Speaker
Description
Weak gravitational lensing has emerged as one of the most powerful probes of cosmology, uniquely tracing the total matter distribution and directly probing the growth of large-scale structure. As a late-time observable, it provides key sensitivity to dark energy that complements early-Universe measurements of the Cosmic Microwave Background. Forthcoming surveys such as $\textit{Euclid}$ and the Legacy Survey of Space and Time will deliver unprecedented weak lensing measurements, but fully exploiting their statistical power requires sub-percent control of astrophysical systematics.
A dominant contaminant is the intrinsic alignment (IA) of galaxies, which correlates galaxy shapes independent of lensing and can bias cosmological constraints if not accurately modelled. In this talk, I present new constraints on IA from the FLAMINGO simulations, analysing millions of Luminous Red Galaxy analogues. By jointly modelling galaxy clustering and alignments, we obtain some of the most precise constraints from hydrodynamic simulations to date. While commonly used IA models like NLA and TATT describe the data well, we introduce a new mass-dependent extension, TATT-M, that provides a significantly improved and more efficient description. This model has been adopted as the fiducial IA model for the \textit{Euclid} DR1 weak lensing analysis.
We then investigate the physical drivers of IA. We find that baryonic feedback does not change the alignment signal beyond its effect on stellar mass, whereas halo assembly history plays a key role: galaxies in earlier-forming haloes exhibit systematically stronger alignments, providing the first clear evidence of assembly bias in IA. We also explore the redshift evolution of the alignment signal, and show that it is more complex than assumed in current models.
Finally, I introduce a method to robustly infer halo shapes from sparse satellite populations by correcting for sampling noise, enabling less biased measurements and extending such studies to lower-mass systems.
Together, these results establish a simulation-driven framework for intrinsic alignment modelling, directly informing precision weak lensing analyses in the era of next generation weak lensing surveys.