0 Assigned ID: 3829 ----- Attempting Submission 1 (Fri Oct 21 01:09:36 GMT 2022) ----- 0 ----- Attempting Submission 1 (Fri Dec 09 15:55:19 GMT 2022) ----- Fast radio bursts (FRBs) are bright millisecond pulses that are prolific in the universe but have uncertain progenitors. The recent localization of FRB 20220610A to a z~1 host galaxy marked a new era in FRB science. As the furthest securely-localized FRB to date by a factor of two and most energetic by a factor of four, this burst is already revolutionizing our understanding of these events. In particular, this FRB directly probed ionized gas and magnetic fields in the intergalactic medium to times approaching the peak of cosmic star formation. Given the uncertainty of FRB progenitors, host galaxy environments offer vital clues. However, current ground-based imaging of FRB 20220610A reveals a complex host galaxy structure, suggestive of multiple galaxies and possibly indicative of an ongoing galaxy merger. As these data cannot resolve the components individually, we propose 4 orbits of HST imaging to discern the true morphology of this system, potentially uncover signs of a galaxy-scale merger, and delineate how this FRB traces its host galaxy's star formation and stellar mass distributions. These observations will give clear context for the proper interpretation of the ground-based observations and provide a key puzzle piece to the first FRB at z~1. This program images a newly discovered FRB host galaxy at z~1 to measure the morphological structure of a complex host galaxy, suggestive of multiple galaxies and possibly indicative of an ongoing galaxy merger. As current ground-based data cannot resolve the components individually, we will use HST imaging to discern the true morphology of this system, potentially uncover signs of a galaxy-scale merger, and delineate how this FRB traces its host galaxy's star formation and stellar mass distributions. The rest-frame NUV (observed optical) will be sampled by the WFC3/UVIS F606W filter tracing recent star formation via young stars and yielding the highest possible resolution to distinguish the nature of the three components. The rest-frame optical (observed NIR) will be sampled by the WFC3/IR F160W filter. This is the reddest wide filter available on HST to measure the distribution of the stellar mass as traced by the old stellar populations. In order to obtain images clean of cosmic rays and image artifacts and sampling the PSF phase, we use custom dither patterns with 6 and 8 positions. For the optical measurements we will obtain six exposures, which also ensures no saturation for our somewhat bright target. Given the high background in the wide F606W bandpass, no post-flash is necessary. To minimize our pixel-based CTE correction, we place the target on chip 2 close to the readout. For the NIR observations, we wish to stay in the linear regime and minimize persistence with our bright target, and therefore will use SPAR50 with NSAMP of 13. Since this sets the exposure time per dither position, we end up with 4 dither positions per orbit for a total of 8 dither positions. As requested in the Phase I, to avoid a larger 4 orbit visit, we break up the filters into two orbits each and tie the orientations of the two filters to each other (within 5 degrees) to ensure sufficient targets in common to align the images to each other preceisely. We use a pos-targ offset of -55" and -45" in x and y, respectively, to align the IR detector on the WFC3/UVIS CTE aperture. For the WFC3/UVIS exposures, we use a dither pattern that is 5 times the custom 6 point dither pattern from WFC3/UVIS ISR 2020-07 (pg 14) to remove residual background patterns. For the WFC3/IR exposures, we use the 8-point wide dither pattern from WFC3/IR ISR 2016-14 (pg 17) to get a good dither for the pixel phase and artifiact mitigation. We multiply those values by 3 to better deal with IR blobs and self persistence. The large number of dithers for the IR is due to the desire to use SPAR50 as described above. We note that there is a 10th magnitude star in the upper north-west of our potential field of view. If scheduled at a PA of around 125 degrees, the star would fall on Chip 1, but the optical ghosts would fall in a favorable direction away from our target. However, if we are at a PA of around 110 degrees, there may be dragons breath on the detector. We believe that it would not impact the science target based on the orientation and the location of our science target, but if this is not the case, please reach out to the observers. We do not forsee any issues if we have to go to 1 gyro mode as we can observe at any absolute orientation, we just need the orientation of our two filters to be matched. If they can't be matched, we can also adjust the post-targ to allow different orientations between the visits as needed. PI Alexa Gordon is a PhD student working to understand the FRB progenitor through studies of their host galaxies. She has experience in photometric and spectroscopic follow-up of FRB host galaxies across multiple size class telescopes and expertise in galaxy stellar population synthesis modeling. Professor Wen-fai Fong is an expert in observations of transients and their host galaxies (FRBs and Short gamma-ray bursts), and will aid with analysis and interpretation. Dr Charles Kilpatrick is an expert in multi-wavelength observations of FRBs and analysis of HST imaging. Professor Jason X. Prochaska is an expert in cosmic baryons and experienced with measuring properties of galaxies hosting transient events. He has expertise in spectroscopic reduction and the follow-up of FRB host galaxies. Sunil Simha is a PhD candidate with considerable experience in spectroscopic and imaging observations of FRB hosts and foreground galaxies. Sunil is also proficient at using GALFIT for morphological fits to galaxy light profiles. Professor Nicolas Tejos is an expert in studies of the intergalactic medium and large-scale structure connection using HST UV spectroscopy and ground-based optical follow-up observations. He is also expert in FRB optical follow-up. Dr Marc Rafelski is an expert at planning for, reducing, and analyzing WFC3 data. He also has experience in measuring photometry and morphology of galaxies. Dr Kameswara Bharadwaj Mantha is a post-doctoral researcher specializing in galaxy mergers and their role in galaxy evolution. He has led studies involving the robust identification of merging systems in {\it HST} imaging. He also has experience in applying galfit for galaxy morphological measurements and subsequent extraction of any merger related tidal signatures from galfit-based data products. Professor Ryan Shannon is an expert in observation of fast radio bursts and burst emission properties in the context of progenitor models. As principal investigator for the ASKAP/CRAFT collaboration, he played a significant role in the detection and localization of this FRB and connecting the radio properties of the burst to that of the host galaxy. Professor Adam Travis Deller is an expert in fast radio burst and astrometry. He has led the FRB localization and will contribute to interpretation of burst position and its radio properties. Dr Stuart Ryder is an experienced user of 8 meter-class optical and infrared telescopes including Gemini, Keck, and the VLT for imaging, spectroscopy, and adaptive optics. He is also lead author of the discovery paper of FRB 20220610A. Lachlan Marnoch is a PhD student using VLT optical and NIR imaging to study the morphologies and properties of FRB host galaxies, and what these can tell us about FRB progenitors and environments. Galaxies 3 false false false false false true false SUPPORTED GALAXY AMORPHOUS IRREGULAR INTERACTING GALAXY MULTIPLE NUCLEI YES J2000 J2000 WFC3/UVIS Imaging No No F606W WFC3/IR Imaging No No F160W 1670600222494 The scheduling data was generated by CASM 30.3. 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