Age is the driving force that changes the way stars move through galaxiesaccording to a publication by an international research team, led by the Australian research center ASTRO 3D, in the journal “Monthly Notices of the Royal Astronomical Society” (MNRAS).

Galaxies begin life with their stars spinning in an organized pattern, however in some the movement of the stars is more random. Until now scientists weren’t sure what causes this, possibly the environment or the mass of the galaxy itself.

The new study finds that the most important factor is neither of these two. It shows that the tendency of stars to have random motion is mainly determined by the age of the galaxy.

“If you find a young galaxy, it will be spinning in whatever environment it’s inwhereas if you find an old galaxy, it will have more random orbits, whether it’s in a dense medium or a vacuum.”notes first author University of Sydney professor Scott Krum.

Second author Jesse van de Sant explains that previous work that suggested environment or mass as more important factors is not necessarily wrong. Young galaxies are super-producers of star production, while in older galaxies star formation ceases. “We know that age is influenced by the environment. If a galaxy falls into a dense medium it will tend to stop forming stars. So, galaxies in denser environments are, on average, older”he says, adding: “Our analysis highlights that it is not living in dense environments that reduces their rotation, but the fact that they are older.”

The image compares a young (top) and an older (bottom) galaxy observed in the survey. On the left are optical images from the Subaru telescope, in the middle rotational velocity maps (blue when coming towards us and red when moving away from us), while on the right are maps measuring random velocities (redder colors for higher random velocity). Both galaxies have the same total mass. The upper galaxy has an average age of 2 billion years, high rotation and low random motion. The lower galaxy has an average age of 12.5 billion years, slower rotation, and much more random motion. Credit: Image from the Hyper Suprime-Cam Subaru Strategic Program