18241( 2) - 30-Apr-2026 14:49:00 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18241 Version: 2 Check-in Time: 30-Apr-2026 14:49:00 STScI Edit Number: 1 Title Unveiling the dark matter structure of the least massive strong lensing galaxy ever found ------------------------------------------------------------------------------------ Type Cycle GO 33 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Yiping Shu Purple Mountain Observatory, CAS Y CoI: Prof. Christopher Fassnacht University of California - Davis N CoI: Hengkai Ding Purple Mountain Observatory, CAS N CoI: Cheng Cheng The National Astronomical Observatories of N the Chinese Acade CoI: Prof. Haojing Yan University of Missouri - Columbia N CoI: Prof. Sherry H. Suyu Technical University of Munich N CoI: Prof. Guoliang Li Purple Mountain Observatory, CAS N ------------------------------------------------------------------------------------ Abstract 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. ------------------------------------------------------------------------------------ Observations Description ------------------------ 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 18241( 2) - 30-Apr-2026 14:49:00 - [ 2] cosmic-ray/bad-pixel removal. ------------------------------------------------------------------------------------ 18241( 2) - 30-Apr-2026 14:49:00 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 J0040+2555 GALAXY, DWARF COMPACT, RA=00H40M18.2600S, J2000 Z = V = 23.0 +/- 0.1 EINSTEIN RING, DEC=+25D55'35.88" 0.143 GRAVITATIONAL LENS Reference Frame: ICRS Extended: YES ------------------------------------------------------------------------------------------------------------------------------------ 18241( 2) - 30-Apr-2026 14:49:00 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.173 Line_Spacing 0.112 Coordinate_Frame POS-TARG Pattern_Orient 23.884 Angle_Between_Sides 81.785 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 18241( 2) - 30-Apr-2026 14:49:00 - [ 5] Visit: 01 Visit Requirements: ORIENT 185D TO 255 D; ORIENT 5D TO 75 D On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 J0040+2555 WFC3/UVI ACCUM UVIS2-C1K F390W FLASH=16 1 525 S PATTERN 1 1 S 1C-CTE (2100 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ J0040+2555 1 WFC3/UVI ACCUM UVIS2-C1 F390W 1 none none none 1 N/A S K1C-CTE J0040+2555 1 WFC3/UVI ACCUM UVIS2-C1 F390W 2 none none none 1 N/A S K1C-CTE J0040+2555 1 WFC3/UVI ACCUM UVIS2-C1 F390W 3 none none none 1 N/A S K1C-CTE J0040+2555 1 WFC3/UVI ACCUM UVIS2-C1 F390W 4 none none none 1 N/A S K1C-CTE 18241( 2) - 30-Apr-2026 14:49:00 - [ 6] Summary Form for Proposal 18241 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2-C1K1C-CTE ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=16 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 185D TO 255 D, ORIENT 5D TO 75 D, PATTERN 1 1 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F390W ------------------------------------------------------------------------------------------------------------------------------------ Target Names J0040+2555 ------------------------------------------------------------------------------------------------------------------------------------