ALMA carries out large census of ‘star factories’ in other galaxies

ALMA carries out large census of ‘star factories’ in other galaxies

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Not all families are the same. Some have many children; others just a few. In some cases, families live close together in their home town; in other towns they are less numerous, and they live further apart. You might wonder about the cause of all these differences – maybe it has something to do with the size or the structure of the town?

Astronomers face a similar question. Not with families of people, but with families of stars. We know that stars are born from giant clouds of cold molecular gas and dust. These clouds are called star-forming regions. One of the best known examples is the Orion Nebula in our own Milky Way galaxy. Each star-forming region gives birth to a large ‘family’ of stars.

There are many such star-forming regions in our own Milky Way. And then there are countless star-forming regions in other galaxies. These ‘star factories’ with their families of newborn stars all have different properties – just like human families here on Earth. And maybe these differences are due to the size or the structure of the galaxy in which they reside – their ‘home towns’, so to say.

To find out, astronomers have used ALMA to study a whopping 100,000 ‘star factories’ in 74 galaxies. For each star-forming region, they studied the size, the internal motions of the molecular gas, and the rate at which new stars are born. Of course, they also looked at the properties of the 74 home galaxies. It is as if the government carries out a census of 100,000 individual families in 74 towns and cities in your home country.

This huge project was only possible because of the extremely sharp vision of ALMA, which made it possible to see the finest details, down to a few tens of light-years. So far, ALMA spent 750 hours studying the distant star-forming regions. But the project isn’t finished yet. In the end, some 300,000 star factories will be studied. Supporting observations are being carried out by the European Very Large Telescope at Cerro Paranal in northern Chile, and by the Hubble Space Telescope.

So, what about the results? Did astronomers discover why some star-forming regions are larger and more efficient in producing new stars than others? Not yet. But it turns out that luminous newborn stars sometimes destroy the clouds from which they are born, preventing the formation of additional sibling stars. This process may be more efficient in larger and older galaxies.

Eventually, thanks to ALMA, astronomers hope to get a much clearer picture of star formation all across the universe.


ALMA studied 74 so-called disk galaxies in the southern sky – galaxies that have a flattened disk of gas, dust and young stars, just like our own Milky Way galaxy. The galaxies were observed when the 66 ALMA dishes were in their ‘extended configuration’, as far apart from each other as possible. Thus, astronomers could study the star-forming regions in the galaxies in much detail. ALMA mapped the distribution of carbon monoxide (CO) molecules. These molecules emit radiation at millimeter and sub-millimeter wavelengths – the type of ‘invisible light’ that ALMA is sensitive to.


The project described here is known as PHANGS-ALMA. PHANGS stands for Physics at High Angular Resolution in Nearby GalaxieS. The project is led by Erik Rosolowsky of the University of Alberta in Canada and Adam Leroy of Ohio State University in the United States. Erik and Adam worked together with a large international team of colleagues. The first results of the project were presented at the 223rd meeting of the American Astronomical Society in Seattle, and a number of research papers have been published in The Astrophysical Journal and Astrophysical Journal Letters.