Galaxies are vast, sprawling cities of stars, gas, and dust, and like any thriving metropolis, they need a constant supply of raw materials to grow and flourish. For galaxies, this vital ingredient is neutral atomic hydrogen, or HI, the fundamental building block from which stars are born. For decades, astronomers have been studying this cosmic recipe, trying to understand how galaxies turn their gas into brilliant new stars. The prevailing wisdom often assumed that more gas simply meant more stars. But what if that wasn’t entirely true? What if the secret lay not just in how much gas a galaxy has, but where that gas is located?
A groundbreaking new study, published in the prestigious Publications of the Astronomical Society of Australia, has thrown a fascinating curveball into our understanding of galactic star formation. Led by PhD student Seona Lee from the International Centre for Radio Astronomy Research (ICRAR) at The University of Western Australia, an international team of scientists peered into the hearts of nearly a thousand nearby galaxies using the revolutionary Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope. Their findings offer a provocative new perspective: simply having a lot of gas doesn’t guarantee a burst of new stars. Instead, it’s the concentration of this star-forming fuel within the galaxy’s stellar disc – the bustling, star-filled central regions – that truly matters.
Unveiling Galactic Secrets with ASKAP
For years, the sheer size of galaxies and the vast distances between them made it difficult to pinpoint exactly where atomic hydrogen gas was distributed within them. Older methods, often relying on single-dish radio telescopes, could provide a general idea of how much gas a galaxy held, but they lacked the sharp focus to map its distribution with enough detail. The WALLABY (Widefield ASKAP L-band Legacy All-sky Blind Survey) project changes that. WALLABY, conducted with the powerful ASKAP telescope in Australia, is designed to map atomic hydrogen gas in hundreds of thousands of galaxies across a vast portion of the southern sky. This pilot study leveraged ASKAP’s high-resolution capabilities to zoom in on 995 nearby, gas-rich galaxies.
The research team began with an initial pool of 1,976 galaxy candidates, meticulously filtering them to focus on 995 galaxies where the stellar disc was larger than the telescope’s “beam” – essentially, the area of the sky that the telescope can see in detail. To understand the galaxies’ stellar properties, such as their size, mass, and color, the astronomers utilized additional data from the Dark Energy Spectroscopic Instrument (DESI) Legacy Survey. By combining the precise HI gas maps from ASKAP with optical images from DESI, the team conducted a sophisticated analysis.
A key part of their methodology involved defining the “stellar disc” of a galaxy using specific measurements called “isophotal radii.” These are measurements of a galaxy’s size based on how bright its stellar light appears. The study primarily used two such radii: R25 and R24. By measuring the amount of HI gas enclosed within these specific boundaries, the researchers could distinguish between the gas actively participating in star formation in the inner regions and the more diffuse gas in the galaxy’s outer halo. This allowed them to move beyond traditional “global” measurements, which considered the entire galaxy, and instead focus on the crucial interplay between gas and stars in the most active regions.
New Insights on Star Formation
The detailed analysis of these 995 galaxies yielded several compelling insights:
First, and perhaps most strikingly, the study confirmed that a galaxy’s total gas content doesn’t directly dictate its star-forming activity. While gas is absolutely necessary, simply having more of it scattered throughout the galaxy doesn’t automatically mean more stars are being born. This crucial distinction underscores the importance of where the gas is concentrated. The survey revealed that having more gas in a galaxy does not automatically mean it will create more stars. Instead, galaxies that are forming stars usually have a higher concentration of gas in the areas where the stars reside.
The research team found that, on average, a significant portion of a galaxy’s total atomic hydrogen gas resides within its stellar disc – approximately 68% within the R25 boundary and 54% within the slightly smaller R24 boundary. The amount of gas within these star-forming zones can vary widely from galaxy to galaxy, ranging from as little as 20% to as much as 100% of its total HI reservoir.
The most potent finding revolved around the “surface density” of atomic hydrogen – essentially, how much gas is packed into a given area. While previous studies looking at the entire galaxy often found little to no correlation between the overall gas density and stellar mass or stellar density, a clear trend emerged when the researchers focused on the inner regions of galaxies. Bluer galaxies, which are typically younger and more actively forming stars, consistently showed higher average HI surface densities in their stellar discs. This trend became even more pronounced when focusing on the R24 boundary, strongly implying that dense pockets of atomic hydrogen within a galaxy’s main body are key to its star-forming prowess. Professor Barbara Catinella, a Senior Principal Research Fellow at ICRAR and co-leader of the WALLABY survey, noted, “Similarly, understanding how stars are formed requires us to measure the atomic gas where stars are actually forming, rather than considering the total gas content, which includes the unused gas in the outer regions.”
The Future of Galactic Astronomy
This pioneering study, made possible by the unprecedented resolution of the ASKAP telescope, marks a significant leap forward in our understanding of how galaxies grow and change over cosmic time. It reinforces the idea that the internal workings of a galaxy, particularly the distribution and concentration of its star-forming gas, are as important as its total gas budget. By resolving these crucial details, astronomers can now paint a much more precise picture of the “gas-star formation cycle” – the continuous process by which galaxies convert their gaseous fuel into the dazzling stars we observe. This research will undoubtedly pave the way for future studies to delve deeper into the complex dance between gas and stars, ultimately helping us unravel the grand narrative of how the universe’s magnificent galaxies came to be.
Paper Summary
Methodology
The study used pilot observations from the WALLABY survey via the ASKAP radio telescope. It analyzed 995 local gas-rich galaxies, selected from an initial 1,976 candidates, whose stellar discs were resolved by the telescope’s beam. Stellar properties were sourced from the DESI Legacy Survey. The research focused on measuring HI mass and surface density within the stellar discs (using R25 and R24 isophotal radii) to understand gas distribution and its correlation with stellar characteristics.
Results
The study found that star formation is more closely linked to the concentration of atomic hydrogen (HI) gas within a galaxy’s stellar disc, rather than its total gas content. On average, 68% of total HI mass was within R25 and 54% within R24, with individual galaxy fractions varying between 20% and 100%. While global HI surface densities showed little correlation with stellar properties, inner region measurements revealed that bluer, actively star-forming galaxies have higher average HI surface densities within their stellar discs, especially within the R24 boundary.
Limitations
Potential limitations include the presence of cluster galaxies in the sample (though key results are distance-independent) and the exclusion of galaxies with stellar discs smaller than one telescope beam, which could introduce resolution uncertainties. Previous studies were limited by lower spatial resolution or smaller sample sizes compared to this WALLABY pilot survey.
Funding and Disclosures
The article is Open Access under the Creative Commons Attribution license (https://creativecommons.org/licenses/by/4.0/), permitting unrestricted use with proper citation. Authors are affiliated with various international institutions, including ICRAR, The University of Western Australia, Peking University, Queen’s University, and CSIRO. No specific funding bodies were detailed in the provided abstract or initial pages of the paper.
Publication Information
- Title: WALLABY pilot survey: Spatially resolved gas scaling relations within the stellar discs of nearby galaxies
- Authors: Seona Lee et al.
- Journal: Publications of the Astronomical Society of Australia
- Volume, Issue, Pages: 42, e046, 1-12
- DOI: https://doi.org/10.1017/pasa.2025.30
- Received: 7 February 2025
- Revised: 21 March 2025
- Accepted: 25 March 2025
- Published: 2025
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