The flare and spiral structure of the Milky Way's disc as traced by young giant stars
We explore the three-dimensional structure of a sample of $\sim$ 16000 young giant stars in the Galactic disc out to $\sim$8 kpc in heliocentric distance. This population traces a thin disc with a local vertical scale height of $h_{Z \odot} = 77 \pm 4$ pc, that progressively thickens toward the outer Galaxy with a prominent Galactic flare, rising exponentially with a radial scale length of $h_{fl} = 3.5 \pm 0.3 \, \rm{kpc}$. Our analysis incorporates both the survey selection function and the vertical displacements caused by the Galactic warp and corrugations, which, if neglected, would lead to significant biases in the derived disc scale height. In the Galactic plane, the young giants trace coherent spiral arm segments, extending previous maps based on upper main sequence (UMS) and OB stars by 2-4 kpc depending on the considered direction. The obtained map supports a pitch angle of roughly 20 degrees for the Perseus Arm, and shows that the Local/Orion arm stretches at least 10 kpc in length. Unlike earlier and more local maps based on UMS and OB stars, where the relatively small sampled portion of the Perseus Arm appeared as a short, nearly straight feature, our map reveals it as an extended structure with a gentle curvature, as expected for spiral arms on large scales. In the inner Galaxy, we also identify a new segment likely associated with the Scutum Arm, clearly detached from the Sagittarius-Carina Arm in the fourth Galactic quadrant.