18035( 2) - 13-Aug-2025 19:19:27 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18035 Version: 2 Check-in Time: 13-Aug-2025 19:19:27 STScI Edit Number: 0 Title Direct search for Lyman continuum in a multiply-lensed low-mass high-redshift analog ------------------------------------------------------------------------------------ Type Cycle GO 33 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Lukas Jonathan Furtak Ben-Gurion University of the Negev Y CoI: Prof. Adi Zitrin Ben-Gurion University of the Negev N CoI: Prof. Daniel P. Stark University of California - Berkeley N CoI: Adele Plat Ecole Polytechnique Federale de Lausanne N CoI: Dr. Michael Topping University of Arizona N CoI: Dr. Dan Coe Space Telescope Science Institute - ESA - N JWST CoI: M. Riley Owens University of Cincinnati Main Campus N ------------------------------------------------------------------------------------ Abstract 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. ------------------------------------------------------------------------------------ Observations Description ------------------------ 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 18035( 2) - 13-Aug-2025 19:19:27 - [ 2] 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. ------------------------------------------------------------------------------------ 18035( 2) - 13-Aug-2025 19:19:27 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 RXC0018-LAE1 GALAXY, GRAVITATIONAL RA=00H18M33.5539S, J2000 Z = 3.22 V = 24.5 +/- 0.05 LENS, IONIZATION FRONT, DEC=+16D26'2.14" LYMAN ALPHA CLOUD, STARBURST Reference Frame: ICRS Comments: This object was initially generated by the targetselector and retrieved from the SIMBAD database, and then adjusted to newer (HST-based) coordinates, magnitudes etc. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2015.5 0.0 0.0 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 18035( 2) - 13-Aug-2025 19:19:27 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-GAP-LINE Pattern_Purpose MOSAIC Number_Of_Points 2 Point_Spacing 2.414 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 85.759 Angle_Between_Sides Center_Pattern YES ------------------------------------------------------------------------------------------------------------------------------------ 18035( 2) - 13-Aug-2025 19:19:27 - [ 5] Visit: 01 (Visit 1) Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 RXC0018-LAE WFC3/UVI ACCUM UVIS2 F336W FLASH=9 1 2568 S PATTERN 1 1 1 S see below (5153 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ RXC0018-LAE1 1 WFC3/UVI ACCUM UVIS2 F336W 1 none none none 1 2588.0 S S RXC0018-LAE1 1 WFC3/UVI ACCUM UVIS2 F336W 2 none none none 2 2565.0 S S 18035( 2) - 13-Aug-2025 19:19:27 - [ 6] Visit: 02 (Visit 2) Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 RXC0018-LAE WFC3/UVI ACCUM UVIS2 F336W FLASH=9 1 2568 S PATTERN 1 1 1 S see below (5153 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ RXC0018-LAE1 1 WFC3/UVI ACCUM UVIS2 F336W 1 none none none 1 2588.0 S S RXC0018-LAE1 1 WFC3/UVI ACCUM UVIS2 F336W 2 none none none 2 2565.0 S S 18035( 2) - 13-Aug-2025 19:19:27 - [ 7] Visit: 03 (Visit 3) Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 RXC0018-LAE WFC3/UVI ACCUM UVIS2 F336W FLASH=9 1 2568 S PATTERN 1 1 1 S see below (5153 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ RXC0018-LAE1 1 WFC3/UVI ACCUM UVIS2 F336W 1 none none none 1 2588.0 S S RXC0018-LAE1 1 WFC3/UVI ACCUM UVIS2 F336W 2 none none none 2 2565.0 S S 18035( 2) - 13-Aug-2025 19:19:27 - [ 8] Visit: 04 (Visit 4) Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 RXC0018-LAE WFC3/UVI ACCUM UVIS2 F336W FLASH=9 1 2568 S PATTERN 1 1 1 S see below (5153 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ RXC0018-LAE1 1 WFC3/UVI ACCUM UVIS2 F336W 1 none none none 1 2588.0 S S RXC0018-LAE1 1 WFC3/UVI ACCUM UVIS2 F336W 2 none none none 2 2565.0 S S 18035( 2) - 13-Aug-2025 19:19:27 - [ 9] Visit: 05 (Visit 5) Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 RXC0018-LAE WFC3/UVI ACCUM UVIS2 F336W FLASH=9 1 2568 S PATTERN 1 1 1 S see below (5153 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ RXC0018-LAE1 1 WFC3/UVI ACCUM UVIS2 F336W 1 none none none 1 2588.0 S S RXC0018-LAE1 1 WFC3/UVI ACCUM UVIS2 F336W 2 none none none 2 2565.0 S S 18035( 2) - 13-Aug-2025 19:19:27 - [ 10] Summary Form for Proposal 18035 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2 ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=9 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 1 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F336W ------------------------------------------------------------------------------------------------------------------------------------ Target Names RXC0018-LAE1 ------------------------------------------------------------------------------------------------------------------------------------