18106( 3) - 23-Oct-2025 14:40:03 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18106 Version: 3 Check-in Time: 23-Oct-2025 14:40:03 STScI Edit Number: 0 Title Stellar Initial Mass Function and Dark Matter from [OII] and Paschen-alpha Emission Maps Of the Giant Dragon Arc in the Hubble Frontier Fields Galaxy Cluster Abell 370 ------------------------------------------------------------------------------------ Type Cycle GO 33 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Patrick Kelly University of Minnesota - Twin Cities Y CoI: Prof. Alex V. Filippenko University of California - Berkeley N CoI: Mr. Amruth Alfred University of Hong Kong N CoI: Dr. Jose M. Diego Instituto de Fisica de Cantabria N CoI: Prof. Jeremy Lim University of Hong Kong N CoI: Prof. Masamune Oguri Chiba University N CoI: Prof. Adi Zitrin Ben-Gurion University of the Negev N CoI: Prof. Liliya L.R. Williams University of Minnesota - Twin Cities N CoI: Dr. Hayley Madeleine Williams Arizona State University N CoI: Allison Keen University of Minnesota - Twin Cities N CoI: Dr. Fengwu Sun Harvard University N ------------------------------------------------------------------------------------ Abstract The ~30 arcsecond Dragon arc at z=0.72 created by the Abell 370 cluster is the most prolific known source of microlensing events of individual stars at a cosmological distance. A total of ~50 stars have been detected across only two ultra-deep UV-optical HST and two shallow JWST visits. Recent work finds that the observed microlensing events may exhibit a broad bias towards the negative-parity side of the critical curve, which holds the promise to distinguish among forms of substructure produced by different dark-matter candidates. Improved constraints on the location of the cluster's critical curve, however, are needed. Furthermore, the microlensing events, if they can be combined with a spatially resolved recent star-formation history, will yield constraints on the upper-end of the initial-mass function (IMF) across the giant arc. Here we propose to take advantage of a pair of coincidences between the WFC3 UVIS F645N filter and [OII] nebular emission from the Dragon arc at z=0.72, and between the JWST NIRCam F323N filter and Paschen alpha emission. Since nebular emission is not, as a rule, found at the same locations as star-forming clumps visible in broadband images, the proposed data yield a new and independent set of constraints on the path of the critical curve. Pairs of counterimages of HII regions identified by their [OII] / Paschen alpha ratio will precisely bracket the location of the critical curve. Moreover, the fluxes of HII regions should not be affected by stellar microlensing, so we can, for the first time, unambiguously detect the existence of millilensing due to dark-matter subhalos in a cluster. ------------------------------------------------------------------------------------ Observations Description ------------------------ The program will acquire a total of five orbits of WFC3 UVIS F645N narrow-band and a single orbit of WFC3 UVIS F625W observations of the Abell 370 galaxy-cluster field. The WFC3 UVIS F645N narrowband filter will have 18106( 3) - 23-Oct-2025 14:40:03 - [ 2] sensitivity to [OII] 3726, 3729 nebular emission from the Dragon arc at z=0.72. Coordinated JWST NIRCam F323N imaging corresponds to Paschen alpha emission from the arc. After subtracting continuum emission estimated from F625W for the WFC3 UVIS imaging, these will allow which will enable identification of counterimages of HII regions which constrain the location of the critical curve. For scheduling flexiblity, the total of six orbits are broken into a set of three two-orbit visits. Each visit include four dithered exposures. POS TARGS are used to accomplish dithering across the full set of three visits for F645N. Visit 2 differs from Visit 1 and Visit 3. The first orbit consists of the four F625W exposures, and the second of four F645N exposures. For each filter, we use the POS TARG values for the WFC3-UVIS-DITHER-BOX from https://hst-docs.stsci.edu/wfc3ihb/appendix-c-dithering-and-mosaicking/c-2-wfc3 -patterns The FLASH level is set to 17 for the F645N exposures, and FLASH is set to 3 for the F625W exposures. We use the UVIS1-FIX aperture to target the center of the Dragon Arc so that the arc falls away from the chip gap at all roll angles. Preferred orient is 85.2, 175.2, 265.2, or 355.2 to match diffraction spike orientation of archival F625W observations for improved subtraction. Orients are set to be the same for three visits. ------------------------------------------------------------------------------------ 18106( 3) - 23-Oct-2025 14:40:03 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 3 ABELL-370-DR CLUSTER OF GALAXIES, RA=02H39M53.0548S +/- 0.1", J2000 V = 27 AGON-ARC GRAVITATIONAL LENS DEC=-01D35'6.09" +/- 0.1" Reference Frame: ICRS Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 0.0 0.0 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 18106( 3) - 23-Oct-2025 14:40:03 - [ 4] Visit: 01 (WFC3 UVIS F645N) Visit Requirements: On Hold Comments: Additional Comments: preferred orient is 85.2, 175.2, 265.2, or 355.2 to match diffraction spike orientation of archival F625W observations for improved subtraction 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 ABELL-370-D WFC3/UVI ACCUM UVIS1-FIX F645N FLASH=17 1 1100 S POS TARG -0.241003125, RAGON-ARC S see below 1.2380015625 (1226 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 ABELL-370-D WFC3/UVI ACCUM UVIS1-FIX F645N FLASH=17 1 1100 S POS TARG 0.127926562, RAGON-ARC S see below 1.29447812 (1226 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 ABELL-370-D WFC3/UVI ACCUM UVIS1-FIX F645N FLASH=17 1 1100 S POS TARG -0.211303125, RAGON-ARC S see below -3.332098 (1213 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 ABELL-370-D WFC3/UVI ACCUM UVIS1-FIX F645N FLASH=17 1 1100 S POS TARG RAGON-ARC S see below 0.24533437500000002, (1213 S) -3.4996984375 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ ABELL-370-DRAGON-AR 1 WFC3/UVI ACCUM UVIS1-FI F645N none none none none 1 1226.0 S C S X ABELL-370-DRAGON-AR 2 WFC3/UVI ACCUM UVIS1-FI F645N none none none none 1 1226.0 S C S X ABELL-370-DRAGON-AR 3 WFC3/UVI ACCUM UVIS1-FI F645N none none none none 2 1213.0 S C S X ABELL-370-DRAGON-AR 4 WFC3/UVI ACCUM UVIS1-FI F645N none none none none 2 1213.0 S C S X 18106( 3) - 23-Oct-2025 14:40:03 - [ 5] Visit: 02 (WFC3 UVIS F625W + F645N) Visit Requirements: SAME ORIENT AS 01 On Hold Comments: Additional Comments: preferred orient is 85.2, 175.2, 265.2, or 355.2 to match diffraction spike orientation of archival F625W observations for improved subtraction 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 ABELL-370-D WFC3/UVI ACCUM UVIS1-FIX F625W FLASH=3 1 548 S (548 POS TARG 0, 0 RAGON-ARC S S) ------------------------------------------------------------------------------------------------------------------------------------ 2 ABELL-370-D WFC3/UVI ACCUM UVIS1-FIX F625W FLASH=3 1 548 S (548 POS TARG 0.158, 0.070 RAGON-ARC S S) ------------------------------------------------------------------------------------------------------------------------------------ 3 ABELL-370-D WFC3/UVI ACCUM UVIS1-FIX F625W FLASH=3 1 548 S (548 POS TARG 0.099, 0.165 RAGON-ARC S S) ------------------------------------------------------------------------------------------------------------------------------------ 4 ABELL-370-D WFC3/UVI ACCUM UVIS1-FIX F625W FLASH=3 1 548 S (548 POS TARG -0.060, 0.095 RAGON-ARC S S) ------------------------------------------------------------------------------------------------------------------------------------ 5 ABELL-370-D WFC3/UVI ACCUM UVIS1-FIX F645N FLASH=17 1 539 S (539 POS TARG 0, 0 RAGON-ARC S S) ------------------------------------------------------------------------------------------------------------------------------------ 6 ABELL-370-D WFC3/UVI ACCUM UVIS1-FIX F645N FLASH=17 1 539 S (539 POS TARG 0.158, 0.070 RAGON-ARC S S) ------------------------------------------------------------------------------------------------------------------------------------ 7 ABELL-370-D WFC3/UVI ACCUM UVIS1-FIX F645N FLASH=17 1 539 S (539 POS TARG 0.099, 0.165 RAGON-ARC S S) ------------------------------------------------------------------------------------------------------------------------------------ 8 ABELL-370-D WFC3/UVI ACCUM UVIS1-FIX F645N FLASH=17 1 539 S (539 POS TARG -0.060, 0.095 RAGON-ARC S 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 | ------------------------------------------------------------------------------------------------------------------------------------ ABELL-370-DRAGON-AR 1 WFC3/UVI ACCUM UVIS1-FI F625W none none none none 1 N/A C S X ABELL-370-DRAGON-AR 2 WFC3/UVI ACCUM UVIS1-FI F625W none none none none 1 N/A C S X ABELL-370-DRAGON-AR 3 WFC3/UVI ACCUM UVIS1-FI F625W none none none none 1 N/A C S X ABELL-370-DRAGON-AR 4 WFC3/UVI ACCUM UVIS1-FI F625W none none none none 1 N/A C S X ABELL-370-DRAGON-AR 5 WFC3/UVI ACCUM UVIS1-FI F645N none none none none 2 N/A C S X 18106( 3) - 23-Oct-2025 14:40:03 - [ 6] 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 | ------------------------------------------------------------------------------------------------------------------------------------ ABELL-370-DRAGON-AR 6 WFC3/UVI ACCUM UVIS1-FI F645N none none none none 2 N/A C S X ABELL-370-DRAGON-AR 7 WFC3/UVI ACCUM UVIS1-FI F645N none none none none 2 N/A C S X ABELL-370-DRAGON-AR 8 WFC3/UVI ACCUM UVIS1-FI F645N none none none none 2 N/A C S X 18106( 3) - 23-Oct-2025 14:40:03 - [ 7] Visit: 03 (WFC3 UVIS F645N) Visit Requirements: SAME ORIENT AS 02 On Hold Comments: Additional Comments: preferred orient is 85.2, 175.2, 265.2, or 355.2 to match diffraction spike orientation of archival F625W observations for improved subtraction 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 ABELL-370-D WFC3/UVI ACCUM UVIS1-FIX F645N FLASH=17 1 1100 S POS TARG -0.104878125, RAGON-ARC S see below 1.4966265625 (1226 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 ABELL-370-D WFC3/UVI ACCUM UVIS1-FIX F645N FLASH=17 1 1100 S POS TARG RAGON-ARC S see below -0.09667968750000001, (1226 S) 1.20078125 ------------------------------------------------------------------------------------------------------------------------------------ 3 ABELL-370-D WFC3/UVI ACCUM UVIS1-FIX F645N FLASH=17 1 1100 S POS TARG -0.0323296875, RAGON-ARC S see below -3.52989375 (1213 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 ABELL-370-D WFC3/UVI ACCUM UVIS1-FIX F645N FLASH=17 1 1100 S POS TARG RAGON-ARC S see below 0.0671343750000000, (1213 S) -3.3340984375000002 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ ABELL-370-DRAGON-AR 1 WFC3/UVI ACCUM UVIS1-FI F645N none none none none 1 1226.0 S C S X ABELL-370-DRAGON-AR 2 WFC3/UVI ACCUM UVIS1-FI F645N none none none none 1 1226.0 S C S X ABELL-370-DRAGON-AR 3 WFC3/UVI ACCUM UVIS1-FI F645N none none none none 2 1213.0 S C S X ABELL-370-DRAGON-AR 4 WFC3/UVI ACCUM UVIS1-FI F645N none none none none 2 1213.0 S C S X 18106( 3) - 23-Oct-2025 14:40:03 - [ 8] Summary Form for Proposal 18106 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS1-FIX ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=17, FLASH=3 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements POS TARG -0.0323296875, -3.52989375, POS TARG -0.060, 0.095, POS TARG -0.09667968750000001, 1.20078125, POS TARG -0.104878125, 1.4966265625, POS TARG -0.211303125, -3.332098, POS TARG -0.241003125, 1.2380015625, POS TARG 0, 0, POS TARG 0.0671343750000000, -3.3340984375000002, POS TARG 0.099, 0.165, POS TARG 0.127926562, 1.29447812, POS TARG 0.158, 0.070, POS TARG 0.24533437500000002, -3.4996984375, SAME ORIENT AS 01, SAME ORIENT AS 02 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F625W, F645N ------------------------------------------------------------------------------------------------------------------------------------ Target Names ABELL-370-DRAGON-ARC ------------------------------------------------------------------------------------------------------------------------------------