Survey Reveals What Makes Galaxies Go Round

This image from the Hubble Telescope gives a clear example of a galaxy losing its gas as its movement through the intracluster medium creates a force that draws material away. NASA, ESA. Acknowledgments: Ming Sun (UAH), and Serge Meunier

A seven-year survey that has created a step-change in galaxy tracking has released its data. Astronomers will be picking over the detailed observations of more than 3,000 galaxies for years to come, but have already learned a lot about how galaxies grow and die as well as what makes them turn, particularly in large clusters.

"The nature of galaxies depends both on how massive they are and their environment," said Professor Scott Croom of the University of Sydney in a statement. "For example, they can be lonely in voids, or crowded into the dense heart of galactic clusters, or anywhere in between.” The clusters he refers to are collections of hundreds of galaxies, operating on a different scale to the local group of which the Milky Way is part.

Noticing that galaxies spin at differing rates, astronomers had their own version of the “nature versus nurture” debate. They argued about whether the spin rate reflected a galaxy's mass and developmental history, or its environment – the presence of other galaxies nearby.

Answering this question was tricky, Croom explained to IFLScience, because to do it properly required a database large enough to detect patterns. There is no shortage of galaxies, but measuring their spin isn't a simple process. It requires the collection of spectra from multiple spots within the galaxy to measure the red and blue shift, revealing whether each portion is turning towards us or away.

“Over a couple of decades astronomers built up samples of a few hundred galaxies,” Croom told IFLScience. Then the Sydney-AAO Multi-Object Integral-Field Spectrograph (SAMI) arrived. Attached to the Anglo-Australian Telescope, SAMI can detect the spectrum of all parts of 13 galaxies at once, although Croom noted most samples use 12 galaxies and one foreground star as a comparison. The dramatic increase in capacity SAMI offers has allowed Croom and colleagues to produce a database of 3068 galaxies, to be published in the Monthly Notices of the Royal of the Astronomical Society

A/Prof Julia Bryant from the University of Sydney inside the SAMI instrument attached to the Anglo Australian Telescope. Image Credit: Scott Croom/University of Sydney

Using these measurements, astronomers have discovered that very large galaxies usually spin quite slowly. Croom said this is because they are formed from the merger of many smaller objects, which were often turning in opposite directions to each other beforehand, causing their angular momenta to partially cancel out. On the other hand, for smaller galaxies, the direction of spin is often dictated by the galaxies around them.

SAMI's observations also expanded opportunities to explore the question of why and how galaxies die. To astronomers, a galaxy is dead not when its stars stop shining as others might assume, but when new ones stop being formed.

Using SAMI, astronomers learned that when galaxies fall into the crowded heart of large clusters, they die within a billion years – short by cosmological standards – as their gas gets sucked out into the intracluster medium by neighbors' gravity.

Although most galaxies die from the inside out – a process our own will undergo – Croom told IFLScience that SAMI has helped astronomers learn the reverse is true within large clusters, with star-forming gas drawn away from the outside in.

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