Congratulations to graduate students Sofia Chiarenza and Cameron Morgan, who were awarded the 2025 WCA Student Paper prize. They were presented with their certificates by Dr Sara Seager, who sits on the WCA Governing Board.
The WCA student paper prize is awarded to the papers judged to be the best graduate student-led papers submitted in the past year (July 1 2024 - June 20 2025). They are evaluated on their importance to their field, originality of conception, difficulty of execution, clarity of the manuscript, and reproducibility.
Here, Cameron and Sofia describe their prize winning work.
Cameron Morgan
"A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE) XVI. The ubiquity of truncated star-forming disks across the Virgo cluster environment".
VESTIGE: truncated disks in the Virgo cluster
Galaxies beyond our own Milky Way inhabit many different environments. Some live quite isolated, relatively unaffected by other galaxies. Many, however, live in groups or clusters, regions of space that are densely packed with galaxies, gas, and dark matter. Galaxies that live in these environments interact with other galaxies and the other material around them – this makes them evolve differently than the more isolated galaxies. Galaxies in clusters tend to be “red and dead” – that is to say, red in colour and lacking in ongoing star formation. This contrasts with isolated galaxies that tend to be blue in colour, have ongoing star formation and features such as disks and spiral arms where this star formation occurs.
A lot of the focus in understanding environmentally-driven evolution of galaxies comes down to what we can observe about these galaxies that gives us clues into the mechanisms that are affecting them. In this work, we chose to probe galaxies in the Virgo cluster – a well-studied and nearby galaxy cluster for which very high-resolution images can be taken. The Virgo Cluster was recently imaged by the Vera C. Rubin Observatory and is shown above (credit: NSF–DOE Vera C. Rubin Observatory/NOIRLab/SLAC/AURA). This paper is one in a series of papers from the Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE).
The most dramatic manifestation of environmental galaxy evolution is the shut down of star formation, known as quenching. It has often been observed that galaxies undergoing environmental evolution are quenched from the outside-in. This can be observed by comparing the size of the galaxy measured in images that show where new stars are formed, and the size measured in images showing where old stars are formed. In this work, we used imaging in the Hα filter – a narrow wavelength of light that traces the youngest, most recently formed stars, compared with broader visible light that traces older stars. We showed that if you measure the brightness of the galaxy from the centre outwards by looking at young stars, and compare it with the same measurement looking at old stars, galaxies in Virgo show a truncation where there are no longer any new stars in the galactic disk, and only old stars are left. This suggests that the galaxy has been quenched in the outer parts at some point in its history, leading to this truncation.
Cartoon showing the brightness profiles of young (blue) and old (red) stars in isolated (left) and cluster (right) galaxies
What was surprising is that galaxy disks are truncated all over the Virgo cluster! This is somewhat unexpected if gas stripping is expected (as it often is) to be the source of this truncation. We showed that gas stripping is not effective in the outer parts of the cluster, so the truncations in that region must be caused by less dramatic mechanisms. The modelling that we did showed that a slow-then-rapid quenching process is likely, where quenching begins beyond the edge of the cluster. This result had been suggested in other studies, but our work used novel techniques to provide a new understanding into the quenching sequence that occurs as galaxies become part of clusters.
The footprint of the Virgo cluster, showing the location of galaxies with truncated disks of varying degrees.
Read the paper as published in Astronomy & Astrophysics.
Sofia Chiarenza
"BLAST: Beyond Limber Angular power Spectra Toolkit. A fast and efficient algorithm for 3x2 pt analysis"
Blast: Making next-generation cosmology analyses fast and accurate
Modern cosmology is entering a new era. Upcoming surveys such as Euclid, LSST, Roman, and DESI will map the positions and measure the shapes of hundreds of millions of galaxies. From these novel datasets, scientists can learn about dark matter, dark energy, and the growth of cosmic structure. One of the most powerful ways to do this is through so-called 3x2pt analyses, which jointly study galaxy clustering, weak gravitational lensing, and their cross-correlations. This approach is known as multi-probe cosmology, where multiple complementary observations are combined to extract more information and reduce systematic uncertainties.
Cartoon illustrating how to combine galaxy observables to probe cosmological parameters. Credit: Dr. Jessie Muir (Perimeter Institute)
While extremely informative, these analyses are also computationally demanding as the theory model for 3x2pt analyses requires evaluating complicated integrals involving rapidly oscillating functions. Because this is so expensive, most current pipelines rely on an approximation (called the Limber approximation) that speeds things up but introduces errors, especially on the largest cosmic scales where sensitivity to fundamental physics is highest.
In this paper, we introduce Blast, an open-source Julia package that makes it practical to compute these theory models accurately without relying on the Limber approximation. The key idea behind Blast is to rewrite the problem in a clever mathematical way: instead of repeatedly evaluating the hardest part of the calculation, we decompose it so that the numerically most challenging component can be pre-computed once and reused, dramatically reducing the cost of each subsequent model evaluation.
We benchmarked Blast against state-of-the-art methods developed by the cosmology community by taking part in a non-Limber algorithm challenge launched by the LSST collaboration. While most current analyses rely on the Limber approximation, Blast computes the full theory model without this simplifying assumption, achieving higher accuracy on large cosmic scales. At the accuracy required for real survey analyses, Blast is approximately 10–15 times faster than the previous best-performing non-Limber method. Importantly, its runtime is largely independent of survey complexity, making it especially well suited for the sophisticated, multi-probe analyses planned for next-generation surveys.
Performance comparison of Blast with other state-of-the-art algorithms. Blast achieves order-of-magnitude speedups while maintaining the accuracy required for next-generation cosmological surveys.
By removing a major computational bottleneck, Blast makes it feasible to analyze upcoming datasets faster and more accurately than ever before. As a result, Blast is now being integrated into the analysis pipelines of major collaborations, including Euclid, as a default approach for next-generation multi-probe cosmology.
Banner image credit: Sofia Chiarenza
Read the paper, as published in the Open Journal of Astrophysics.