Monday 31 March 2025
A new study has shed light on a long-standing puzzle in astrophysics: how to define the size of a galaxy. Researchers have been struggling to pinpoint a clear and consistent way to measure galaxy sizes, which is crucial for understanding their formation and evolution.
The problem lies in the fact that galaxies come in all shapes and sizes, making it difficult to determine what constitutes their boundaries. Astronomers have traditionally used methods such as light concentration or isophotal densities to define galaxy sizes, but these approaches often rely on observational limitations and may not provide a clear physical boundary for galaxy outskirts.
Enter a new definition proposed by researchers that takes into account the current or past radial position of the star formation threshold as the size of the galaxy. In practice, this means using the radial position of the stellar mass density contour at 1 M⊙pc-2, defined as R1, to measure galaxy sizes.
To test the validity of this new definition, researchers analyzed three state-of-the-art hydrodynamical simulation suites: AURIGA, HESTIA, and NIHAO. These simulations model the formation and evolution of galaxies in a cosmological context, allowing researchers to study the properties of simulated galaxies at different redshifts.
The results show that the new definition is consistent across different redshifts and galaxy formation models. The authors found that the R1 size metric correlates well with traditional size metrics, such as the stellar half-mass radius, but provides a more physically motivated boundary for galaxy outskirts.
One of the key benefits of this new definition is its ability to distinguish between different types of galaxies based on their sizes. By using R1, researchers can identify galaxies that are more or less massive than expected at a given redshift, which can help us better understand how they formed and evolved over time.
The study also highlights the importance of simulating galaxy formation in a cosmological context. The authors found that the scatter in the stellar-to-halo mass relation (SHMR) and the total mass within R1 (STMR) are closely linked, suggesting that the SHMR scatter may be used as an alternative metric to understand galaxy properties.
The findings of this study have significant implications for our understanding of galaxy formation and evolution. By developing a more accurate and consistent way to measure galaxy sizes, researchers can gain a better understanding of how galaxies formed and evolved over billions of years.
The next step is to apply this new definition to real-world observations of galaxies.
Cite this article: “Defining Galaxy Size: A New Approach to Understanding Cosmic Evolution”, The Science Archive, 2025.
Galaxies, Size, Definition, Astrophysics, Galaxy Formation, Evolution, Hydrodynamical Simulations, Redshifts, Cosmological Context, Galaxy Properties
