Kinematics of the molecular interstellar medium probed by Gaia: steep velocity dispersion-size relation, isotropic turbulence, and location-dependent energy dissipation

Abstract

The morphology and kinematics of the molecular interstellar medium are controlled by processes such as turbulence, gravity, stellar feedback, and Galactic shear. Using a sample of 15149 young stellar objects (YSOs) with Gaia Data Release 2 (DR2) astrometric measurements, we study morphology and kinematic structure of their associated molecular gas. We identify 150 YSO associations with distance 𝑑 3 kpc. The YSO associations are oriented parallel to the disk midplane, with a median angle of 30 • , and they have a median aspect ratio of 1.97. Along the Galactic longitude direction, the velocity dispersion is related to the separation by 𝜎 𝑣 𝑙 = 0.58 (𝑟 𝑙 /pc) 0.66±0.05 (km s −1 ), along the Galactic latitude direction, 𝜎 𝑣 𝑏 = 0.54 (𝑟 𝑏 /pc) 0.64±0.04 (km s −1 ), and overall 𝜎 𝑣 ,2D = 0.74 (𝑟/pc) 0.67±0.05 (km s −1 ). The slope is on the stepper side, yet consistent with previous measurements. The energy dissipation rate of turbulence 𝜖 = 𝜎 3 𝑣,3D /𝐿 decreases with the Galactocentric distance 𝑟 gal by 𝜖 = 1.77 × 10 −4 𝑒 −0.45 (𝑟 gal /kpc) (erg g −1 s −1 ) for clouds with 40 pc < 𝑟 < 130 pc, which corresponds to a gradient of 0.2 dex kpc −1 . This decrease can be explained if turbulence is driven by cloud collisions, as the clouds located in the inner Galaxy have higher chances to accrete smaller clouds. Although the density structures of the complexes are anisotropic, the turbulence is consistent with being isotropic. Thus, the clouds are long-lived, stationary structures shaped by the Galactic motion where turbulence is maintained by a continuous injection.

Energy balance and Alfvén Mach numbers in compressible magnetohydrodynamic turbulence with a large-scale magnetic field

“…vations using Herschel (e.g., the Herschel Gould Belt Survey -André et al 2010) generally cover much smaller fields of view; over size scales typically probed by such observations, there is no sign of a flattening in the velocity dispersion-size relation (Federrath et al 2021;Yun et al 2021;Zhou et al 2021), clear evidence that the region being studied is much smaller than the correlation scale.…”

Weather Forecast of the Milky Way: Shear and Stellar feedback determine the lives of Galactic-scale filaments

“…We start with a sample of Young Stellar Object (YSO) associations identified in a previous paper (Zhou et al 2021). We identify the molecular cloud counterparts of the YSO associations from the CO observations (Dame et al 2001).…”

“…Another assumption adopted in the current work is that the clouds needs to be long-lived, with 𝑡 cloud > 𝑡 simulation = 𝑡 shear . This condition can be satisfied, as the molecular clouds are estimated to live up to a few tens of Myr (Meidt et al 2015;Koda 2021;Zhou et al 2021), which is comparable or longer than the shear time 1 . Our evolution forecast results are plotted in Fig.…”

“…Our sample is taken from our previous paper (Zhou et al 2021), which contains a sample of YSO associations within ∼ 3kpc from the Sun. We identify their molecular cloud counterparts from the CO observations (Dame et al 2001) by visual inspection.…”

“…This, combined with radial velocity measurements, allow us to derive the 3D velocities of YSOs. In this paper, we trace the evolution of the molecular ★ gxli@ynu.edu.cn † bchen@ynu.edu.cn ISM using a sample of Young Stellar Objects (YSO) association (Zhou et al 2021)-molecular cloud complex (YSO-MC complex). We derive their three-dimensional (3D) velocities by combining the Gaia astrometric measurements (Gaia Collaboration et al 2018a) of the YSO associations and the CO observations of the associated molecular clouds (Dame et al 2001).…”