0 Assigned ID: 7720 ----- Attempting Submission 1 (Thu Apr 16 22:18:46 GMT 2026) ----- HST Phase I Proposal 7720 successfully submitted. Receipt: # 7720-1 ----- Attempting Submission 2 (Thu Apr 16 22:23:36 GMT 2026) ----- HST Phase I Proposal 7720 successfully submitted. Receipt: # 7720-2 ----- Attempting Submission 3 (Thu Apr 16 22:42:06 GMT 2026) ----- HST Phase I Proposal 7720 successfully submitted. Receipt: # 7720-3 The list of Investigators does not match the STScI records for this program. 0 ----- Attempting Submission 1 (Mon Aug 10 17:48:53 GMT 2026) ----- HST Phase II Proposal 18391 successfully submitted. Receipt: # 18391-1 ----- Attempting Submission 2 (Mon Aug 10 18:26:01 GMT 2026) ----- We have requested the addition of a CoI - and corrected the mismatched PI Very Massive Stars (VMS ;M*>~100Msol) are increasingly invoked to explain the hard ionizing spectra, strong HeII emission, and elevated ionizing photon production efficiencies observed in early galaxies, yet their extreme-ultraviolet (EUV) spectra remain essentially unconstrained. We propose ACS/SBC imaging of nine UV-luminous star-forming galaxies at z=2.3-3.6 with spectroscopic signatures consistent with VMS populations, and high escape fraction of ionizing radiation (~40-90%). At these redshifts, rest-frame 300-500 AA emission is shifted into the HST far-UV window, enabling the first direct constraints on escaping stellar radiation between the H I and He II ionization edges. Combining new SBC photometry with existing rest-UV and optical spectroscopy and HST measurements near rest-frame 800 AA, we will infer the hardness and shape of the ionizing continuum using forward modeling of stellar populations and IGM transmission. These observations will test whether VMS dominate the EUV output of extreme starbursts and provide an empirical calibration of ionizing spectra crucial for interpreting JWST galaxies and cosmic reionization. Imaging in F140L C. Scarlata & M. Hayes bring extensive expertise in the physics of ionizing-photon escape and the ultraviolet properties of star-forming galaxies. Their work has focused on the observational identification and interpretation of Lyman-continuum leaking galaxies, the structure and kinematics of the interstellar medium that regulate photon escape, and the connection between UV spectral diagnostics and ionizing radiation fields in both nearby and distant systems. They have led major HST programs targeting LyC emitters and Green Pea analogs, where rest-frame UV spectroscopy and imaging were used to constrain escape fractions and feedback-driven channels in the ISM, and a number of SBC-based imaging and spectroscopic programs. 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