2022MNRAS.513..638Z

The evolution of the molecular interstellar medium is controlled by processes such as turbulence, gravity, stellar feedback, and Galactic shear. AL a part of the ISM-6D project, using Gaia astrometric measurements towards a sample of young stellar objects (YSOs), we study the morphology and kinematic structure of the associated molecular gas. We identify 150 YSO associations with distance d ≲3 kpc. The YSO associations are elongated, with a median aspect ratio of 1.97, and are oriented parallel to the disc mid-plane, with a median angle of 30°. The turbulence in the molecular clouds as probed by the YSOs is isotropic, and the velocity dispersions are related to the sizes by σ_v, 2D_ = 0.74 (r/pc)^0.67 (km s^–1) . The slope is on the steeper side, yet consistent with previous measurements. The energy dissipation rate of turbulence {dot}ε= σ_v, 3D_³/L decreases with the Galactocentric distance, with a gradient of 0.2 dex kpc^–1, which can be explained if turbulence is driven by cloud collisions. In this scenario, the clouds located in the inner Galaxy have higher chances to accrete smaller clouds and are more turbulent. Although the density structures of the complexes are anisotropic, the turbulence is consistent with being isotropic. If the alignment between density structures and the Galactic-disc mid-plane is due to shear, we expect t_ cloud_ ≳t_ shear_~30 Myr. This cloud lifetime is longer than the turbulence crossing time, and a continuous energy injection is required to maintain the turbulence.