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The Milky Way

Hazed and confused - what gas tells us about galactic evolution

1 February 2017
Gas disturbances key to understanding galactic evolution

Thanks to new technologies developed at the University of Sydney we might be on the verge of a breakthrough in understanding how the universe evolved, PhD student and science communicator Jess Bloom writes.

Merging galaxies in Galaxy Zoo

Merging galaxies in Galaxy Zoo (Source: Galaxy Zoo)

Space may seem cold, remote and unchanging, but the universe is actually a dynamic place and constantly evolving. Looking back in time at the distant universe, we see a picture looking very different from the modern universe.

But how and why did galaxies develop from small, fluffy blobs to have the structure and variety that we see today? Answering this question is one of the fundamental problems of astronomy.

Using new technology developed at the University of Sydney and the Anglo-Australian Observatory, the SAMI (Sydney-AAO Multi-object Integral field spectrograph) Galaxy Survey team are assembling deep, physical data for a statistically robust sample size of galaxies.

Previous galaxy surveys have had one of two major difficulties: either the sample size was very small, or they were only able to probe the central region of a galaxy, ignoring the full spatial extent of the disk. The SAMI instrument circumvents these two problems simultaneously.

Instead of a single fibre being placed to image the centre of each galaxy, 61 fibres image the galaxy’s full face. Multiple galaxies are imaged during each pointing, so a large sample of galaxies can be compiled relatively quickly.

Galaxies are understood to contain several components: gas, stars, dust and dark matter. Of these, gas is the most turbulent and responsive to change. Its perturbations provide a key laboratory for investigating galactic evolution. In particular, the star-forming gas traces and influences the changes which determine the evolutionary paths of galaxies across a wide range of masses and morphologies.

Examining the star-forming gas in these galaxies, it became apparent that, despite their sometimes unimpressive appearance, the smallest galaxies were amongst the strangest and most disturbed.

Low mass galaxies (only 109 solar masses or so!) contain much more gas than larger galaxies (up to 1011 solar masses) when proportion is taken into consideration. This excess gas is unstable and prone to form large clumps, which themselves tend to collapse and cause disturbances. In effect, the gas in small galaxies is constantly bubbling and warping. In comparison the gas in larger galaxies tends to be more sedate. Thus, small galaxies are, in effect, self-perturbing, without outside influence.

Examining the star-forming gas in these galaxies, it became apparent that, despite their sometimes unimpressive appearance, the smallest galaxies were amongst the strangest and most disturbed.

However, there is another major source of disturbance. Interactions are critically important drivers of galaxy evolution, and cause some of the most spectacular warping of Hα gas.

The current model of modern galaxy formation is one of hierarchical merging. Smaller clumps of gas, dust, and stars gradually coalesce into ever larger, cumulative systems. Understanding these mergers and their impact on galaxies on various timescales is therefore of crucial importance in testing and refining theories of galaxy evolution.

We find that the closer galaxies are to their nearest neighbours, the more disturbed their gas is. This is true, even when galaxies are very far from their nearest neighbours, telling us that the gravitational pull of neighbouring systems has a clear signature in gas perturbation.

However, the influence of galaxy size is even stronger than that of distance. Even when large galaxies are very close to their neighbours, the large amount of gas in small galaxies is enough to make them even more disturbed. This tells us that disturbance in low mass galaxies may have a different source from that in high mass galaxies. This has fascinating implications for how galaxies of different masses evolve.

About the author

Jessica is an astrophysics PhD student at Sydney University - studying galaxy mergers and their impact on galaxy evolution, Jessica is particularly interested in physics outreach.

Teaching first and second year Physics at the University, over the past two years Jessica has spent four weeks as Astronomer in Residence at Uluru, leading star tours and giving talks. When she is not talking physics and the universe or writing her thesis, Jessica is taking the most boss selfies ever, and is also circus performer - a contortionist, hooper, acrobat and fire spinner - freelancing at nightclubs, parties and festivals.

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