The outflow of galaxies is much dirtier than the gas that goes into them, according to research by an international team of astronomers.
The research, published in The Astrophysical Journal, examines the ‘accretion’ and ‘outflow’ of galaxies – the atoms that flow in, and the atoms that are eventually expelled.
“Enormous clouds of gas are pulled into galaxies and used in the process of making stars,” explains Deanne Fisher, associate professor at the Centre for Astrophysics and Supercomputing at Swinburne University, and co-author on the paper.
“On its way in, it is made of hydrogen and helium. By using a new piece of equipment called the Keck Cosmic Web Imager, we were able to confirm that stars made from this fresh gas eventually drive a huge amount of material back out of the system, mainly through supernovas.
“But this stuff is no longer nice and clean – it contains lots of other elements, including oxygen, carbon and iron.”
These heavier elements are formed in the centre of stars through nuclear fusion. When stars collapse in on themselves, or explode in supernovas, these elements are expelled.
“We found there is a very clear structure to how the gases enter and exit,” says Dr Alex Cameron, co-author on the paper and now a researcher at the University of Oxford, UK.
“Imagine the galaxy is a spinning frisbee. The gas enters relatively unpolluted from the cosmos outside, around the perimeter, and then condenses to form new stars. When those stars later explode, they push out other gas – now containing these other elements – through the top and bottom.”
The researchers examined a galaxy called Mrk 1486, which is a ‘starburst’ galaxy – it is forming stars very rapidly. Mrk 1486 sits at an angle to Earth that makes it ideal for viewing both the accreting gases and the outflow.
“This work is important for astronomers because for the first time we’ve been able to put limits on the forces that strongly influence how galaxies make stars,” says Fisher.
“It takes us one step closer to understanding how and why galaxies look the way they do – and how long they will last.”