HST Imaging of A Nearby Dwarf Galaxy Surrounded by an Ionized Ring of Pristine Circumgalactic Gas

0 Assigned ID: 7692 ----- Attempting Submission 1 (Thu Apr 16 21:50:07 GMT 2026) ----- HST Phase I Proposal 7692 successfully submitted. Receipt: # 7692-1 ----- Attempting Submission 2 (Thu Apr 16 22:47:38 GMT 2026) ----- HST Phase I Proposal 7692 successfully submitted. Receipt: # 7692-2 ----- Attempting Submission 3 (Thu Apr 16 23:58:20 GMT 2026) ----- HST Phase I Proposal 7692 successfully submitted. Receipt: # 7692-3 This is a first submission but we have several questions we would appreciate guidance on before final submission. The first regards the orbit planning: we have scheduled 2 exposures per orbit across all visits to maximize count accumulation, particularly for the narrowband filters, and minimize overheads. We are aware this might be somewhat atypical, and just want to ensure this does not have additional downsides --- while CR accumulation will be higher, with 4 dithered positions per visit we should be able to remove these. Is there anything else we should be aware of? The second regards post-flash: we are aware low counts in the NB almost always require post-flash, we want to confirm this matches the expectations for this proposal, and ask what mitigations are currently recommended for the ~2x variation in illumination pattern, in order to conduct flux calibrations. Is the FLSHCORR reduction treatment via a reference file considered by STScI as adequate for removal of the gradient and to what precision? We note that our "object" is large, but the individual clumps being targeted are small (<0.5") and faint. Alternatively does HST currently recommend programs obtain post-flash oriented calibration images as part of programs? Finally, as a minor question, to fit the extent of the ring within the dithers, we left LEDA135162's base coordinates as the TARGET but used the ALADIN tool to generate the nominal offset that best captures the object. These offsets are in the special instructions for the exposure under POS TARG; if it is much preferrable to instead encode that offset into the direct target coordinate, we can do so. We are happy to discuss any aspect of the proposal plan that might improve our overall data quality or robustness of ensuring data is gathered. -1089908011 ----- Attempting Submission 1 (Fri Aug 07 23:07:57 GMT 2026) ----- HST Imaging of A Nearby Dwarf Galaxy Surrounded by an Ionized Ring of Pristine Circumgalactic Gas It was recently discovered that the nearby dwarf galaxy LEDA 135162 hosts a large, low surface brightness ring surrounding the galaxy, well beyond its optical extent. Followup spectroscopy has revealed that the gas, clumps of which have been ionized by triggered star formation in the ring, is remarkably lacking in almost any [NII] emission, despite being bright in [OIII] and H-alpha, suggesting this ring is composed of low metallicity, diffuse gas from the circumgalactic medium of the system. Unexpectedly, the [OIII]/H-beta line ratios vary considerably from clump to clump in the ring (and even from spaxel to spaxel) as measured from KCWI IFU spectroscopy, suggesting sub-seeing variation in metallicity or ionization conditions. We propose 12 orbits of HST narrowband and broadband imaging to characterize the properties of this unique ring, which provides a rare opportunity to study the spatially resolved circumgalactic medium of a nearby dwarf galaxy in situ, as well as to study the nature of star formation in the low-density, low metallicity regime. We aim to carry out a 12 orbit ACS/WFC program, split between 8 orbits of narrowband imaging (4 orbits in F502N, 4 orbits in F660N), with accompanying 4 orbits of broad band coverage (2 orbits in F475W, 2 orbits in F625W). To maximize signal accumulation per subexposure, particularly in the narrowband filters, we have scheduled 2 images per orbit, maximizing availability at around 1000-1200 seconds per exposure. We complete 1 full 4 point dither pattern (large, at 9.5" scale --- we are aiming to ensure uniform filling of the field, rather than high subpixel aligned sampling) per visit/orient, with each visit comprising 2 orbits. There are no roll angle constraints for any individual visit. Per ETC recommendations, we have input 10e- of post flash per NB exposure, though we seek guidance on this front. ETC: ACS.im.2363734 (see submission comments). Updating from the proposal, we reran the ETC using the same clump input spectrum (0.4" source with a 0.4" extraction aperture) but with an updated exposure time of 1099 seconds (based on the 2 exposure per orbit packing), and determined 10e- of post flash would be sufficient compared to the 27e- for the initial proposal. PI Pasha has extensive experience with spectroscopic and imaging studies of galaxies, and has worked specifically on ring galaxies using HST: they were the PI of a previous HST imaging study in Cycle 30 (GO 17301) to study the properties of a many-ringed galaxy. They are an expert in the use of ground based IFU systems (e.g., KCWI) to measure spatially resolved emission properties. Co-PI van Dokkum has been the PI of 20 HST programs (awarded 559 orbits), and Co-I of dozens of more successful HST programs and is an expert in the reduction and analysis of HST imaging (and spectra); they were the PI of the 3D-HST large program and were a leading member of the 3D-DASH program. Co-I Chloe Neufeld is an expert in the study of dwarf galaxies and the baryonic processes in dwarfs, including the interpretation of diagnostic line ratios. Galaxies Intergalactic Medium and the Circumgalactic Medium 6 false false false false false false false SUPPORTED GALAXY DWARF COMPACT INTERACTING GALAXY KNOT RING STAR FORMING REGION YES This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 ACS/WFC Imaging No No F660N ACS/WFC Imaging No No F502N ACS/WFC Imaging No No F475W ACS/WFC Imaging No No F625W POS-TARG DITHER 9.5 LINE 9.5 false false 69.02 4 20.7 ACS-WFC-DITHER-BOX 1

false

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