Bird’s eye view of molecular clouds in the Milky Way

Abstract

Context. The kinematics of molecular gas are crucial for setting the stage for star formation. One key question related to the kinematic properties of gas is how they depend on the spatial scale. Aims. We aim to describe the CO spectra, velocity dispersions, and especially the linewidth-size relation of molecular gas from cloud (parsec) scales to kiloparsec scales in a complete region within the Milky Way disk. Methods. We used the census of molecular clouds within 2 kpc from our earlier work, together with CO emission data for them from the literature. We studied the kinematics and the Larson relations for the sample of individual clouds. We also mimicked a face-on view of the Milky Way and analysed the kinematics of the clouds within apertures of 0.25–2 kpc in size. In this way, we describe the scale-dependence of the CO gas kinematics and Larson’s relations. Results. We describe the spectra of CO gas at cloud scales and in apertures between 0.25 and 2 kpc in our survey area. The spectra within the apertures are relatively symmetric, but show non-Gaussian high-velocity wings. At cloud scales, our sample shows a linewidth-size relation σv = 1.5 · R0.3±0.1 with a large scatter. The mass-size relation in the sample of clouds is MCO = 794 · R1.5±0.5. The relations are also present for the apertures at kiloparsec-scales. The best-fit linewidth-size relation for the apertures is σv = 0.5 · R0.35±0.01, and the best-fit mass-size relation is MCO = 229 · R1.4±0.1. A suggestive dependence on Galactic environment is seen. Apertures closer to the Galactic centre and the Sagittarius spiral arm have slightly higher velocity dispersions. We explore the possible effect of a diffuse component in the survey area and find that such a component would widen the CO spectra and could flatten the linewidth-size relation. Understanding the nature of the possible diffuse CO component and its effects on observations is crucial for connecting Galactic and extragalactic data.