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- ItemThe SEDIGISM survey: The influence of spiral arms on the molecular gas distribution of the inner Milky Way(Les Ulis : EDP Sciences, 2022) Colombo, D.; Duarte-Cabral, A.; Pettitt, A.R.; Urquhart, J. S.; Wyrowski, F.; Csengeri, T.; Neralwar, K.R.; Schuller, F.; Menten, K.M.; Anderson, L.; Barnes, P.; Beuther, H.; Bronfman, L.; Eden, D.; Ginsburg, A.; Henning, T.; König, C.; Lee, M.-Y.; Mattern, M.; Medina, S.; Ragan, S.E.; Rigby, A. J.; Sánchez-Monge, Á.; Traficante, A.; Yang, A. Y.; Wienen, M.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 the 13CO (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 ~105-106 M⊙ 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.
- ItemOGHReS: Large-scale filaments in the outer Galaxy(Les Ulis : EDP Sciences, 2021) Colombo, D.; König, C.; Urquhart, J. S.; Wyrowski, F.; Mattern, M.; Menten, K. M.; Lee, M.-Y.; Brand, J.; Wienen, M.; Mazumdar, P.; Schuller, F.; Leurini, S.Filaments are a ubiquitous morphological feature of the molecular interstellar medium and are identified as sites of star formation. In recent years, more than 100 large-scale filaments (with a length > 10 pc) have been observed in the inner Milky Way. As they appear linked to Galactic dynamics, studying those structures represents an opportunity to link kiloparsec-scale phenomena to the physics of star formation, which operates on much smaller scales. In this Letter, we use newly acquired Outer Galaxy High Resolution Survey (OGHReS) 12CO(2-1) data to demonstrate that a significant number of large-scale filaments are present in the outer Galaxy as well. The 37 filaments identified appear tightly associated with inter-arm regions. In addition, their masses and linear masses are, on average, one order of magnitude lower than similar-sized molecular filaments located in the inner Galaxy, showing that Milky Way dynamics is able to create very elongated features in spite of the lower gas supply in the Galactic outskirts.
- ItemA Kiloparsec-scale Molecular Wave in the Inner Galaxy: Feather of the Milky Way?(London : Institute of Physics Publ., 2021) Veena, V.S.; Schilke, P.; Sánchez-Monge, Á.; Sormani, M.C.; Klessen, R.S.; Schuller, F.; Colombo, D.; Csengeri, T.; Mattern, M.; Urquhart, J. S.We report the discovery of a velocity coherent, kiloparsec-scale molecular structure toward the Galactic center region with an angular extent of 30° and an aspect ratio of 60:1. The kinematic distance of the CO structure ranges between 4.4 and 6.5 kpc. Analysis of the velocity data and comparison with the existing spiral arm models support that a major portion of this structure is either a subbranch of the Norma arm or an interarm giant molecular filament, likely to be a kiloparsec-scale feather (or spur) of the Milky Way, similar to those observed in nearby spiral galaxies. The filamentary cloud is at least 2.0 kpc in extent, considering the uncertainties in the kinematic distances, and it could be as long as 4 kpc. The vertical distribution of this highly elongated structure reveals a pattern similar to that of a sinusoidal wave. The exact mechanisms responsible for the origin of such a kiloparsec-scale filament and its wavy morphology remains unclear. The distinct wave-like shape and its peculiar orientation makes this cloud, named as the Gangotri wave, one of the largest and most intriguing structures identified in the Milky Way.