Galaxies are polluting the cosmos

A team of astronomers led by Dr Alex Cameron and Associate Professor Deanne Fisher from the ARC Centre of Excellence for All-Sky Astrophysics in 3 Dimensions (ASTRO 3D) has used a new imaging system on the WM Keck Observatory in Hawaii to analyse the flow of gases both into and out of a galaxy 500 million light-years away. While the gases drawn into a galaxy are ‘clean’ (i.e. containing the hydrogen and helium formed from nucleosynthesis during the Big Bang) the flow out of the galaxy contains elements such as carbon, oxygen and iron (what astronomers often term as 'metals'), effectively polluting the cosmos. 

This is the first time the full cycle has been confirmed in a galaxy other than the Milky Way. Until now, the composition of the inward and outward flows could only be guessed.

“Enormous clouds of gas are pulled into galaxies and used in the process of making stars,” said co-lead author Deanne Fisher, Associate Professor at the Centre for Astrophysics and Supercomputing at Swinburne University in Australia.

“On its way in, the gas is made up 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.”

So what makes gases containing elements such as oxygen and carbon, pollution?

“Hydrogen and helium were created in the Big Bang, and so if you only see these elements in a region of the Universe, that is essentially the original chemistry of the Universe. We call gas ‘polluted’ when different elements such as carbon, nitrogen and iron are added to the mix since they weren't there originally,” explained Professor Fisher. 

“We know that galaxies grow over time. We also know that galaxies need gas to make new stars. Therefore, a galaxy must acquire gas through accretion in order to grow. The loss of gas through outflows will slow down this process. We don't yet know how much this is slowed down. Our observations are a ground-breaking step towards understanding the relative ratio of gas leaving the galaxy, to gas entering the galaxy.”

This research focused on the galaxy Mrk 1486, 500 million light-years away. Not only is it going through a period of very rapid star formation, but Mrk 1486 lies edge-on to Earth, meaning that the outflowing gas could be easily viewed, and its composition measured. 

“We found there is a very clear structure to how the gases enter and exit,” explained Dr. Alex Cameron, who has recently moved from the University of Melbourne in Australia to the UK’s University of Oxford.

“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 polluted gases containing more than half the elements (metals) of the periodic table are pumped out of the galaxies, mainly by supernovas, polluting the cosmos and surrounding regions.

“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,” added Professor Fisher.

“It takes us one step closer to understanding how and why galaxies look the way they do – and how long they will last.”

Other scientists contributing to the work are based at the University of Texas at Austin, the University of Maryland at College Park, and the University of California at San Diego – all in the US – plus the Universidad de Concepcion in Chile.