The current Call for Proposals is for HST Cycle 34. 0 Assigned ID: 6955 ----- Attempting Submission 1 (Fri Mar 27 02:30:54 GMT 2026) ----- HST Phase I Proposal 6955 successfully submitted. Receipt: # 6955-1 We are submitting this proposal as a part of our approved JWST-HST joint program in JWST Cycle 5 (ID: 11465). 0 ----- Attempting Submission 1 (Thu Apr 30 14:31:08 GMT 2026) ----- The nature of dark matter (DM) represents one of the most fundamental open questions in astrophysics. Although the cold dark matter (CDM) paradigm remains the prevailing framework, a number of alternative DM models have recently attracted increasing attention. Examining the spatial distribution of DM provides a promising means of constraining its nature, as different models predict distinct structural signatures. We serendipitously identified a remarkable system in which a $10^{8.5}$ solar mass dwarf galaxy, J0040-SL, acts as a strong lens. As the least massive strong lensing galaxy known, J0040-SL provides a unique laboratory to place the most stringent constraints on the DM profile at these scales. To fully exploit its uniqueness, we request coordinated JWST and HST imaging observations to precisely determine the stellar content of J0040-SL, a critical input required to robustly disentangle DM from stars. These observations will enable a direct test of DM models in the dwarf galaxy regime. This is a part of our approved JWST+HST program on dissecting the least massive strong lensing galaxy ever found, J0040-SL. The multi-band imaging observations from JWST and HST will provide high-quality photometry covering from rest-frame ultraviolet (UV) to near-infrared (NIR), essential for determining the stellar mass of J0040-SL to a precision comparable to that on the total mass. For the HST, we propose the following observations: One orbit in the WFC3/UVIS F390W filter. Given the lens redshift of 0.143, this will provide high-resolution imaging data for extracting rest-frame UV photometry for J0040-SL. Based on our modeling results, an on-target exposure time of 2100s is needed to achieve the required signal-to-noise ratio. We will split the observations to four sub-exposures of equal time (525s) and use the 4-step dithering pattern (i.e. WFC3-UVIS-DITHER-BOX) to improve the sampling of the PSF and perform cosmic-ray/bad-pixel removal. Our team combines expertise in high-resolution imaging analysis, strong-lens modeling, and stellar population synthesis. We are well equipped to efficiently process the JWST and HST data, perform precise lens modeling, and derive robust stellar and dark matter mass measurements for the dwarf lensing galaxy. Prof. Christopher Fassnacht, Dr. Cheng Cheng, and Prof. Haojing Yan have extensive experience in JWST data reduction and analysis. Dr. Yiping Shu and Prof. Christopher Fassnacht have experience in HST/WFC3 data processing-the instrument corresponding to the F390W band in this proposal. Prof. Guoliang Li specializes in upsampling and PSF deconvolution coaddition algorithms (UPDC; Wang, L., Li, G., & Kang, X. 2022, MNRAS, 517, 787). The UPDC technique has been demonstrated to successfully detect low-mass strong lenses in JWST imaging (Deng, L., Shu, Y. et al. 2025). Prof. Haojing Yan and Dr. Cheng Cheng have extensive experience in SED fitting and stellar mass estimation for high-redshift galaxies (Yan et al. 2023, ApJ, 942, L9; Cheng et al. 2023, ApJ, 942, L19). Prof. Sherry H. Suyu is an expert in gravitational lensing and the developer of the widely used lens modeling software GLEE (Suyu & Halkola 2010, A&A, 524, A94; Suyu et al. 2012, ApJ, 750, 10; Schuldt et al. 2019, A&A, 631, A40). Dr. Yiping Shu has developed an automated pipeline and successfully produced lens models for hundreds of strong-lensing systems based on HST imaging (Shu et al. 2016, ApJ, 833, 264; Shu et al. 2017, ApJ, 851, 48). Hengkai Ding has experience in strong-lens modeling using pixel-level analysis with software such as PyAutoLens (Ding et al. 2025, RAA, 25, 065013). Prof. Christopher Fassnacht has extensive experience in investigating the nature of dark matter through the Strong-lensing at High Angular Resolution Program (SHARP) (e.g. Hsueh et al. 2016, MNRAS, 463, L51; Tajalli et al. 2025, MNRAS, 543, 540). Galaxies 6 false false false false false false false true SECONDARY JWST 11465 false SUPPORTED GALAXY DWARF COMPACT EINSTEIN RING GRAVITATIONAL LENS YES J2000 J2000 WFC3/UVIS Imaging No No F390W POS-TARG DITHER 0.173 LINE 0.112 false false 81.785 4 23.884 WFC3-UVIS-DITHER-BOX 1 false 1777558070647 The scheduling data was generated by CASM 34.0. CASM was run in Phase II mode. 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