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What flows into a galaxy ‘cleaner’ than what flows out

What flows into a galaxy: caption
Artist’s concept of an edge-on galaxy, with flows of gas suffusing from it. Image via Science in Public

What flows into a galaxy

On August 30, 2021, astronomers described a process by which clouds of pure hydrogen and helium gas are pulled into galaxies, to be used in the process of making new stars. The stars convert those very simple elements – hydrogen and helium – into more complex elements including oxygen, carbon, and iron. And then the galaxies pump this “exhaust,” as the astronomers called it, out again into intergalactic space. Supernova explosions cause most of the outward flow. In other words, these astronomers said:

… What flows into a galaxy is a lot cleaner than what flows out.

Astronomers Alex Cameron and Deanne Fisher of ASTRO 3D in Australia – a consortium of 9 Australian universities, plus international partners – led this research. The team used a new piece of equipment called the Keck Cosmic Web Imager at the W.M. Keck Observatory in Hawaii. The peer-reviewed Astrophysical Journal Letters published the new work on August 30.

So it’s much the same story we’ve known for decades. It’s about elements being forged inside stars and then released via supernovae. We are star stuff, right? But this story focuses on the process as it occurs galaxy-wide.

Astronomers describe the process of atoms flooding into galaxies as accretion. They use the word outflow for the eventual expulsion of atoms from galaxies. Until now, these astronomers said, they could only guess at the composition of this inward and outward flow of material. Yet accretion and outflow are vital processes within galaxies. They govern the growth, mass and size of galaxies. And so astronomers want to understand them. In this new research, for the first time, astronomers were able to confirm the full cycle in a galaxy other than our home galaxy, the Milky Way.

Mrk 1486: Perfect for study

The researchers studied a galaxy called Mrk 1486, which is going through a period of very rapid star formation. Alex Cameron said:

We found there is a very clear structure to how the gases enter and exit. 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.

Mrk 1486 was the perfect candidate for observation, these astronomers said, because it lies edge-on to Earth. And that means the astronomers could see the outflowing gas more easily than if, for exmaple, they were looking at a face-on galaxy. Deanne Fisher added:

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

Bottom line: For the first time, astronomers were able to measure the inflow of hydrogen and helium atoms, and the outflow of more complex atoms, in a galaxy other than our Milky Way.

Source: The DUVET Survey: Direct Te-based metallicity mapping of metal-enriched outflows and metal-poor inflows in Mrk 1486

Via Science in Public

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