Accelerated calibration of semi-analytic galaxy formation models

Thu, 01 Jan 2026 00:00:00 +0000

Understanding how galaxies form is one of the major challenges in astrophysics. Researchers often use “semi-analytic models” — simplified but physically motivated simulations — to explore how processes such as gas cooling, star formation, and feedback from stars and black holes shape galaxy populations.

These models contain many uncertain parameters that must be calibrated so that the simulated galaxies resemble real ones observed in the Universe. Unfortunately, traditional calibration methods require running very large simulations, which can be computationally expensive and slow.

In this work we introduce a faster way to calibrate these models. Instead of matching the full distribution of galaxy stellar masses directly, we use the relationship between the stellar mass of a galaxy and the mass of the dark matter halo that hosts it. This allows us to evaluate each model using only a small number of simulated galaxies rather than thousands.

Using this approach with the galaxy formation model, we show that it can reproduce key observational measurements while allowing rapid exploration of model assumptions. This makes it much easier to test different physical prescriptions and identify promising models for more detailed simulations.

Not So Dark, Not So Dense: An Alternative Explanation for the Lensing Subhalo in SDSS J0946+1006

Wed, 01 Oct 2025 00:00:00 +0000

New development: This system will be observed with JWST in Cycle 5 ( ). The multi-wavelength imaging will help disentangle emission from the lensed source and any putative satellite galaxy, providing a decisive test of the interpretation presented here.

Strong gravitational lensing provides one of the most powerful ways to detect small clumps of dark matter in distant galaxies. By studying how these substructures perturb the shapes of lensed images, astronomers can measure the mass and density of otherwise invisible objects.

Previous analyses of the lens system SDSS J0946+1006 suggested the presence of a dark matter subhalo with an unusually high central density. Such an object would be difficult to reconcile with predictions from the standard cold dark matter model and attracted significant attention.

In this work we revisit the system and explore a different possibility: that the perturber may host a faint satellite galaxy. Once this possibility is included in the lens modelling, the data can be explained without requiring an extreme dark matter density.

While this provides a compelling alternative interpretation, it is currently difficult to determine conclusively whether the perturber contains a faint galaxy or is truly dark. Future observations at multiple wavelengths could help resolve this question: colour differences between the lensed background source and any satellite galaxy would allow the two components to be more cleanly separated, providing a stronger test of the model.

Publications | Andrew Robertson

Accelerated calibration of semi-analytic galaxy formation models

Not So Dark, Not So Dense: An Alternative Explanation for the Lensing Subhalo in SDSS J0946+1006