From Einstein Radii to Halo Masses: Dark Matter, Stellar Matter and the Galaxy–Halo Connection at Cosmic Noon
| Program ID | 19036 |
|---|---|
| Science Category | Galaxies |
| Program Type | Analysis |
| Category | Medium |
| Principal Investigator | Zhiyuan Ji |
| PI Institution | University of Arizona |
| Co-Investigators |
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| Abstract | We propose an archival program to exploit the unprecedented number of galaxy-galaxy strong lenses that Roman's HLWAS Medium and Deep tiers are predicted to deliver at z > 1, increasing existing samples by nearly two orders of magnitude. With this sample, we will (1) reveal how dark matter is distributed within and around massive (logM_stellar>10.7) galaxies at cosmic noon, and (2) obtain, for the first time, a statistically significant direct constraint on the stellar-to-halo mass relation. The central dark matter fraction of massive galaxies at these redshifts remains unsettled: kinematic studies have reached conflicting conclusions, and no independent mass probe has yet to be applied to a sufficiently large sample. At the same time, constraints on the galaxy–halo connection at z > 1 rest largely on indirect, model-dependent halo mass estimates. Roman's wide-field near-infrared imaging, ~0.11'' resolution, and grism spectroscopy will enable two linked investigations with this lens sample. First, we will combine lensing-derived total masses within the Einstein radius with stellar masses from joint Roman+Rubin SED fitting to measure the central dark matter fraction as a function of stellar mass, galaxy size, and redshift, providing the first lensing-based test of the conflicting kinematic results at these redshifts. Second, we will compare halo masses inferred statistically from forward modeling of the Einstein radius distribution with independent estimates from large-scale clustering, testing galaxy–halo connection models near the epoch and mass scale where star formation efficiency peaks. Our analysis combines deep-learning lens identification, automated lens modeling in a hierarchical Bayesian framework, and clustering measurements on scales of 1–100 Mpc. This program will deliver the first empirical, multi-scale view of how massive galaxies and their dark matter halos are connected during the peak era of galaxy assembly. |