0 Assigned ID: 2958 ----- Attempting Submission 1 (Thu Mar 24 10:22:36 GMT 2022) ----- 0 ----- Attempting Submission 1 (Thu Jul 14 13:42:16 GMT 2022) ----- Rise of the Titans: Stellar Properties of a Binary Hyper-Luminous Starburst Within the First Billion Years We propose HST/WFC3-IR and ACS observations in 4 bands to characterize the nature and the environment of HeLMS-54, the most distant binary hyper-luminous starburst known, at a redshift of 5.9. This rare, gas-rich (Mgas>2x10^11Msun), dusty starburst (SFR~2900Msun/yr) system was recently discovered with ALMA, NOEMA, and Herschel. It consists of two merging galaxies ~1-2kpc in diameter with dynamical masses of >10^11Msun each, separated by only 19kpc, resolved in dust emission by ALMA at 0.15" (1kpc) resolution. We here request imaging of the rest-frame ultraviolet light at matching resolution to measure the unobscured star formation rate, stellar mass and star formation history of this system through a complete SED analysis, as well as the morphology and sizes of the stellar disks and the UV extinction profile. We will also use the multi-band data to search for dropout galaxies in its environment on Mpc scales using the Lyman-break technique, which will put constraints on its formation history and the dark matter halo mass scale. This investigation could reveal the presence of a massive proto-cluster of galaxies within the first billion years of the Big Bang. Finally, the HST imaging will unambiguously address the possibility of a weak gravitational magnification of this system. Given its extreme cosmic rarity, this is key to properly place the existence and evolution of HeLMS-54 into context with cosmological simulations and models of starburst galaxies at different epochs. This study will be an important pathfinder for detailed investigations of the most exceptional star-forming environments in the early universe with JWST. Program uses 6 orbits, 4 with WFC3/IR in the F098M, F125W, and F160W filters, and 2 with ACS/WFC and the F606W filter. It is broken up into three visits, one consisting of the two ACS orbits, and the others consisting of the two WFC3 IR orbits each. There is no restriction on the orientation angle. We would prefer to schedule all 4 WFC3-IR orbits together to have similar orientation angles, but we do not make it a requirement given the recommendations for this cycle. All observations are of the same target field (HeLMS-54). 4-point position/readout dither patterns are employed, primarily to improve the PSF through subpixel sampling, i.e., to mitigate the effects of the large detector pixel sizes (especially for WFC3/IR), as well as to offset by a sufficiently large amount to move persistent pixels to different locations on the sky in each exposure, and to provide good cosmic ray rejection. For ACS/WFC, this is also important to cover the 50 pixel gap in y direction between the two detector chips. The ACS/WFC dither pattern consists of 2 positions with offset large enough to cover the inter-chip gap, and 2 subpositions each to dither on subpixel scales. The WFC3/IR dither patterns consist of 2 postions large enough to cover the "death star" feature, and 2 subpositions each to dither on subpixel scales. For the single F098M/F125W orbits, 4 positions are targeted. For the two F160W orbits, each orbit contains two subpixel dither positions, and the larger re- positioning is done between orbits. The PI has published ~300 peer-reviewed publications (h-index 68, >16,000 citations), of which ~30 are first author publications. Many of these publications have made use of HST data, and many focus on observations of dusty starburst galaxies (e.g., Riechers et al. 2010, ApJ 720, L131; Riechers et al. 2013, Nature, 496, 329; Riechers et al. 2014, ApJ, 796, 84; Riechers et al. 2017, ApJ, 850, 1; Riechers et al. 2020, ApJ, 895, 81; Riechers et al. 2021, ApJ, 907, 62; Riechers et al. 2022, Nature, 602, 58) and galaxy proto-clusters (e.g., Capak, Riechers et al. 2011, Nature, 470, 233; Pavesi, Riechers et al. 2018, ApJ, 861, 43; Riechers et al. 2021, ApJ, 907, 62) and lens modeling (e.g., Riechers et al. 2008, ApJ, 686, 851; Bussmann, Riechers et al. 2015, ApJ, 812, 43) and they include those that invented and establish the techniques used to discover the target of this proposal. The entire proposal team has ample experience in these areas, and with HST data analysis, including de-blending, photometry, SED fitting, and Lyman-break selection (see, e.g., Cooray et al. 2014, ApJ, 790, 40 and Laporte et al. 2015, ApJ, 810, 130 for a similar analysis). We have ample computing resources available to analyze the modest amount of data. Galaxies 6 false false false false false false false false SUPPORTED GALAXY HIGH REDSHIFT GALAXY INTERACTING GALAXY STARBURST J2000 J2000 ACS/WFC Imaging No No F606W WFC3/IR Imaging No No F098M WFC3/IR Imaging No No F125W WFC3/IR Imaging No No F160W POS-TARG DITHER BACKGROUND 2 3.034 LINE false false 0.149 2 85.29 POS-TARG ACS-WFC-DITHER-LINE 34.25 1 true The scheduling data was generated by CASM 30.3. 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CASM was run in Phase II mode. /private/var/folders/q5/pgd2_mqs78b2rc6c4f68fd900005mw/T/OPVisit5962674835589953183.xml Repeat of failed visit 02. 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