0 Assigned ID: 6440 ----- Attempting Submission 1 (Thu Apr 10 13:31:23 GMT 2025) ----- HST Phase I Proposal 6440 successfully submitted. Receipt: # 6440-1 ----- Attempting Submission 2 (Thu Apr 10 13:35:29 GMT 2025) ----- 0 ----- Attempting Submission 1 (Wed Aug 13 18:45:21 GMT 2025) ----- Direct search for Lyman continuum in a multiply-lensed low-mass high-redshift analog Early star-forming galaxies at high redshifts have been shown to most likely be the sources that drove the reionizantion of the Universe with deep JWST observations. Direct observation of their ionizing Lyman continuum (LyC) emission however remains impossible due to IGM opacity. Instead, low-redshfit analogs of high-redshift galaxies are studied to constrain their LyC escape and link it to other observables, e.g. Lyman-alpha (Lya), the ultra-violet (UV) continuum slope, or rest-frame optical emission lines and line ratios. We here propose to use the HST's unrivaled UV sensitivity to directly detect the LyC emission of a particularly unique, gravitationally lensed, analog of the faint low-mass galaxy population that reionized the Universe. Situated at z=3, this galaxy has an unusual double-peaked Lya emission profile where the blue peak is much stronger than the red peak which suggests a significant LyC escape fraction. It moreover presents an extremely blue UV continuum slope such as has thus far only been observed in the highest-redshift (z>10) galaxies with JWST. Thanks to the strong lensing, we benefit from a factor ~10 in flux magnification and will be able to stack the multiple images to achieve greater depth and probe LyC escape fractions as low as 0.015, establishing a link between this object's LyC escape fraction, the unique Lya emission profile and other properties. The timing to conduct these observations in cycle 33 is ideal since the cluster will be imaged with JWST in cycle 4, which will add crucial additional constraints to our analysis. Our proposed deep UV observations will in turn ideally complement JWST near-infrared imaging of the lensing cluster. This program aims at detecting the Lyman-Continuum emission from a multiply-imaged, double-peaked strong Lyman-alpha emitter at z=3.2, located in the strong lensing cluster RXC0018. To this end, we will image the RXC0018 cluster field with WFC3/UVIS over 10 orbits in order to achieve sufficient depth in the F336W-band. The target is placed optimally on the UVIS2 detector. Note that this set-up covers the entire RXC0018 cluster field, including the positions where our lens model predicts the counter images of our target. The observations are split into five visits, with exposures of 2x 2058s split over two orbits each. This is to maximize depth and minimize overheads. We apply a simple, standard UVIS dither pattern to bridge the detector gap, in order to provide uniform imaging of the RXC0018 lensing cluster field, and thus leverage the legacy value of our observations. PI Dr. Lukas Furtak is an expert in high-redshift galaxy physics and observations, and gravitational lensing with a particular focus on faint gravitationally lensed galaxies and AGN. He has extensive expertise in data reduction, catalog assembly, HST, JWST, and ground-based imaging and spectroscopy and will lead the main study of this program. Prof. Adi Zitrin is an expert in gravitational lensing modeling of galaxy clusters and in high-redshift galaxy observations. He has played key roles in various lensing surveys to date such as CLASH, the Hubble Frontier Fields, RELICS, and various spetroscopic campaigns of high-redshift objects. Prof. Daniel Stark is an expert in high-redshift galaxies and their spectroscopic study, physical properties, formation and evolution and their contribution to the reionization of the universe. Dr. Dan Coe is an expert in gravitational lensing and high-redshift galaxies and the PI of the RELICS lensing survey with Hubble. Dr. Adele Plat is an expert in stellar population synthesis and galaxy SED-modeling with extensive expertise in photoionization modelling and interpretation of physical properties of Lyman continuum leakers. Dr. Michael Topping has extensive experience in analyzing the rest-frame UV properties of stellar populations within high-redshift star-forming galaxies. Our team has sufficient expertise in data reduction and analysis to achieve the science goals of this proposal. Galaxies Intergalactic Medium and the Circumgalactic Medium 3 false false false false false false SUPPORTED GALAXY GRAVITATIONAL LENS IONIZATION FRONT LYMAN ALPHA CLOUD STARBURST This object was initially generated by the targetselector and retrieved from the SIMBAD database, and then adjusted to newer (HST-based) coordinates, magnitudes etc. RXCJ0018.5+1626 CL0016+1609 J2000 J2000 WFC3/UVIS Imaging No No F336W POS-TARG MOSAIC DITHER 4 2.414 WFC3-UVIS-DITHER-BOX false 81.785 true 0.112 0.173 2 85.759 POS-TARG WFC3-UVIS-GAP-LINE 23.884 1 false /private/var/folders/wg/60btczbd6bqgl597n3nwrpw80000gp/T/OPVisit15989984825558945529.xml The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. 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