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Age is the Key Factor Controlling How Stars Move in Galaxies, New Study Finds

Galaxies are enormous collections of stars, gas, and dust. Scientists have long studied how stars move within galaxies, but the cause of these movements has been a mystery. A new study, published in Monthly Notices of the Royal Astronomical Society (MNRAS), has shed light on this question.


A comparison of a young (top) and old (bottom) galaxy observed as part of the SAMI Galaxy Survey. Panels on the left are regular optical images from the Subaru Telescope. In the middle are rotational velocity maps (blue coming towards us, red going away from us) from SAMI. On the right are maps measuring random velocities (redder colors for greater random velocity). Both galaxies have the same total mass. The top galaxy has an average age of 2 billion years, high rotation and low random motion. The bottom galaxy has an average age of 12.5 billion years, slower rotation and much larger random motion. Credit: Subaru credit: Image from the Hyper Suprime-Cam Subaru Strategic Program
A comparison of a young (top) and old (bottom) galaxy observed as part of the SAMI Galaxy Survey. Panels on the left are regular optical images from the Subaru Telescope. In the middle are rotational velocity maps (blue coming towards us, red going away from us) from SAMI. On the right are maps measuring random velocities (redder colors for greater random velocity). Both galaxies have the same total mass. The top galaxy has an average age of 2 billion years, high rotation and low random motion. The bottom galaxy has an average age of 12.5 billion years, slower rotation and much larger random motion. Credit: Subaru credit: Image from the Hyper Suprime-Cam Subaru Strategic Program

The research team, led by Professor Scott Croom of the University of Sydney, discovered that the primary factor influencing stellar motion is the age of the galaxy itself. Younger galaxies exhibit stars with a more orderly, rotating pattern. In older galaxies, however, stellar motion becomes increasingly random.


This finding surprised scientists, who previously believed that the surrounding environment or the mass of the galaxy played a more significant role. While these factors can influence a galaxy's age, the study suggests that age is the dominant driver of stellar motion.


"Regardless of the environment, younger galaxies will rotate in a more organized way," explained Professor Croom. "Conversely, older galaxies exhibit more random motion in their stars, irrespective of whether they reside in a dense or sparse environment."


The study utilized data from the SAMI Galaxy Survey, a project that has examined 3,000 galaxies across diverse environments. This vast dataset allowed the researchers to rule out many previously hypothesized explanations for stellar motion.


These findings have significant implications for our understanding of galaxy formation and evolution. Astronomers can now refine models of the universe's development by focusing on the role of age.


The next steps for this research involve creating more detailed simulations of galaxy evolution. Current simulations lack the necessary resolution to capture the intricate dynamics within galaxies.


Professor Croom and his team plan to leverage the Hector Galaxy Survey, which will observe 15,000 galaxies with even greater detail. This expanded dataset will provide a clearer picture of how galaxies change over time.

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