A three-dimensional image of the Milky Way

In our Milky Way, there are about 200 billion suns and vast amounts of gas, some of which serve as raw materials for star birth. The gas collects in compact chunks but also appears as extensive molecular clouds. Astronomers have used the Apex submillimeter telescope 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 part of the Milky Way with unprecedented accuracy. The researchers have cataloged more than 10,000 interstellar clouds. They found that currently only about 10% of them contain 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 mapping includes 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 observational project is now coming to fruition,” said Frederic Schuller of the Max Planck Institute for Radio Astronomy, SEDIGISM’s project leader. .

Scientists have been able to observe the southern part of the inner Milky Way with an angular resolution of 30 arc seconds; this is equivalent to 1/60 of the apparent diameter of the full moon in Earth’s sky. They have also obtained valuable information about structure, distance and speed for all galactic molecular clouds in about two-thirds of the Milky Way’s inner disk.

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 were found, such as filaments and recesses; these are the result of various physical effects.

Molecular clouds contain the raw material from which new stars are formed. Mapping these clouds is therefore necessary to determine important parameters, such as the efficiency of star formation in the Milky Way. Structures and physical conditions in the clouds are the fundamental basis for the theories of star formation. It is therefore important to spatially resolve the individual clouds and to distinguish them from each other.

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

The new data complements a series of images of the galactic plane produced in the mid to far infrared 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 for a more detailed study of star formation – and thus of the structure and dynamics of the Milky Way itself.