This video (or alternative link) scrolls across the full extent of the SEDIGISM (“Structure, Excitation and Dynamics of the Inner Galactic Interstellar Medium”) data, which span from -0.5° to +0.5° Galactic latitude, and 18° to 0° to 300° Galactic longitude. The data are color-encoded by the receding velocity of the clouds with respect to the Sun. A few interesting regions are indicated: “RCW120", an emission nebula; “Nessie”, one of the longest molecular gas filaments ever observed; and several high mass star forming regions, “W31”, “W33", “G333”, and “G305". In the bottom, on the left, the Galactic coordinate span of the data is shown; on the middle, the compass follows the region of the Galactic disk shown in the upper part; on the right, the distribution of the CO intensity from the clouds at the different velocities is indicated. Credit: D. Colombo, V. Kalinova & the SEDIGISM consortium.
In our Milky Way, there are about 200 billion suns as well as large quantities of gas, some of which serves as raw material for star births. The gas collects in compact clumps but also appears as extended molecular clouds. An international research team of more than 50 astronomers (including KIAA faculty Ke Wang) led by the Max Planck Institute for Radio Astronomy in Bonn, Germany, used the APEX submillimeter telescope at 5100 m altitude in Chile to look deep into the galactic plane and measure the interstellar medium. They studied the distribution of the cold molecular gas in the inner region of the Milky Way with unprecedented accuracy. The researchers catalogued more than 10,000 interstellar clouds. They found out that currently only about 10% of them are forming stars. The project is called SEDIGISM (Structure, Excitation and Dynamics of the Inner Galactic Interstellar Medium) and covers an area of 84 square degrees in the southern sky. The first data release makes these data available to the scientific community (see link below). The initial results from this survey are presented in three publications.
The mapping contains data from 2013 to 2017, which was collected by the 12-metre APEX telescope in the Chilean Andes. “With the publication of this unprecedentedly detailed map of cold clouds in our Milky Way a huge observational effort comes to fruition”, says Frederic Schuller from the Max Planck Institute for Radio Astronomy (MPIfR), the principal investigator of the SEDIGISM survey and lead author of Paper I. “The team did a great job in delivering a new roadmap for molecular gas in the Galaxy as legacy of APEX for years to come.”
Scientists have been able to observe the southern part of the inner Milky Way with an angular resolution of 30 arcseconds; this corresponds to ¹⁄₆₀ of the apparent diameter of the full moon in the Earth’s sky. They have also gained valuable information on structure, distance, and velocity for all galactic molecular clouds in about two thirds of the inner disc of the Milky Way.
The researchers observed the spectral lines of the carbon monoxide molecule – including the rare isotopes 13CO and C18O – and deduced the mass and three-dimensional distribution of cold and dense molecular gas in the interstellar medium. Various structures such as filaments and recesses were found; these are the result of different physical effects.
Molecular clouds contain the raw material from which new stars are formed. The mapping of these clouds is therefore necessary to determine important parameters such as the efficiency of star formation in the Milky Way. Structures and physical conditions within the clouds provide the fundamental basis for the theories of star formation. It is therefore important to spatially resolve the individual clouds and distinguish them from each other.
"Based on these data, a catalog of over 10,000 of these clouds in our Milky Way has been compiled, which show a highly structured Galactic distribution, albeit with relatively homogeneous physical properties, with only hints for potential environmental dependency of some cloud properties”, explains Ana Duarte-Cabral from Cardiff University, the lead author of Paper II. James Urquhart from the University of Kent, the lead author of Paper III, adds: “In conjunction with the previous survey of cold dust emission in the Galaxy (ATLASGAL), the fraction of clouds associated with dense gas could be estimated: only 10% of the clouds are sites of ongoing star formation".
The SEDIGISM survey is not only interesting on its own, but also complements a number of other outstanding Galactic plane surveys, conducted in the last decade in the mid- to far-infrared wavelength ranges with space-based telescopes such as Spitzer and Herschel and, at longer wavelengths with APEX itself, which are all lacking the velocity information. The data of these surveys can now be re-analysed in conjunction with the new carbon monoxide line data. This will significantly enhance their role in the ongoing quest to understand the formation of stars, stellar clusters and ultimately the structure and dynamics of the Milky Way.
“Our survey represents a significant step towards understanding the structure of the Galaxy in which we live”, concludes Dario Colombo from MPIfR, co-author of all three publications, who is currently preparing another analysis of the data to establish the influence of spiral arms on molecular cloud properties.
“The SEDIGISM survey adds the velocity information to previously known dusty clumps galaxy wide, at a comparable resolution. This is the key to study the distribution of molecular clouds in the context of Galactic structure”, says Ke Wang from KIAA, co-author of the Paper I and II, who is leading a project to study the largest filaments in our Galaxy.
Text credit: MPIfR, MPE
SEDIGISM Data Server
SEDIGISM (Structure, Excitation and Dynamics of the Inner Galactic Interstellar Medium) Data Server
Video: The clouds beyond the sky: a travel through the Galaxy with SEDIGISM (or: alternative video link)
The video scrolls across the full extent of the SEDIGISM (“Structure, Excitation and Dynamics of the Inner Galactic Interstellar Medium”) data, which span from -0.5° to +0.5° Galactic latitude, and 18° to 0° to 300° Galactic longitude.
Paper I: The SEDIGISM survey: first data release and overview of the Galactic structure
F. Schuller et al., Monthly Notices of the Royal Astronomical Society, Vol. 500, Issue 3, Pages 3064-3082. DOI: 10.1093/mnras/staa2369
Paper II: The SEDIGISM survey: Molecular clouds in the inner Galaxy
A. Duarte-Cabral et al., Monthly Notices of the Royal Astronomical Society, Vol. 500, Issue 3, Pages 3027-3049. DOI: 10.1093/mnras/staa2480
Paper III: SEDIGISM-ATLASGAL: Dense Gas Fraction and Star Formation Efficiency Across the Galactic Disk
J.S. Urquhart et al., Monthly Notices of the Royal Astronomical Society, Vol. 500, Issue 3, Pages 3050-3063. DOI: 10.1093/mnras/staa2512