2025MNRAS.542L..94M

Mapping the Milky Way spiral arms in the vertical direction remains a challenging task that has received little attention. Taking advantage of recent results that link the position of the Galactic spiral arms to metal-rich regions in the disc, we analyse a sample of young giant stars from Gaia DR3 and use their metallicity distribution to produce a 3D metallicity excess map. The map shows signatures of the spiral arms, whose vertical height vary across the Galactic disc, reaching up to 400 ${\rm \, pc}$ in amplitude and exhibiting vertical asymmetries with respect to the mid-plane. Specifically, the Perseus arm displays a high vertical asymmetry consistent with the Galactic warp. Moreover, we find evidence of a metal-rich stellar structure that undulates vertically, nearly in phase with the arrangement of star-forming regions named the Radcliffe Wave. This new structure is larger and extends beyond the Radcliffe Wave, reaching vertical amplitudes of $\sim$270 ${\rm \, pc}$ and extending for at least 4 ${\rm \, kpc}$ in length. We confirm that for at least half of its length this Extended Radcliffe Wave is the inner edge of the Local Arm. The finding of a metal-rich stellar counterpart of the Radcliffe Wave shows that mapping the three-dimensional metallicity distribution of young stellar populations reveals key information about the structures and chemical enrichment in the Galactic disc.