KIPAC Tea Talk: Statistical Predictions of the Accreted Stellar Halos around Milky Way-Like Galaxies / Lifting astrophysical barriers to small scale cosmology

Monzon Abstract: In the ΛCDM paradigm, the abundance of satellite galaxies and the amount of stellar material in the outskirts of a galaxy both arise from the hierarchical assembly of dark matter subhalos. Using a sophisticated semi-analytic framework, we develop new machinery for forward-modeling satellite galaxy populations and building up accreted stellar halos while fully accounting for halo-to-halo (cosmic) variance. Our approach links observable galaxy properties to their underlying dark matter substructure at a fraction of the computational cost of hydrodynamic simulations. At the Milky Way mass scale we show that intrinsic host-to-host variability remains significant even under idealized assumptions, implying that larger observational samples are required to robustly constrain galaxy–halo connections. Our model provides statistically rigorous tests of stellar halo formation scenarios and can be used to deliver predictions for future low-surface-brightness extragalactic surveys.

Siegel Abstract: Weak lensing cosmology is now limited by our understanding of astrophysical systematics: intrinsic alignments and baryon feedback. To lift these barriers to small scale cosmology and resolve key questions in galaxy evolution, we made direct measurements of intrinsic alignments and baryon feedback. With over 2 million spectroscopic galaxies from DESI Data Release 1 and imaging from four lensing surveys, we developed a tailored IA modeling approach: building a library of IA measurements across color, luminosity, stellar mass, and redshift. To constrain how baryon feedback reshapes the matter distribution, we jointly analyzed the SDSS/DESI+ACT Sunyaev-Zel’dovich effect profiles and eROSITA X-ray gas mass fractions, each characterized with galaxy-galaxy lensing. Across group and cluster masses and at 0<z<1, we find consistent evidence of more efficient gas expulsion beyond the virial radius than predicted by most state-of-the-art simulations, implying significant suppression of the matter power spectrum on small scales. By incorporating our intrinsic alignment and baryon feedback constraints into data-driven priors for cosmic shear, we significantly improve cosmological constraining power on small scales.