Why does this monster galaxy keep on forming new stars

Why does this monster galaxy keep on forming new stars

Read time: 3 minutes

Sometimes, scientists are like little children. They always ask ‘why?’. That’s because they want to understand everything they see. A lot of ‘why questions’ have already been answered, like ‘why is the sky blue?’ and ‘why is the sun hot?’. But many remain.

So when astronomers discovered a distant galaxy with huge numbers of newborn stars, they wondered: why is the birth rate of stars in this galaxy so much higher than in other galaxies? Thanks to ALMA, they have now found the answer.

Stars are born from large clouds of gas. Because cold gas contains a lot of molecules, these clouds are often called ‘giant molecular clouds’. Normally, when stars begin to form, the radiation from the newborn stars will blow away the surrounding cloud. Eventually, the birth of new stars will peter out.

But in some galaxies, star formation just goes on and on. These are called ‘starburst galaxies’. ALMA has now observed an extreme starburst galaxy at a distance of many billions of light-years. The birth rate of stars in this galaxy is thousand times higher than in our own Milky Way galaxy! No wonder astronomers wanted to know why.

The new ALMA observations have provided an answer. ALMA was able to measure the distribution and the motion of the gas in the ‘monster galaxy’. Most of the molecular gas turned out to be located in two huge clouds, a few thousand light-years away from the galaxy’s center. That was a surprise: usually, the largest clouds are located right at the galaxy’s core.

ALMA also discovered that the giant molecular clouds in the galaxy are unstable: the gas in the clouds is moving chaotically. It is not so easily blown away into space. Instead, it continues to collapse into new stars.

The astronomers think that new stars will continue to form for at least hundred million years. By then, most of the gas in the giant molecular clouds will have been consumed.

The new observations shed light on the early life of galaxies. The starburst galaxy studied by ALMA is so far away that its light took 12.4 billion years to reach Earth. In other words: the galaxy is seen as it was 12.4 billion years ago, when the universe was still very young.

Of course, astronomers are now faced with a new ‘why question’: why are the gas clouds in this galaxy so unstable? They don’t know the answer yet. Maybe it’s because the galaxy has collided with another galaxy.


The ‘monster starburst galaxy’ studied by ALMA is known as COSMOS-AzTEC-1. It was first studied a few years ago by the James Clerk Maxwell Telescope on Mauna Kea, Hawaii, and by the Large Millimeter Telescope in Mexico. These observations revealed the high rate of star formation and the distance of the galaxy: 12.4 billion light-years. But it took the high sensitivity and the sharp vision of ALMA to reveal the distribution and the motion of gas clouds in the remote galaxy. To achieve this, the 66 ALMA dishes were positioned in their widest possible configuration, up to 16 kilometers from each other.


The study of the monster galaxy COSMOS-AzTEC-1 was carried out by a large group of mainly Japanese astronomers. The group was led by Ken-ichi Tadaki of the National Astronomical Observatory of Japan. The new ALMA results were published in the weekly scientific magazine Nature.