0 Assigned ID: 6546 ----- Attempting Submission 1 (Thu Apr 10 17:28:21 GMT 2025) ----- HST Phase I Proposal 6546 successfully submitted. Receipt: # 6546-1 ----- Attempting Submission 2 (Thu Apr 10 20:19:01 GMT 2025) ----- The multiphase circumgalactic medium (CGM) surrounding galaxies serves as a crucial laboratory for understanding galaxy formation and evolution. Despite significant observational progress, our understanding of this medium-particularly its highly ionized components traced by OVI-remains elusive due to uncertainties in ionization mechanisms and spatial distribution. We present a systematic forward modeling framework to fully characterize the physical and chemical properties of multiphase CGM gas using the publicly available COS CGM Compendium (CCC), a HI-selected dataset comprising ~170 absorption systems with OVI coverage at z<1. Our approach will: 1) Apply rigorous forward modeling techniques to simultaneously analyze multiple gas phases, incorporating both equilibrium and non-equilibrium ionization models across the full suite of UV+EUV absorption lines tracing gas at T < 10^6 K; 2) Establish quantitative relationships between high and low ionization phases to reveal underlying CGM structure properties; 3) Correlate derived gas properties with host galaxy characteristics through comparisons with galaxy-selected and OVI-selected surveys; 4) Benchmark our results against state-of-the-art cosmological simulations to validate our methodology and provide feedback for theoretical models; and 5) Deliver public high-level data products, including an efficient forward modeling toolkit for characterizing multiphase absorption spectra. This comprehensive analysis will provide unprecedented constraints on the baryon cycle during a critical epoch of galaxy evolution. PI Dr. Sameer has significant experience in handling COS, STIS, and FUSE spectra, as well as HIRES and UVES optical spectra. He has developed a Bayesian inference-based multiphase ionization modeling algorithm that provides insights into the physics underlying absorption line systems. He will be instrumental in modeling the physicochemical properties of the absorption systems. He will guide another postdoc/grad student in deriving the physical properties of the gas. CoIs Prof. Lehner and Prof. Howk have over 2 decades of experience analyzing UV spectra and have used COS spectra for the last 13 years to study the CGM of galaxies from our local group of galaxies (Milky Way, Magellanic Clouds, M31) to z<1 galaxies. They are specialists in the high ions and multiple gas phases in the Milky Way, Magellanic Clouds, and the low- and high-redshift universe. Both have been PI on over 20 GO and archival HST programs. The group has published several tens of papers on the CGM of galaxies over the last 10+ years in major scientific journals, including Science and Nature. CoI Dr. Bart Wakker has an extensive background in studies of the interstellar and intergalactic medium, having published over 100 papers on these subjects, using data from ground-based radio and optical telescopes, as well as many of the UV space instruments (IUE, FUSE, FOS, GHRS, STIS, COS), being PI on 11 HST observing and 5 HST archival projects, as well as Co-PI or Admin-PI on 7 others. CoI Dr. Oppenheimer, a leading expert in hydrodynamic simulations of the chemical evolution, IGM, galaxies, and their gas flows, will be key in connecting our observations to theoretical models and cosmological simulations. CoI Ms. Deepak, a graduate student in the Howk-Lehner group at Notre Dame, will help with the analysis, modeling, and interpretation. She has experience in ionization modeling and analyzing COS G130M and G160M spectra. CoI Prof. Charlton is an expert in quasar absorption line (QAL) 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. CoI Prof. Nielsen is an expert on examining the absorption properties and quantifying their kinematics. Nielsen has experience with Keck/KCWI data and has published papers using these data, which will be important for future Keck/KCWI follow-up observations. CoI Prof. Kacprzak is a leading CGM expert focused on connecting the CGM to their host galaxies. His expertise involves 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. Intergalactic Medium and the Circumgalactic Medium Galaxies 0 false false false true false false false SUPPORTED