Sownak Bose

Accurate modelling of redshift-space distortions (RSD) in galaxy clustering is essential for extracting cosmological information from current and forthcoming large-scale structure surveys. While perturbation theory is reliable on large scales, much of the constraining power lies at intermediate and small separations, where non-linear dynamics within and between dark matter haloes dominate. We present a halo streaming model for non-linear galaxy clustering in redshift space that is accurate and physically interpretable. Our framework combines the streaming model for RSD with a halo-model decomposition of the galaxy clustering into central/satellite and one-/two-halo contributions. We build dedicated emulators for the key physical ingredients, trained on a suite of N-body simulations: halo mass functions, real-space halo two-point correlation functions, and pairwise velocity moments. By emulating these modular building blocks rather than the final redshift-space observable, this approach preserves physical transparency, enables targeted optimization for each ingredient, and remains flexible to changes in tracer populations and galaxy–halo connection models. The resulting halo streaming model reproduces the simulated non-linear anisotropic clustering signal down to highly non-linear scales, while achieving the computational efficiency required for cosmological parameter inference. This framework is designed to support full-shape RSD analyses for surveys such as DESI and Euclid, facilitating precision measurements of structure growth and tests of gravity. All codes and trained emulators are publicly available in the freyja repository (https://github.com/chzruan/freyja).

We present a combined strong and weak gravitational lensing mass model of the massive merging galaxy cluster Abell 2744 ($z = 0.308$). The mass reconstruction combines strong-lensing constraints in the cluster core with weak-lensing measurements derived from deep imaging with the Hubble Space Telescope and the James Webb Space Telescope, enabling a reconstruction of the projected mass distribution from the core to the cluster outskirts. Using the resulting mass map, we identify cluster substructures and construct the subhalo mass function for Abell 2744. To interpret the observed substructure population, we apply identical detection methods to projected mass maps of massive galaxy clusters from the BAryons and HAloes of MAssive Systems (BAHAMAS) simulations. We observe a systematic excess of detected substructures in Abell 2744 compared to BAHAMAS. We interpret this excess as likely reflecting Abell 2744’s exceptionally disturbed dynamical state, characterized by complex ongoing mergers, rather than tension with Lambda cold dark matter or self-interacting dark matter predictions. However, confirming this interpretation will require extending the analysis to a larger sample of clusters spanning a range of dynamical states.

Baryonic physics is anticipated to be a major source of systematic uncertainty in current and future large-scale cosmological surveys. We investigate how baryonic effects on halo density profiles vary with secondary halo properties at fixed halo mass, using the large-volume MillenniumTNG hydrodynamical simulation and its dark matter-only counterpart. We focus on the impact of halo concentration and large-scale environment on the ratio of density profiles of matched halos in the hydrodynamical and dark matter-only simulations. At redshift z = 0.0, we find a strong dependence on halo concentration, especially at lower halo mass (12.5 h / h ^-1 M _⊙ ) < 13.0), where more concentrated halos exhibit weaker inner enhancement and stronger intermediate-radius suppression at fixed halo mass, with variations reaching ∼ 15% at small scales and decreasing toward larger scales. This trend weakens and reverses at higher halo mass. In contrast, the secondary dependence on large-scale environment is weaker (∼ 2%) and largely scale-independent, with halos in denser regions exhibiting slightly weaker intermediate suppression. By separating internal profile redistribution from total mass suppression, we show that concentration impacts both components, whereas the environmental dependence is primarily associated with an overall mass shift. These secondary dependencies persist at z = 0.5 and correlate with variations in internal baryonic properties. We examine additional halo properties, including halo spin and velocity dispersion, and find significant secondary dependence. Overall, our results highlight the important role of secondary halo properties in modulating baryonic effects on halo density profiles, with potential implications for future modeling efforts.