April 13, 2024

Study reports that age is the driving force in changing the way stars move within galaxies

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A comparison between a young galaxy (top) and an old galaxy (bottom) observed as part of the SAMI Galaxy Survey. The panels on the left are regular optical images from the Subaru Telescope. In the middle are the rotational velocity maps (blue coming toward us, red moving away from us) from SAMI. On the right are maps measuring random speeds (redder colors for higher random speed). 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 greater random motion. Credit: Subaru Credit: Hyper Suprime-Cam Subaru Strategic Program Image

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A comparison between a young galaxy (top) and an old galaxy (bottom) observed as part of the SAMI Galaxy Survey. The panels on the left are regular optical images from the Subaru Telescope. In the middle are the rotational velocity maps (blue coming toward us, red moving away from us) from SAMI. On the right are maps measuring random speeds (redder colors for higher random speed). 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 greater random motion. Credit: Subaru Credit: Hyper Suprime-Cam Subaru Strategic Program Image

Galaxies begin life with their stars rotating in an ordered pattern, but in some the stars’ movement is more random. Until now, scientists haven’t been sure what causes this – possibly the surrounding environment or the mass of the galaxy itself.

A new study, published in MNRAS (Monthly Notices of the Royal Astronomical Society), found that the most important factor is none of those things. This shows that the tendency of stars to have random movements is mainly driven by the age of the galaxy – things get complicated with time.

“When we did the analysis, we found that age, consistently, no matter how we slice or dice it, is always the most important parameter,” says first author Prof Scott Croom, ASTRO 3D researcher at the University of Sydney.

“After taking age into account, there is essentially no environmental trend, and it is similar for mass.

“If we find a young galaxy, it will be rotating no matter what environment it is in, and if we find an old galaxy, it will have more random orbits, whether it is in a dense environment or a void.”

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

The study updates our understanding of previous studies that suggested environment or mass as more important factors. But previous work is not necessarily incorrect, says second author Dr. Jesse van de Sande.

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

“We know that age is affected by the environment. If a galaxy falls into a dense environment, it will tend to stop star formation. Therefore, galaxies in denser environments are, on average, older,” says Dr. van de Sande.

“The point of our analysis is that it is not living in dense environments that reduces their spin, it is the fact that they are older.”

Our own galaxy, the Milky Way, still has a thin star-forming disk, so it is still considered a fast-spinning rotational galaxy.

“But when we look at the Milky Way in detail, we see something called the thick disk of the Milky Way. It’s not dominant, in terms of light, but it’s there and they appear to be older stars, which may well have been heated from the thin disk in previous epochs, or was born with more turbulent movements at the beginning of the universe”, says Professor Croom.

The research used data from observations made within the scope of 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 the Siding Spring Observatory near Coonabarabran, New South Wales. It has surveyed 3,000 galaxies in a wide variety of environments.

The study allows astronomers to rule out many processes when trying to understand the formation of galaxies and thus adjust models of how the Universe developed.

The next steps will be to develop simulations of the evolution of galaxies with more granular details.

“One of the challenges of getting the simulations right is the high resolution needed to predict what is happening. Typical current simulations are based on particles that have the mass of perhaps 100,000 stars and it is not possible to resolve small-scale structures in galaxy disks.” , says Prof. Croom.

The Hector Galaxy Survey will help Prof Croom and his team expand this work using a new instrument on the Anglo-Australian Telescope.

“Hector is observing 15,000 galaxies, but with higher spectral resolution, allowing the age and rotation of galaxies to be measured even in galaxies of much lower mass and with more detailed environmental information,” says Professor Julia Bryant, Hector Galaxy leader Survey, University of Sydney.

Professor Emma Ryan-Weber, director of ASTRO 3D, says: “These findings answer one of the key questions posed by ASTRO 3D: how do mass and angular momentum evolve in the universe? This careful work by the SAMI team reveals that the age of a galaxy determines how stars orbit. This critical information contributes to a clearer view of the universe.”

More information:
Scott Croom et al, The SAMI Galaxy Survey: Galaxy spin is more strongly correlated with the age of the stellar population than with mass or environment, Monthly Notices of the Royal Astronomical Society (2024). DOI: 10.1093/mnras/stae458. academic.oup.com/mnras/article… 0.1093/mnras/stae458

Diary information:
Monthly Notices of the Royal Astronomical Society

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