The study suggests that age is a driving force in changing how stars move within galaxies

Galaxies begin life with their stars rotating in an orderly fashion, but in some the stars' motion is random. Until now, scientists were unsure what caused this—perhaps the surrounding environment or the galaxy's mass.

A new study, published MNRAS (Monthly Notices of the Royal Astronomical Society), found that the most important factor was neither of these. This shows that the random motion of stars is largely driven by the age of the galaxy – things get messy over time.

„When we did the analysis, we found that age was consistently the most important parameter no matter which way you slice it or dice it,” says first author Professor Scott Groom, an Astro3D researcher at the University of Sydney.

„If you factor in age, there's basically no environmental trend, and it's the same for mass.

„If you see a young galaxy, it's going to be spinning no matter what environment it's in, and if you see an old galaxy, it's going to have highly random orbits, whether it's in a dense environment or a vacuum.”

The research team also includes scientists from Macquarie University, Swinburne University of Technology, University of Western Australia, Australian National University, University of New South Wales, University of Cambridge, University of Queensland and Yonsei University of the Republic of Korea. Korea.

This study advances our understanding from previous studies that suggested environment or mass as the most important factors. But that doesn't necessarily mean the earlier work is wrong, says second author Dr. Jesse Van de Sande.

Young galaxies are star-forming superfactories, while in older ones, star formation ceases.

„We know that age is influenced by environment. If a galaxy falls into a dense environment, it shuts down star formation. So galaxies in dense environments are older on average,” says Dr van de Sande.

„The point of our analysis is that it's not living in a dense environment that slows down their circulation, it's the fact that they're old.”

Our own galaxy, the Milky Way, has an even thinner star-forming disk, so it is still considered a high-spin galaxy.

„But when we look at the Milky Way in detail, we see something called the Milky Way thick disk. It's not dominant in terms of light, but it's there and they're old stars, they're probably well-heated. Born from a thin disk at earlier times, or with more turbulent motion in the early universe.” Professor Groome says.

The research used data from observations made under the SAMI Galaxy Survey. The SAMI instrument was built in 2012 by the University of Sydney and the Anglo-Australian Observatory (now Astralis). SAMI uses the Anglo-Australian Telescope at Siding Spring Observatory near Coonabarabran, New South Wales. It has surveyed over 3,000 galaxies in a wide range of environments.

The study allows astronomers to rule out many processes as they try to understand better models of how galaxies formed and how the universe evolved.

Next steps are to build simulations of galaxy evolution with more granular detail.

„One of the challenges in getting the simulations right is the high resolution you need to predict what's going on. Typical current simulations are based on particles with the mass of 100,000 stars, and you can't resolve small-scale structures in galactic disks,” says Professor Groome.

The Hector Galaxy Survey will enable Professor Groome and his team to extend this work using a new instrument at the Anglo-Australian Telescope.

„HECTOR observes over 15,000 galaxies, but with high spectral resolution, allowing us to measure the ages and cycles of galaxies in much lower mass galaxies and with more detailed environmental information,” says Professor Julia Bryant, head of the Hector Galaxy Survey. Sydney.

Professor Emma Ryan-Weber, Director of ASTRO 3D, says, „These findings answer one of the key questions raised by ASTRO 3D: How do mass and angular momentum develop in the Universe? This careful work by the SAMI team determines how old stars orbit a galaxy. This important information provides a clear, larger picture of the Universe.” Contributes to the image.”