Efficient selection of Lyman Continuum emitters at z

Submission 2: Thur 10th Apr Deadline-day submission -1347261796 Assigned ID: 6225 ----- Attempting Submission 1 (Wed Apr 09 12:17:26 GMT 2025) ----- HST Phase I Proposal 6225 successfully submitted. Receipt: # 6225-1 ----- Attempting Submission 2 (Thu Apr 10 14:52:14 GMT 2025) ----- Submission 1 (4th Aug 2025): First submission - targets, orientations, orbit and visit scheduling checked Only visit 08 requires F625W, all others require F814W -1330921515 ----- Attempting Submission 1 (Mon Aug 04 19:11:42 GMT 2025) ----- Efficient selection of Lyman Continuum emitters at z > 3 Understanding which galaxies dominated the ionizing flux (Lyman continuum, LyC) that reionized the early Universe requires measuring the escape fraction of galaxies during that epoch. This cannot be done directly, so instead we need to use indirect indicators calibrated on galaxies observed shortly after reionization. The Parallel Ionizing Emissivity (PIE) survey is an ongoing effort to address this by detecting over 400 galaxies across ~50 independent fields that can be stacked to measure how LyC emission varies with galaxy properties. This survey uses F336W imaging to probe pure LyC at 3.1<z<3.5, alongside F625W and F814W to select U-dropout galaxies for spectroscopic follow-up, which will be used to confirm redshifts and measure emission-line properties. Unfortunately, 8 fields with >3 orbits of F336W data are missing one of these bands, so cannot be used. Therefore, we propose a very small investment of WFC3 imaging to complement 40 orbits of existing data in these fields. These observations will allow us to efficiently detect dropout galaxies and thereby add over 100 galaxies at 3.1<z<3.5 to the PIE sample, of which ~30 will be confirmed as LyC emitters. Leveraging the existing data makes this program far more efficient than observing new fields. Confirmed LyC emitters will likely become prime targets for future studies with JWST. Aided by the increased sample, we will calibrate LyC indicators out to z~3.5 by measuring escape fractions as a function of stellar mass, morphology, star-formation surface density, Ly-alpha properties, UV luminosity, and UV colors, helping to understand which galaxies emitted the flux that reionized the early Universe. AIMS: The Parallel Ionizing Emissivity (PIE) survey is an ongoing effort to build a sample of over 400 galaxies across ~50 independent fields that can be stacked to measure how Lyman continuum (LyC) emission varies with galaxy properties. This survey uses F336W imaging to probe pure LyC at 3.1<z<3.5, alongside F625W and F814W to select U-dropout galaxies for spectroscopic follow-up, which will be used to confirm redshifts and measure emission-line properties. Unfortunately, 8 fields with >3 orbits of F336W data are missing one of the optical bands, so we cannot measure the colors required to select candidates for follow-up. This program aims to replace the missing F625W and F814W data and thereby substantially increase the number of candidate galaxies that we can target with spectroscopy. EXPOSURES: We use WFC3/UVIS observations so that we can utilize our existing pipeline to easily reduce the data and produce photometric catalogs including both the new and existing data. One orbit of F625W or F814W in each field is sufficient to match or exceed the depth of the existing observations. In order to ensure that CTE degradation does not affect our observations, we observe each field with three exposures (each of 750-800s depending on orbital visibility). These relatively long exposures allow for sufficient background to avoid the worst effects of poor CTE. We also add a small post-flash (2e-) to our F814W observations, ensuring that we exceed the recommended background levels. DITHER: We use the 'UVIS-GAP-LINE' dither pattern to cover the chip gap with two of our three exposures. This will allow reasonable cleaning of cosmic rays from the images and provide usable data at every location. This is particularly important for this program as the existing PIE data are not dithered, so do not cover the chip gap. The chip gap of the new data will be at a different orienation to that of the earlier data. Covering the chip gap in the new data ensures that we do not further restrict the available area in which there is coverage with all three filters. SPECIAL REQUIREMENTS: In order to maximize the overlap with the existing PIE data, we request ORIENT constraints. In the Phase I, we requested constraints such that at least 90% of the existing field is covered by the new observations (wherever this could be done with minimal impacts to schdeuling). This requires that the orientation of the new images matches that of the existing images to within ~15 degrees. For most of our observations these constraints can be applied and there are still large windows available during which they can be scheduled. However, for fields 52 and 54 (visits 07 and 08), this constraint leaves quite small windows for scheduling, so we remove the ORIENT constraints for these visits. The proposing team includes investigators with expertise in all necessary components of the program, both technical and scientific. Scientific expertise: Our team includes experts on the problem of the measurement of the escape of ionizing radiation, its evolution with redshift, its link with the escape of Lya radiation, and the study and characterization of indirect indicators from low redshift galaxies to be used at z>4. We have consistently published on this and related issues, with more than 50 papers over the past decade. Drs. Kim, Malkan, Rafelski, Scarlata, have led a number of works on constraining the escape fraction from different types of galaxies, using both HST UV imaging and spectroscopy, as well as ground based imaging and spectroscopy. Our work spans from galaxies in the nearby universe to objects at z~3. Drs. Goovaerts, Hayes, Henry, Scarlata are expert on the properties of Lyman alpha from galaxies, its resonant nature and how it can be used to trace the distribution of neutral hydrogen gas within and the outskirt of galaxies. Drs. Bunker, Hayes, Henry, Rafelski, Scarlata have substantial experience with observations and interpretation of rest frame UV spectra, both of local analogs of high redshift galaxies (e.g., the Green Pea extreme emission line galaxies, the Lyman alpha Reference Sample, etc) and high redshift Lyman Break Galaxies. Additionally, we have substantial experience in rest-frame optical diagnostics. Technical Expertise: Dr. Rafelski lead the reduction effort for the UV-UDF project (PI-Teplitz) and is a collaborator on CANDELS, and the UV-Frontier Fields (CoPI Scarlata). He is an expert in the reduction and analysis of WFC3 UV imaging data. Many of the team members are part of the large WFC3 IR Spectroscopic Parallel (WISP) Survey (PI: Malkan), UV-UDF (PI: Teplitz) as well as UV-CANDELS (PI: Teplitz). Drs. Beckett, Rafelski, Mehta also have experience in color selecting galaxies via the Lyman Break technique and photometric redshift estimates. We have a strong track record of delivering reduced data and catalogs to the public, via both MAST and IRSA archives. 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