Abstract
The morphology of the Milky Way is still a matter of debate. In order to shed light on uncertainties surrounding the structure of the Galaxy, in this paper, we study the imprint of spiral arms on the distribution and properties of its molecular gas. To do so, we take full advantage of the SEDIGISM (Structure, Excitation, and Dynamics of the Inner Galactic Interstellar Medium) survey that observed a large area of the inner Galaxy in the13CO (2–1) line at an angular resolution of 28′′. We analyse the influences of the spiral arms by considering the features of the molecular gas emission as a whole across the longitude–velocity map built from the full survey. Additionally, we examine the properties of the molecular clouds in the spiral arms compared to the properties of their counterparts in the inter-arm regions. Through flux and luminosity probability distribution functions, we find that the molecular gas emission associated with the spiral arms does not differ significantly from the emission between the arms. On average, spiral arms show masses per unit length of ~105–106M⊙kpc−1. This is similar to values inferred from data sets in which emission distributions were segmented into molecular clouds. By examining the cloud distribution across the Galactic plane, we infer that the molecular mass in the spiral arms is a factor of 1.5 higher than that of the inter-arm medium, similar to what is found for other spiral galaxies in the local Universe. We observe that only the distributions of cloud mass surface densities and aspect ratio in the spiral arms show significant differences compared to those of the inter-arm medium; other observed differences appear instead to be driven by a distance bias. By comparing our results with simulations and observations of nearby galaxies, we conclude that the measured quantities would classify the Milky Way as a flocculent spiral galaxy, rather than as a grand-design one.
Highlights
Spiral galaxies dominate the star formation budget of the local Universe
Two methodologies are used for the analyses described here (Sect. 4): we first study the full distribution of molecular gas in longitude–velocity space (Sect. 4.1) and we use the locations of discretised molecular clouds extracted from the SEDIGISM survey (Sect. 4.2)
Summary We present an analysis of the molecular gas distribution in the inner Galaxy and in particular its relationship with the Milky Way spiral arms
Summary
Grand-design galaxies (such as M51 or NGC 628) are characterised by two long and fairly symmetric arms while flocculent galaxies (such as NGC 7793 or NGC 7331) have multiple, fragmented, and generally shorter arms. Flocculent spirals are supposedly generated by local disc instabilities, while the grand-design character is associated with large-scale quasi-stationary density waves or tidal interactions, or with the presence of a bar (Dobbs & Baba 2014). These two arm classes are not mutually exclusive. The M51 galaxy, which is often put forward as an archetypal example of a grand-design galaxy, shows flocculent-type arms in its outer region, which can no longer be associated with a density wave (Meidt et al 2013; Colombo et al 2014b)
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