18351( 3) - 17-Aug-2026 15:25:52 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18351 Version: 3 Check-in Time: 17-Aug-2026 15:25:52 STScI Edit Number: 0 Title A New Faint Companion to the Fornax Dwarf Galaxy? ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Teymoor Saifollahi Observatoire Astronomique de Strasbourg Y CoI: Prof. Ariane Lancon Universite de Strasbourg I N CoI: Prof. Annette Ferguson University of Edinburgh, Institute for N Astronomy CoI: Dr. Chiara Crociati University of Edinburgh, Institute for N Astronomy CoI: Ms. Jess Howell University of Edinburgh, Institute for N Astronomy CoI: Dr. Mattia Libralato INAF - Osservatorio Astronomico di Padova N CoI: Dr. Nicolas Martin Observatoire Astronomique de Strasbourg N CoI: Dr. Pierre-Alain Duc Observatoire Astronomique de Strasbourg N CoI: Dr. Rodrigo Ibata Universite de Strasbourg I N CoI: Dr. Karina Voggel Observatoire Astronomique de Strasbourg N CoI: Dr. Michele Cantiello INAF - Osservatorio Astronomico d'Abruzzo N CoI: Dr. Massimiliano Gatto INAF - Osservatorio Astronomico di N Capodimonte CoI: Prof. Francine Marleau University of Innsbruck N CoI: Dr. Michele Bellazzini INAF - Osservatorio di Astrofisica e N Scienza dello Spazio CoI: Dr. Francesca Annibali INAF - Osservatorio di Astrofisica e N Scienza dello Spazio CoI: Dr. Florian Niederhofer Leibniz-Institut für Astrophysik Potsdam N (AIP) CoI: Dr. Soeren S. Larsen Radboud University Nijmegen N CoI: Dr. Davide Massari INAF - Osservatorio di Astrofisica e N Scienza dello Spazio CoI: David Valls-Gabaud Observatoire de Paris N CoI: Dr. Joanna Diana Sakowska Instituto de Astrofisica de Andalucia N (IAA) CoI: Dr. David Martinez-Delgado Centro de Estudios de Fisica del Cosmos de N Aragon ------------------------------------------------------------------------------------ Abstract Thanks to the unprecedented depth and precision of the Euclid wide-field survey, we have identified a new faint stellar system, resolved into individual stars, in the vicinity of the Fornax dSph galaxy, hereafter Fornax-7. The available data suggest that Fornax-7 may be physically associated with Fornax; however, they are insufficient to determine its nature conclusively. The most effective approach to address this question is to obtain deep optical imaging with HST. Achieving photometric quality comparable to existing HST observations of Fornax globular clusters will allow us to (i) measure the distance 18351( 3) - 17-Aug-2026 15:25:52 - [ 2] to Fornax-7 with high precision and establish whether it is located at the distance of Fornax, and (ii) probe its stellar population in significantly greater detail than currently possible, providing strong constraints on its age and metallicity. Fornax-7 may represent either a very remote star cluster associated with Fornax, a dwarf satellite of Fornax (i.e., a "satellite of a satellite"), or an independent ultra-faint galaxy in the Local Group. In all cases, it constitutes an exceptionally interesting system that probes the lowest-mass regime of galaxy formation and the structure of dwarf galaxy halos. ------------------------------------------------------------------------------------ Observations Description ------------------------ Fornax-7 is a recently discovered faint and compact stellar system identified in deep Euclid imaging, located in the vicinity of the Fornax dwarf spheroidal galaxy. Current Euclid observations indicate that it is an old, metal-poor stellar population, but are insufficient to determine whether the system is physically associated with Fornax. If confirmed, Fornax-7 may represent either an extremely low-mass remote star cluster or a dark matter-dominated satellite of the Fornax dSph ("satellite of a satellite"). Alternatively, it could be an independent ultra-faint dwarf galaxy in the Local Group. The primary objective of this program is to obtain a deep F606W-F814W colour-magnitude diagram extending approximately 2.5 magnitudes below the main-sequence turn-off, enabling an accurate determination of the distance modulus and significantly improved constraints on the age and metallicity of the stellar population. These observations will establish whether Fornax-7 is consistent with being associated with the Fornax dSph and provide the basis for distinguishing between the proposed scenarios. The observations consist of six HST orbits divided into three independent two-orbit visits: 1 visit in F606W (2 ORBITS IN TOTAL) and two visits in F814W (4 ORBITS IN TOTAL). The target is positioned on the WFC3/UVIS1 detector, well away from the inter-chip gap. Each visit consists of four science exposures of approximately 1200s (two exposures per orbit), providing a total integration time of about 4800 s per visit. A 5 arcsec box dither pattern is employed between successive exposures to improve cosmic-ray rejection, mitigate detector defects, and enhance image quality while keeping the target on the same detector. No UVIS post-flash is requested. ETC calculations predict a total natural background (sky + dark current) of approximately 58 e/pixel in F606W and 34 e/pixel in F814W for the adopted exposure times (~1200s), both exceeding the recommended minimum background of ~20 e/pixel for optimal CTE performance. ------------------------------------------------------------------------------------ 18351( 3) - 17-Aug-2026 15:25:52 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 FORNAX-7 STELLAR CLUSTER, RA=02H35M17.8000S, J2000 V = 21.1 +/- 0.2 UNDESIGNATED DEC=-33D57'23.30" Reference Frame: ICRS Extended: YES Comments: The object is "spatially extended", meaning that it is a collection of point sources within 30". The V-band magnitude corresponds to the estimated total magnitude of the object. 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 ------------------------------------------------------------------------------------------------------------------------------------ 18351( 3) - 17-Aug-2026 15:25:52 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 5.02 Line_Spacing 5.02 Coordinate_Frame POS-TARG Pattern_Orient 23.884 Angle_Between_Sides 81.785 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 18351( 3) - 17-Aug-2026 15:25:52 - [ 5] Visit: 01 (Fornax-7-Visit01) Visit Requirements: 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 FORNAX-7 WFC3/UVI ACCUM UVIS1-FIX F606W 1 1200 S PATTERN 1 1 S see below (4914 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 | ------------------------------------------------------------------------------------------------------------------------------------ FORNAX-7 1 WFC3/UVI ACCUM UVIS1-FI F606W 1 none none none 1 1235.0 S S X FORNAX-7 1 WFC3/UVI ACCUM UVIS1-FI F606W 2 none none none 1 1235.0 S S X FORNAX-7 1 WFC3/UVI ACCUM UVIS1-FI F606W 3 none none none 2 1222.0 S S X FORNAX-7 1 WFC3/UVI ACCUM UVIS1-FI F606W 4 none none none 2 1222.0 S S X 18351( 3) - 17-Aug-2026 15:25:52 - [ 6] Visit: 02 (Fornax-7-Visit02) Visit Requirements: 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 FORNAX-7 WFC3/UVI ACCUM UVIS1-FIX F814W 1 1200 S PATTERN 1 1 S see below (4914 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 | ------------------------------------------------------------------------------------------------------------------------------------ FORNAX-7 1 WFC3/UVI ACCUM UVIS1-FI F814W 1 none none none 1 1235.0 S S X FORNAX-7 1 WFC3/UVI ACCUM UVIS1-FI F814W 2 none none none 1 1235.0 S S X FORNAX-7 1 WFC3/UVI ACCUM UVIS1-FI F814W 3 none none none 2 1222.0 S S X FORNAX-7 1 WFC3/UVI ACCUM UVIS1-FI F814W 4 none none none 2 1222.0 S S X 18351( 3) - 17-Aug-2026 15:25:52 - [ 7] Visit: 03 (Fornax-7-Visit03) Visit Requirements: 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 FORNAX-7 WFC3/UVI ACCUM UVIS1-FIX F814W 1 1200 S PATTERN 1 1 S see below (4914 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 | ------------------------------------------------------------------------------------------------------------------------------------ FORNAX-7 1 WFC3/UVI ACCUM UVIS1-FI F814W 1 none none none 1 1235.0 S S X FORNAX-7 1 WFC3/UVI ACCUM UVIS1-FI F814W 2 none none none 1 1235.0 S S X FORNAX-7 1 WFC3/UVI ACCUM UVIS1-FI F814W 3 none none none 2 1222.0 S S X FORNAX-7 1 WFC3/UVI ACCUM UVIS1-FI F814W 4 none none none 2 1222.0 S S X 18351( 3) - 17-Aug-2026 15:25:52 - [ 8] Summary Form for Proposal 18351 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS1-FIX ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 1 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names FORNAX-7 ------------------------------------------------------------------------------------------------------------------------------------