An international team led by the Australian research center ASTRO 3D has discovered that age plays a key role in altering the movement of stars within galaxies.
A comparison of a young (top) and old (bottom) galaxy observed as part of the SAMI Galaxy Survey. Image Credit: Hyper Suprime-Cam Subaru Strategic Program
Galaxies initially have stars rotating in a structured manner, yet in certain cases, the motion of stars appears more sporadic. Until recently, scientists have been uncertain about the cause, speculating whether it could be influenced by the surrounding environment or the galaxy's mass itself.
In a recent study published in MNRAS (Monthly Notices of the Royal Astronomical Society), researchers discovered that neither of these factors is the most significant. Instead, the study reveals that the tendency of the stars to have random motion is driven mostly by the age of the galaxy – things tend to just get messy over time.
When we did the analysis, we found that age, consistently, whichever way we slice or dice it, is always the most important parameter. Once you account for age, there is essentially no environmental trend, and it’s similar for mass. If you find a young galaxy it will be rotating, whatever environment it is in, and if you find an old galaxy, it will have more random orbits, whether it’s in a dense environment or a void.
Scott Croom, Study First Author and Professor, University of Sydney Scott Croom is an ASTRO 3D researcher
Scientists from the Australian National University, Macquarie University, Swinburne University of Technology, University of Western Australia, University of New South Wales, University of Cambridge, University of Queensland, and Yonsei University in the Republic of Korea were also part of the research team.
This study provides a fresh perspective, updating our understanding from previous research that has alternately highlighted environment or mass as predominant factors. However, the earlier findings are not necessarily incorrect, notes second author Dr. Jesse van de Sande.
Young galaxies are star-forming super-factories, while in older ones, star formation ceases.
We do know that age is affected by environment. If a galaxy falls into a dense environment, it will tend to shut down the star formation. So galaxies in denser environments are, on average, older. The point of our analysis is that it’s not living in dense environments that reduces their spin, it’s the fact that they’re older.
Dr. Jesse Van de Sande, Study Second Author, Sydney Institute for Astronomy, School of Physics, University of Sydney
The Milky Way, our own galaxy, still has a thin star-forming disk, so it is still considered a high-spin rotational galaxy.
Prof Croom says, “But when we look at the Milky Way in detail, we do see something called the Milky Way thick disk. It’s not dominant, in terms of light, but it is there and those look to be older stars, which may well have been heated from the thin disk at earlier times, or born with more turbulent motion in the early Universe.”
The researchers utilized data from observations made under the SAMI Galaxy Survey. The SAMI instrument was built back 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 3000 galaxies across a large range of environments.
The study allows astronomers to rule out many processes when trying to understand galaxy formation and so fine-tune models of how the Universe has developed.
Creating more intricately detailed simulations of galaxy evolution will be the next step in the researchers' journey.
Prof. Croom says, “One of the challenges of getting simulations right is the high resolution you need in to predict what’s going on. Typical current simulations are based on particles which have the mass of maybe 100,000 stars and you can’t resolve small-scale structures in galaxy disks.”
The Hector Galaxy Survey, led by Prof Croom and his team, will further advance this research by leveraging a novel instrument on the Anglo-Australian Telescope.
Hector is observing 15,000 galaxies but with higher spectral resolution, allowing the age and spin of galaxies to be measured even in much lower mass galaxies and with more detailed environmental information.” Julia Bryant, Professor and Lead, Hector Galaxy Survey, University of Sydney
Emma Ryan-Weber, Professor and Director of ASTRO 3D, says, “These findings answer one of the key questions posed by ASTRO 3D: how does mass and angular momentum evolve in the Universe? This careful work by the SAMI team reveals that the age of a galaxy determines how the stars orbit. This critical piece of information contributes to a clearer big-picture view of the Universe.”
Journal Reference:
Croom, M., S., et al. (2024) The SAMI Galaxy Survey: galaxy spin is more strongly correlated with stellar population age than mass or environment. Monthly Notices of the Royal Astronomical Society. doi.org/10.1093/mnras/stae458