Science Keywords: Select from 2 to 5 items. 0 Your proposal could not be saved. Submission aborted. Assigned ID: 4879 ----- Attempting Submission 1 (Thu Jun 01 12:59:25 GMT 2023) ----- JOint FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser FLASH level may be too high for this exposure or a long subexposure. See extended explanation in the diagnostic browser FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser FLASH level may be too high for this exposure or a long subexposure. See extended explanation in the diagnostic browser FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser I adjusted the pointing centers for both targets so that they fall near the center of a ACS/WFC chip as both are extended objects. They are both ultra-faint dwarf galaxies that are marginally resolved into stars, therefore the targets themselves are essentially invisible in the target confirmation images, but I have confirmed these are the correct coordinates. There are warnings regarding the WFC3/UVIS exposures and the amount of FLASH used. I found that the estimated e- counts from the APT and ETC were quite different. I went with the estimates from the ETC to work out how much FLASH to use, but I would appreciate input of whether this is suitable or not. 594744454 ----- Attempting Submission 1 (Fri Jun 02 01:14:59 GMT 2023) ----- See submission comments. I would appreciate input on the FLASH values. Quenching in the lowest mass galaxies at the edge of the Local Group We request 10.5 h of VLA D-array time and 2 HST orbits in order to constrain the neutral gas content and stellar populations of three recently discovered ultra-faint dwarf galaxies (UFDs) at the edge of the Local Group. UFDs are the lowest mass galaxies that exist and our standard framework of galaxy evolution and cosmology predicts that they should be permanently quenched by cosmic reionization. However, the discovery of Pegasus W, a UFD that has evidence for recent star formation, has called this longstanding prediction into question. Here we target Pegasus W and two other recently discovered UFDs to conclusively address whether or not they contain HI gas, which could provide fuel for ongoing star formation, and whether there is evidence for populations of young stars in their color--magnitude diagrams. If they do contain gas, these UFDs may be fainter analogs of the Phoenix dwarf, which appears to be in the process of losing its gas. Discovering HI gas or young stellar populations in (or around) any of these systems would reinforce the recent findings for Pegasus W and would force a rethink of our longstanding assumptions about reionization and the evolution of the lowest mass galaxies. These observations focus on two ultra-faint dwarf galaxies at the edge of the Local Group, and near M31's virial radius in particular. The goal is to study their star formation history with HST and improve our understanding of quenching of the smallest galaxies in this unique environment. We model our strategy after the recent successful observations of Pegasus W. We request the ACS/WFC imager for its wider field of view than WFC3 and its better sensitivity in our chosen imaging bands. Also, we will image in the F606W and F814W filters to produce a color magnitude diagram (CMD) of our dwarfs; these filters are the standard for nearby resolved stellar population work, and are identical to those used in Pegasus W. We also request WFC3-UVIS parallel imaging in the same filters (F606W, F814W). This will allow us to observe a field region next to our two dwarf targets (albeit to slightly brighter limits), and get a better statistical handle on the number of background/foreground contaminants in the CMD (which we know will have some Milky Way contamination, given the galactic latitude of -30 degrees). To get both bands in a single orbit (with ~1200s total integration per filter), we will obtain two exposures per filter, with a standard dither line offset between them to aid with cosmic ray and chip defect removal. If a given stellar system can safely fit on a single chip, we will position it accordingly. We may place orient constraints on our ACS imaging to avoid bright stars and potential image artifacts. n/a Galaxies 6 false false false false false false false false SUPPORTED GALAXY DWARF SPHEROIDAL J2000 J2000 GALAXY DWARF SPHEROIDAL J2000 ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W POS-TARG DITHER 3.034 LINE false false 2 85.29 ACS-WFC-DITHER-LINE 1 false /tmp/OPVisit8154637537851132347.xml The scheduling data was generated by CASM 31.0. CASM was run in Phase II mode. 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