In some countries, more babies are born than in others. With galaxies, it’s the same: some galaxies have a much higher birth rate of new stars than others. Astronomers have studied six of those so-called starburst galaxies with the ALMA observatory. In five of them, they discovered large amounts of cool turbulent gas, surrounding the galaxies.
‘Turbulent’ means that the gas atoms are not moving smoothly and steadily like cars on a highway. Instead, the gas is moving with a wide range of velocities in many directions – more or less like walkers, cyclists and motorists on a large town square without lines and traffic lights.
The turbulence in the gas can actually lead to more new stars being born. Thus, the stellar birth rate in the galaxies – many dozens of new stars per year – can remain high for a very long period: many tens of millions of years. Without the inflow of cool molecular gas, the starburst phase of a galaxy couldn’t last that long.
Because the galaxies are so remote, the cool, turbulent gas remains hidden from the view of a normal telescope. Astronomers used a trick to detect the gas with ALMA. They measured the millimeter radiation produced by carbon hydride molecules (CH+). These molecules consist of one carbon atom (C) and the nucleus of a hydrogen atom (H). To form them, quite a lot of energy is needed, so they can only exist in regions of strong turbulence.
It’s a bit like tracking down the location of an invisible hurricane by mapping the distribution of debris from demolished buildings: you will only find large amounts of debris in regions where there’s enough hurricane energy.
So where does the gas in the turbulent reservoirs come from? Probably from two sources. Part of the gas was first blown away into space by the active star-forming regions in the galaxy’s core, and is now falling back into the galaxy. Another part of the gas may be coming from another galaxy that collided and merged with the starburst galaxy.
ALMA was used to study six remote starburst galaxies. The six relatively small galaxies have distances between 10.7 and 11.3 billion light-years. One of them is known as the ‘Cosmic Eyelash’. This funny-looking galaxy is located in the constellation Aquarius, the Water-bearer. Surrounding the starburst galaxies, ALMA detected large amounts of cool, turbulent gas, out to distances of some 30,000 light-years.
The team that made the discovery was led by Edith Falgarone. Edith is an astronomer at the Paris Observatory in France. For this project, she worked together with colleagues from France, Germany, the United Kingdom and the United States. The results of the research were published on 24 August 2017 in the scientific weekly magazine Nature.