Galaxies grow more chaotic over time

Galaxies grow more chaotic over time
by Simon Mansfield
Sydney, Australia (SPX) Apr 08, 2024

A new study led by Professor Scott Croom of the ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) and the School of Physics has demonstrated that the age of a galaxy is the principal factor influencing its increasing disorganization over time, challenging previous beliefs that mass and environmental conditions were predominant influences.

This research, which meticulously analyzed data from over 3000 galaxies, discovered that older galaxies tend to exhibit more random star motions compared to their younger counterparts, which display more structured and rotational movement. The findings of this study were published in the prestigious Monthly Notices of the Royal Astronomical Society.

Professor Croom explained the significance of these findings: "Our data consistently shows that regardless of how we analyze it, age is the most significant factor in the dynamics of galaxies. Environmental factors and mass, which were once thought to be influential, actually have minimal impact once the age of the galaxy is accounted for."

This revelation has profound implications for our understanding of galactic evolution. In younger galaxies, stars generally rotate in a coherent, flat disk-like pattern, contributing to what can be considered a 'high-spin' galaxy, such as our own Milky Way. As galaxies age, this rotational movement gives way to more erratic and random star orbits, contributing to what is termed a 'low-spin' state.

"The Milky Way still retains a high-spin structure thanks to its ongoing star formation in the thin disk," Professor Croom added. "However, we also observe the Milky Way's thick disk, which is less luminescent and contains older stars that likely moved from the thin disk or were born in more turbulent conditions early in the universe's history."

The research utilized data from the SAMI Galaxy Survey, conducted using the SAMI instrument on the Anglo-Australian Telescope at Siding Spring Observatory. This instrument, developed by the University of Sydney and the former Anglo-Australian Observatory (now Astralis), has enabled astronomers to explore the properties of galaxies across varied environments.

Dr. Jesse van de Sande, the study's second author, noted, "The insights from the SAMI survey challenge previous studies that suggested environmental factors like galaxy density play a larger role in the aging process of galaxies. Our findings indicate that the intrinsic age of a galaxy is a more critical determinant of its structural evolution."

As part of ongoing efforts to refine our understanding of how galaxies evolve, the researchers are looking forward to advancements from the Hector Galaxy Survey. This project will employ a new instrument with higher spectral resolution to observe 15,000 galaxies. This upgrade will allow for more detailed analysis of galaxy age and spin, even in less massive galaxies.

Professor Julia Bryant, leader of the Hector project, is enthusiastic about the future insights this survey will provide: "With Hector, we aim to extend the foundational work of the SAMI survey by examining additional properties and a larger sample of galaxies, enhancing our ability to model galactic evolution with greater accuracy."

Emma Ryan-Weber, Director of ASTRO 3D, highlighted the broader implications of this research: "Understanding the relationship between age, mass, and angular momentum in galaxies is essential for painting a comprehensive picture of the universe's evolution. The careful work by our team is vital in answering some of the most perplexing questions in astrophysics today."

These findings not only refine our understanding of galactic behavior but also underscore the importance of considering a galaxy's age when studying its structure and dynamics.

Research Report:The SAMI Galaxy Survey: galaxy spin is more strongly correlated with stellar population age than mass or environment