A three-dimensional view of the Milky Way

The Apex telescope makes it possible to observe molecular clouds and star births in the galactic plane.


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The Milky Way is a spiral galaxy. The Milky Way is a barred spiral galaxy. It is about 150,000 to 200,000 light-years across, about 2,000 light-years deep, and has 100 to 400 billion stars.

There are about 200 billion suns as well as large quantities of gas in our milky way. The gas collects in compact lumps but also appears as extended molecular clouds.

Using the Apex sub-millimeter telescope in Chile, astronomers 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.

Scientists cataloged more than 10,000 interstellar clouds and found that only about 10% contain stars.

This work comes under the project 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 section shows interstellar clouds in a small area of about 5% of the total SEDIGISM mapping
Colourful variety: The section shows interstellar clouds in a small area of about 5% of the total SEDIGISM mapping; each of these clouds is coloured differently. The small picture in the upper left diagrams the course of the spiral arms in the Milky Way. The grey region marks the complete area of the SEDIGISM mapping. In this illustration, the direction of the sector is light blue. Credit: Ana Duarte-Cabral, Alex Pettitt, and James Urquhart

Frederic Schuller from the Max Planck Institute for Radio Astronomy said, “The mapping contains data from 2013 to 2017, collected by the 12-meter Apex telescope in the Chilean Andes. With the publication of this most detailed map of cold molecular clouds in the Milky Way to date, a long-term observation project is now coming to fruition.” 

Scientists have had the option to notice the southern part of the inner Milky Way with an angular goal of 30 arcseconds; this relates to 1/60 of the full moon’s apparent diameter in the Earth‘s sky. They have also gained significant data on the structure, distance, and velocity for all galactic molecular clouds in around 66% of the inner disc of the Milky Way.

They observed the carbon monoxide molecule’s spectral lines—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 essential to determine significant parameters, such as the efficiency of star formation in the Milky Way. Structures and physical conditions within the clouds give the basic premise to the speculations of star formation. It is consequently imperative to spatially resolve the individual clouds and separate them from one another.

One key to the success was the 12-meter Apex telescope with its highly accurate surface and one of the world’s best sub-millimeter astronomy locations. The instrument is located at an altitude of 5100 meters on the Chajnantor Plain in the Chilean Atacama Desert. Here, there is extremely low water vapor content and thus excellent transparency of the atmosphere.

The new data complement a series of mappings of the galactic plane produced in the mid to far infra-red wavelength range over the past decade. This was done with space telescopes such as the Spitzer, Herschel, and—for longer wavelengths—the Apex itself. 

However, these projects lacked the speed information that SEDIGISM has now provided. The re-analysis of the data allows a more detailed study of star formation—and thus of the structure and dynamics of the Milky Way itself.

Journal Reference:
  1. F Schuller et al. The SEDIGISM survey: first data release and overview of the Galactic structure*, Monthly Notices of the Royal Astronomical Society (2020). DOI: 10.1093/mnras/staa2369


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