0 Assigned ID: 7524 ----- Attempting Submission 1 (Thu Apr 16 18:25:25 GMT 2026) ----- HST Phase I Proposal 7524 successfully submitted. Receipt: # 7524-1 ----- Attempting Submission 2 (Thu Apr 16 20:08:49 GMT 2026) ----- HST Phase I Proposal 7524 successfully submitted. Receipt: # 7524-2 ----- Attempting Submission 3 (Thu Apr 16 21:51:50 GMT 2026) ----- The circumgalactic medium (CGM) is the primary reservoir of baryons around galaxies and the canvas on which the competing processes of gas accretion, stellar feedback, and AGN activity leave their signatures. Yet the ionization modeling methods used to extract these signatures from HST/COS absorption spectra - the principal observational window onto this gas - have never been validated against cosmologically self-consistent CGM gas. Without this validation, we cannot know whether the physical picture emerging from CGM observations reflects reality or the imprint of our assumptions. We propose to close this gap by applying cloud-by-cloud multiphase Bayesian ionization modeling to synthetic spectra from two complementary simulation suites: CATS, which provides controlled MHD turbulence environments to calibrate thermal and non-thermal line broadening, and Project GIBLE, which for the first time achieves pc-scale resolution within a fully cosmological framework. In a modeler-blind experiment, we recover density, temperature, and metallicity for individual gas clouds and compare directly to known simulation truth - delivering the first bias-characterized uncertainties for CGM ionization modeling as a function of gas complexity. All products including the Bayesian framework will be publicly released as open-source, establishing a simulation-validated framework for the interpretation of HST/COS CGM spectroscopic data. PI Dr. Sameer is an expert in multiphase modeling of CGM absorption line profiles. He has developed a Bayesian cloud-by-cloud multiphase modeling code that takes advantage both of the observed optical depths and the velocity information from absorption profiles to derive the physical properties of individual absorbing components. He and a graduate student at OU will lead the investigations as well as the bulk of the analysis and interpretation of the results. Dr. Rahul Ramesh is an expert in galaxy formation simulations and one of the leaders of Project GIBLE. He will be instrumental in the generation of blinded synthetic spectra from the Project GIBLE and in interpretation of the results. Dr. Dylan Nelson is a leading expert in hydrodynamic simulations of galaxies with an emphasis on cosmological gas accretion, the circumgalactic medium, the baryon cycle and energetic feedback processes. He is a leader of the IllustrisTNG Project. He will be key in interpretation of our inferences and connecting them to the ground truth in the simulations. Prof. Nielsen is an expert on examining CGM absorption properties and connecting gas flow kinematics to host galaxies. She developed methods to quantify the internal velocity dispersions of the CGM using COS and HIRES/UVES data. She is also experienced in fitting absorption-line systems and modeling their metallicities. Prof. Charlton is an expert in quasar absorption line spectroscopy and ionization modeling of quasar absorbers. She has been PI (Co-I) on 9 (over 20) HST programs. She is very experienced with cloud-by-cloud, multiphase photoionization modeling. Prof. Lehner has over 2 decades of experience analyzing UV spectra and have used COS spectra for more than a decade to study the CGM of galaxies, from Local Group galaxies (Milky Way, Magellanic Clouds, M31) to z<1 galaxies. Both have been PI on over 20 GO and archival HST programs. He has published several tens of papers on the CGM of galaxies over the last 10+ years in major scientific journals, including Science and Nature. Prof. Kacprzak is a leading CGM expert, with a focus on connecting the CGM to their host galaxies. His expertise involves modeling galaxy orientations, and he discovered the warm and hot CGM azimuthal bimodalities. He is also an expert in understanding how the kinematics of the CGM are connected to host galaxies. Prof. Churchill has 25 years of experience observing and studying the CGM. He is highly knowledgeable and experienced with multiphase ionization modeling of quasar absorption line systems. He is also highly knowledgeable and experienced with absorption line studies of CGM simulations. Ms. Deepak is a graduate student in the Howk-Lehner group at Notre Dame. She will help with the interpretation of this program. She developed the tools to identify QSOs around galaxies on the basis of a Gaia astrometric selection. She has experience in ionization modeling and analyzing COS G130M and G160M spectra. Ms. DeFrancesco is a graduate student in the Charlton group at Penn State. She will help with the analysis and modeling of synthetic spectra from simulations, and in their interpretation. She has experience in ionization modeling and analysis of spectroscopic data. Intergalactic Medium and the Circumgalactic Medium 0 false false true false false false false SUPPORTED