The current Call for Proposals is for HST Cycle 34. This is an APT Phase I proposal for an HST program that was awarded jointly with an approved JWST Cycle 5 program (JWST-GO-10329) -1150947725 Assigned ID: 6954 ----- Attempting Submission 1 (Thu Mar 26 21:20:38 GMT 2026) ----- HST Phase I Proposal 6954 successfully submitted. Receipt: # 6954-1 The email from STScI requesting this joint HST program Phase I submission specificall instructed us to assign this proposal to Cycle 33, which we have done. This is the Phase II for an HST program awarded jointly with cycle 5 JWST program GO-10329. 1584843301 ----- Attempting Submission 1 (Wed Apr 22 00:43:20 GMT 2026) ----- High Mass X-ray Binaries (HMXBs) trace some of the youngest, most extreme, stellar populations. They produce large amounts of high energy emission and feedback in galaxies that are dominated by young stellar populations, a stage that describes all galaxies at some point in their evolutionary history. Furthermore, these extreme but short-lived X-ray sources can clear out the interstellar medium (mechanically), and ionize gas - both important processes for driving cosmic re-ionization. However, HMXBs and their associated stellar populations are not well-understood, especially in the distant universe. Characterizing these sources and the stellar populations that host them is crucial for a wide range of problems in observational astrophysics, from high-mass stars to galaxy evolution to re-ionization. We propose a unique study of the only highly magnified X-ray emitting dwarf starburst galaxy known. Hubble imaging and ground based spectroscopy reveal that the lensed dwarf (logM/MSun<8) galaxy hosts an X-ray bright HMXB population. The strong lensing magnification enables a zoomed-in, spatially resolved study of the HMXB-hosting region within the lensed galaxy. These data will measure the isolated emission from the HMXB population, undiluted by other regions of the galaxy, enabling a study that connects star-formation, nebular gas in the ISM, stellar populations, and HMXBs on the star cluster scale during the epoch of peak star-formation in the Universe. Joint HST UV imaging and archival MIRI imaging will multiply the science value of the JWST observations, constraining the gas and stellar populations. This HST program was awarded jointly with JWST program JWST-GO-10329. This is a program awarded jointly with a cycle 5 JWST program that is targeting a highly magnified, strongly lensed dwarf galaxy with confirmed low-luminosity X-ray emission, believed to originate from a high mass X-ray binary population in the lensed galaxy. The goal of the observations is to measure the spatially resolved properties of the stellar populations and nebular gas across the lensed galaxy, and in particular to zero in on the properties of the region of the galaxy that produces the X-ray emission. The JWST data will deliver rest-frame optical through-NIR imaging and spectroscopy of the lensed galaxy, which will be combined with HST imaging spanning the rest-frame UV. Three archival HST imaging bands for this field cover ~4800-8000 angstroms, immediately blueward of the JWST data. This HST program will obtain new imaging in two blue WFC3/UVIS filters (F336W and F390W) that provide complementary coverage of the lensed galaxy's spectral energy distribution extending to the far rest-frame UV, where the hottest, most massive stars dominate the continuum emission. Members of this proposal team have extensive expertise using strong lensing to study the properties of highly magnified galaxies across the entire observable electromagnetic spectrum. Many members of this proposal are also members of the TEMPLATES ERS program team, as well as several approved C1 and C2 NIRSpec IFU program teams, and have a lot of experience analyzing the instrument modes proposed for in this program. Several team members hve also written public data guides and cookbooks for NIRCam and NIRSpec data reduction and analysis. PI Dr. M. Bayliss is an expert on spectroscopic diagnostics of star formation, gravitational lensing, and galaxy clusters, including a published HST + Chandra study of the X-ray emitting giant arc that is the focus of this proposal. Dr. Bayliss has led several HST and JWST studies of highly magnified strong lensing systems, and will serve as the overall project lead. All CoIs below will also contribute to some or all of: data reduction, analysis, and paper-writing. Dr. K. Sharon and Dr. G. Mahler, and Dr. C. Cerny are experts at constructing strong gravitational lens models from multi- band HST imaging. Dr. Sharon has already generate a precision strong lens model for the target in this proposal, and will support lensing-assisted analyses of the new HST and JWST data. Dr Gourav Khullar has extensive experience with stellar populatoin modeling, and has applied SPS models to a variety of high-redshift galaxies. He is a member of the TEMPLATES and UNCOVER ERS and Cycle 1 JWST teams. He will contribute to the SPS modeling effort incorporating both broadband SED and emission line spectroscopic constraints. Dr. M. Florian iwas a member of the JWST/NIRCam instrument team and has been reducing JWST data from ERS, Cycle 1, and Cycle 2 programs. He is also an expert in the morphologies, stellar populations, and physical conditions inside distant galaxies, particularly in those that have been gravitationally lensed and has extensive experience building morphological models using space-based imaging data. He will contribute to data reduction and the morphological analysis for this program. Dr. Rigby is the Senior JWST Project Scientist, the PI of the TEMPLATES ERS program, and an expert on studying distant star-forming galaxies magnified by gravitational lensing. Dr. Keuhno Kim has expertise analyzing extreme star forming galaxies in the local universe, and also ditting the morphological properties of galaxy samples across a wide range of redshifts. Co-I Roberson is a graduate student at the University of Cincinnati working with PI Bayliss, and has experience working on SED model analyses of high redshift star-forming galaxies. Dr. M. Gladders is an expert on galaxy clusters, gravitational lensing, and galaxy evolution. Dr. H. Dahle is an expert on galaxy clusters and gravitational lensing, as well as studies of strongly lensed quasars and galaxies. Dr. M. McDonald is an expert in X-ray astrophysics and the properties of massive galaxy clusters. Co-I Owens is a student working with Dr. Bayliss who has experience analyzing spectra and imaging of strongly lensed star-forming galaxies. Dr. Hlavacek-Larrondo is an expert in black hole feedback processes of clusters of galaxies. She has worked with dozens of telescopes targeting clusters across all wavelengths (Chandra/XMM/JVLA/ALMA/KECK/CFHT/etc) and has published more than 90 papers in the field. Dr. Forman and Dr. Calzadilla are experts with Chandra X-ray observations and are both familiar with X-ray properties of compact X-ray sources (HMXBs) and their evolution. Michael Calzadilla has extensive experience with multiwavelength observations (Chandra/HST/Magellan/ALMA/ATCA/ASKAP) of high-redshift galaxies and galaxy clusters, including star formation and broadband SED modeling in BCGs Co-I E. Solhaug is a graduate student at the University of Chicago and an expert on LyC-leaking galaxies and Lyman-alpha radiative transfer. Solhaug also has experience in data reduction of ground-based VLT/MUSE IFU cubes and gravitational lens modeling. 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