Title: A Free-Form Analysis of the Subhalo in the 'Jackpot' Strong Lensing System

Speaker: Dr. Xiaoyue Cao（University of Chinese Academy of Sciences）

Time: 10:00am, December 26, 2024

Location: 5-516, PMO Xianlin Campus

Abstract: Characterizing the population and internal structure of sub-galactic haloes is essential for constraining the nature of dark matter. While galaxy-galaxy strong lensing with extended arcs offers a promising avenue for detecting these haloes within lens galaxies, it is susceptible to systematic uncertainties, particularly the degeneracy between subhalo signals and other mass complexities, often leading to spurious detections. We present a novel free-form lens modeling code, developed using the open-source software \texttt{PyAutoLens}, designed to mitigate these limitations. Our approach models mass perturbations within the lens galaxy as pixelized potential corrections, employing a novel regularization scheme inspired by Gaussian process regression. This methodology enables the recovery of diverse mass perturbations, including subhaloes, line-of-sight haloes, external shear, and multipole components representing the complex mass distribution of the lens galaxy. Furthermore, our fully Bayesian framework allows for objective hyperparameter determination, eliminating manual fine-tuning. By applying our code to the "Jackpot" lens, SLACS0946+1006, we robustly detect a highly concentrated subhalo with properties that challenge standard cosmological models. This represents the first robust subhalo measurement using a fully free-form approach, providing compelling evidence that the subhalo in SLACS0946+1006 poses significant tension with the standard cosmological paradigm.