0 Assigned ID: 7592 ----- Attempting Submission 1 (Thu Apr 16 19:59:12 GMT 2026) ----- The resubmission is done after updating the porposal for the "orentation contrasints" requested, following the negative decision made by the HST-TTRB for such a request. 1177228614 ----- Attempting Submission 1 (Thu Aug 06 17:30:21 GMT 2026) ----- HST Phase II Proposal 18351 successfully submitted. Receipt: # 18351-1 ----- Attempting Submission 2 (Mon Aug 17 14:23:35 GMT 2026) ----- A New Faint Companion to the Fornax Dwarf Galaxy? 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 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. 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. The team brings extensive expertise in wide-field imaging, resolved stellar populations, and HST data analysis, ensuring the successful execution and interpretation of the proposed observations. The PI has experience in wide-field optical and near-infrared imaging (Saifollahi), complemented by leading expertise in extragalactic globular clusters and stellar populations (Lancon), and in imaging of resolved stellar populations (Ferguson). Extensive expertise in HST photometry of star cluster systems is provided by several team members (Annibali, Bellazzini, Crociati, Howell, Larsen, Libralato, Massari). Direct experience with HST observations and analysis of dwarf galaxies and their satellites, including systems at the lowest mass scales, is contributed by other members (Gatto, Ibata, Martin, Martinez-Delgado, Marleau, Niederhofer, Sakowska, Voggel). Expertise in distance determination and stellar population modeling, including TRGB and SBF methods, is provided by Cantiello, while advanced analysis of color-magnitude diagrams using Bayesian techniques is contributed by Valls-Gabaud. The team represents experts within the Euclid Consortium in the study of resolved stellar systems and photometric analysis. Galaxies 6 false false false false false false false SUPPORTED STELLAR CLUSTER UNDESIGNATED YES 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. J2000 J2000 WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W POS-TARG DITHER 5.02 LINE 5.02 false false 81.785 4 23.884 WFC3-UVIS-DITHER-BOX 1 false /private/var/folders/sj/ww4d6d1167x7cpxddyzg59dw0000gp/T/OPVisit10988185291893357197.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18351 :visit-id "01" :proposal-version -1 :trans-version "20260618" :date "17-Aug-2026 16:21:35.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1154976 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18351 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18351 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2606 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2987 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18351 :visit-id "01" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2606 :al-overhead-at-end 116 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2987 :pcs-gap 0 ) true 2 0 /private/var/folders/sj/ww4d6d1167x7cpxddyzg59dw0000gp/T/OPVisit2259188874609839829.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18351 :visit-id "02" :proposal-version -1 :trans-version "20260618" :date "17-Aug-2026 16:21:35.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1154976 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18351 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18351 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2606 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2987 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18351 :visit-id "02" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2606 :al-overhead-at-end 116 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2987 :pcs-gap 0 ) true 2 0 /private/var/folders/sj/ww4d6d1167x7cpxddyzg59dw0000gp/T/OPVisit5894490497930338098.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18351 :visit-id "03" :proposal-version -1 :trans-version "20260618" :date "17-Aug-2026 16:21:36.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1154976 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18351 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18351 :visit-id "03" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2606 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2987 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18351 :visit-id "03" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2606 :al-overhead-at-end 116 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2987 :pcs-gap 0 ) true 2 0