $R\approx 11\,$kpc; for larger R, the disc is increasingly inclined, reaching i ∼ 3° at $R\ge 14\,$kpc. With larger R, the azimuth of the warp’s ascending node shifts from φlon ≈ −15° at 11 kpc by about 14° kpc−1 in the direction of Galactic rotation, implying a leading spiral of nodes, the general behaviour of warped galaxies. From the method of fitting planes to the positions, we also obtain ${\dot{\varphi }}_{\mathrm{lon}}$ and find prograde precession of ${\dot{\varphi }}_{\mathrm{lon}}\sim 12\, \rm{km{}\,s^{-1}{}\,kpc^{-1}}$ at 12 kpc decreasing to ${\sim}\,6\, \rm{km{}\,s^{-1}{}\,kpc^{-1}}$ at 14 kpc and beyond. This would unwind the leading spiral of nodes in ${\sim}\,100\,$Myr, suggesting that our instantaneous measurements of ${\dot{\varphi }}_{\mathrm{lon}}$ reflect transient behaviour. This is consistent with existing simulations, which show oscillations in ${\dot{\varphi }}_{\mathrm{lon}}$ overlaying a long-term retrograde differential precession that generates the leading spiral of nodes.