0 0 ----- Attempting Submission 1 (Thu Aug 14 14:20:01 GMT 2025) ----- The Fate of the Leading Arm of the Magellanic Stream The Leading Arm (LA) is a fragmented, multiphase gaseous region of the Magellanic System resulting from tidal and hydrodynamic interactions between the Large and Small Magellanic Clouds (LMC and SMC). It traces the orbital trajectory of the LMC-SMC system and also serves as a potential reservoir of cold gas accreting onto the MW fuel that could sustain ongoing star formation in our Galaxy. Unlike the trailing Magellanic Stream, the LA leads the Magellanic Clouds and is the only Magellanic component currently interacting with the MW disk offering an opportunity to study active gas accretion. However once this gas plunges through the MW disk, how it survives or succumbs to the harsh ionizing environment of the MW remains unclear. We propose studying two regions, LA II and LA III, to investigate the survival of this gas by measuring ionization levels. We request 22 orbits of HST/COS G130M spectroscopy to observe four quasars. Together with three archival sightlines, these observations will map the LA III region over 20 degrees on the sky, sampling a range of distances from the MW disk. Our goals are: (1) calculate the ionized-to-neutral hydrogen ratio, (2) investigate spatial variations in gas properties, and (3) compare our findings with simulations. This study will enhance our understanding of satellite gas accretion, its role in sustaining star formation in the MW, and overall the mechanisms through which galaxies acquire and utilize baryonic matter. This is a COS FUV program to observe four QSO targets on the Leading Arm III using G130M. The science goal is to estimate the effect of MW ionizing radiation on to the accreting gas from the Leading Arm by detecting Si metal absorption lines. Each target will be acquired with a NUV ACQ/IMAGE with the PSA and MIRRORB, followed by science observations with the G130M/1291 setting. We used two FP-POS 3 and 4 in each visit. These four objects are spread across 8 visits. All visits are up to 3 orbits in length, following the guidance from the PC. Dr Elena D'Onghia is an expert on galaxy evolution and on the dynamical evolution of the Magellanic System. She led a 2016 Annual Reviews article on the Magellanic Stream. Co-PI Dr Andrew Fox is an expert in the gaseous halos of galaxies and the analysis of UV spectra. He has led seven HST GO and two AR proposals, and has first-authored over 20 refereeed journal papers in related fields. He will coordinate and oversee the analysis and manage the Co-I contributions. Dr Dhanesh Krishnarao is an expert with modeling the Magellanic CGM. He is leading the analysis of the high ions in the Magellanic Corona from HST archival program 16363. Dr Scott Lucchini is an expert on simulations of the Magellanic System. He has published two papers on this subject (one in Nature and one in ApJL). He will lead the simulations in this program. PI Dr Sapna Mishra is an expert in UV and optical quasar absorption line spectroscopy, with a focus on studying the Magellanic systems, CGM, IGM and galaxy clusters. She is leading the analysis on the cool CGM of the LMC from HST archival program 17053. She has led HST-GO proposal on the Magellanic clouds program 17757. She has two first author papers mainly on Magllanic Stream. Dr Jonathan Smoker is the CRIRES instrument scientist and former UVES instrument scientist at the VLT. He has decades of experience with point-source spectroscopy, and is an expert in the high-velocity clouds in the MW halo. He will advise on the technical aspects of the program. Dr Jason Tumlinson is expert in the UV and optical absorption line studies both in observation and simulation. He is also expert in the halo gas physics in galaxies of different halo masses. He will assist us in interpreting our results. Stellar Populations and the Interstellar Medium Galaxies 6 false true false false true false false false false false SUPPORTED GALAXY QSO QUASAR NO J2000 J2000 FUV = 16.44 ISM ABSORPTION LINE SYSTEM ABSORPTION LINE SYSTEM - GALACTIC HALO NO J2000 J2000 FUV =16.50 GALAXY QSO QUASAR NO magnitude given is R-band J2000 J2000 FUV =17.66 GALAXY QSO QUASAR NO J2000 J2000 FUV = 18.00 COS/FUV Spectroscopic No No G130M 1291 COS/FUV Spectroscopic No No G130M 1291 COS/FUV Spectroscopic No No G130M 1291 COS/FUV Spectroscopic No No G130M 1291 /Users/smishra/Dropbox/HST_Cy33/LAIII/PhaseII/OP-cache/LAIII-v01.xml The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. 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