ALMA peers into star-forming gas regions outside our Milky Way

March 13, 2017
An international team of astronomers led by MPE has used the Atacama Large Millimeter/submillimeter Array (ALMA) to zoom into sites of star formation in the nearby galaxy NGC 6822. The new ALMA observations reveal the structure of star-forming gas clouds with a high level of detail making in possible to compare it to similar regions in our home galaxy. They indicate that the physics of star formation may be the same in low mass, pristine galaxies - the building blocks of more massive galaxies - as in our own Galaxy.

Observations in the Milky Way have shown that stars form in the dense cores of giant gas clouds, where the gas can reach temperatures cold enough to become susceptible to gravitational collapse. The same conditions foster the formation of molecules which are indispensable observational tracers of gas in galaxies.

So far, observations were unable to resolve the sites of star formation outside the Milky Way. Not only are those galaxies much farther away and therefore appear smaller, the galaxies closest to the Milky Way are of low mass with slow stellar build-up and limited heavy elemental enrichment of their gas reservoir - rendering observations even harder. These pristine conditions result not only in a lack of molecules, they may also result in a hostile environment for the formation of cold gas, making star formation much harder.

ALMA has now overcome this observational limit by providing images two orders of magnitude sharper than commonly observed. The nearby low-mass galaxy NGC 6822 has been targeted, a small galaxy at a distance of 1.5 million lightyears with 500 times less mass than the Milky Way. The data reveal a plethora of small dense cores in the star-forming sites, with molecular tracers constrained to the densest cores. They fill a much smaller volume than in Galactic star-forming clouds such as the Orion cloud.

"The difference in appearance of star-forming gas clouds in NGC 6822 as compared to those in our Galaxy is striking,” remarks Andreas Schruba, who led the team at MPE. “The observed molecules can only be found in very small, dense cores, which explains why previous observations often remained unsuccessful."

The high spectral resolving power of ALMA lead to a second important discovery: Unexpectedly, despite the different distribution of the molecules, the dense gas cores show the same kinematics as similar-sized structures in our own Galaxy. “From the width of the molecular lines we can infer the kinematic properties of the gas in these cores,” explains Andreas Schruba. “This finding is today's strongest observational evidence that the time evolution and the physics of star formation in these low-mass galaxies resemble those of the Milky Way.”

These observations therefore provide important clues to understanding star formation in low-mass galaxies, which are the building blocks of more massive galaxies such as the Milky Way. They can guide the interpretation of less-resolved observations of more distant galaxies.

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