0 Assigned ID: 3251 ----- Attempting Submission 1 (Fri Mar 25 14:53:56 GMT 2022) ----- HST Phase I Proposal 3251 successfully submitted. Receipt: # 3251-1 ----- Attempting Submission 2 (Fri Mar 25 17:01:38 GMT 2022) ----- re-submission with updatet ASC-WFC-DITHER-BOX pattern 1134212126 ----- Attempting Submission 1 (Fri Jul 15 01:08:01 GMT 2022) ----- ----- Attempting Submission 2 (Tue Oct 18 09:41:47 GMT 2022) ----- DDO68 C: the actual appearance of a ghost satellite dwarf Extremely metal-poor galaxies (XMPs) in the local Universe are of fundamental interest since they provide a window on conditions similar to those of primordial galaxies. An iconic example of a nearby XMP is the gas-rich dwarf DDO68, at ~13 Mpc distance, with a metallicity of ~3% solar, discrepantly low compared with its relatively large stellar mass of 10^8 Msun. What makes DDO68 even more exceptional is the presence of two confirmed interacting satellites and a third candidate, dubbed DDO68-C, with a systemic HI velocity similar to that of DDO68, but with no confirmed distance. Indeed this "ghost" satellite has been poorly studied so far because of the presence of a bright foreground star close to its line of sight, thus very little is known about its stellar mass and stellar population. We propose to image DDO68-C for the first time with ACS/WFC in F606W (V) and F814W (I) to resolve its individual stars and derive its distance through the tip of the red giant branch. With a new robust distance for DDO68-C, the ACS data will allow us to also infer its star formation history, stellar mass, and constrain its metallicity through comparison of the empirical (V,I) color-magnitude diagrams with synthetic ones. These results will serve as input to N-body hydrodynamical simulations aimed at reconstructing the merging history of DDO68 with its satellites, and at possibly explaining its extremely low metal content. If DDO68-C turns out to be at the same distance as DDO68, this system would be a unique case of a dwarf interacting with three satellites, challenging the predictions from cosmological models for the satellite population around galaxies with the mass of DDO68. We will observe the faint galaxy DDO68C wiht the ACS WFC for ~3.5 orbits in F606W and ~2.5 orbits in F814W, for a total of 6 orbits. DDO68C is located very close to a bright foreground star, thus much attention is payed to avoid this contaminant while sampling at the same time a large enough portion of the galaxy. We organize our observations in 2 separate visits. - Vis01. This will last for 3 orbits during which we will observe the target in F606W. We choose a primary 3 point dither line pattern coupled with a 4 point dither box. For the primary dither, we adopt large 8" offsets running parallel to the gap. This is because we need artifacts from the bright foreground star to fall in different regions of the ACS detector in order to remove them during data reduction. The secondary dither will instead improve PSF sampling. We request an orient in the range 265-275 to avoid the bright foreground star. - Vis02. This will last for 3 orbits during which we will observe the target in F814W for 2.5 orbits and in F606W for 0.5 orbits. For the F814W observations, the dither is the same as in visit 1. The F606W exposures are instead acquired according to a 3 point line dither pattern as the primary one (but no secondary dither pattern is adopted). We request the same orient as in Visit 01. Due to the stringent scheduling constraints needed to execute this program, any reduction of the visibility windows from a reduced gyro operations would highly affect the possibility of completing this project. Drs Aloisi, Annibali, Bellazzini, and Correnti have strong expertise in data reduction for deep and accurate stellar photometry in crowded fields within the Local Universe. Together with Drs Cignoni, Sacchi and Tosi, they have led, or participated to, several ACS and WFC3 projects devoted to the understanding of the formation and evolution of dwarf galaxies and to the analysis of their star clusters, streams and satellites. We pioneered (Tosi et al 1991) the method now widely used worldwide to derive the star formation history of a galactic region from the color-magnitude diagram of its resolved stellar populations (see Tolstoy, Hill & Tosi 2009 ARAA 47, 371 for a review), and updated and improved it over the years (e.g. Cignoni & Tosi 2010, Cignoni et al 2015). In the last decade, our team has published more than 70 refereed papers based on proprietary HST data. Prof. J. Cannon is an expert on HI studies of dwarf galaxies, and discovered DDO68C, which is the target of this proposal. Our work on DDO68 itself (Sacchi+ 2016, Annibali+ 2016, Annibali+ 2019), and, e.g., on IZw18 (Aloisi+ 2007, Fiorentino+ 2013, Annibali+ 2013) at a distance of ~18 Mpc, i.e. 5 Mpc farther away than DDO68, demonstrates the feasibility of our project. The CMDs of our target galaxy will be obtained and analyzed exactly as those shown in Fig.1 of the proposal and obtained with the same observational setup. Galaxies Stellar Populations and the Interstellar Medium 6 false false false false false false false SUPPORTED GALAXY AMORPHOUS IRREGULAR J2000 J2000 ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F606W POS-TARG DITHER DITHER 4 8 primary dither: 3 point dither line with large 8" offsets runnin parallel to the gap (purpose is to have artifacts from the bright foreground stars falling within different region of the ACS) secondary dither: 4 point dither box (to increase PSF sampling) ACS-WFC-DITHER-BOX false 69.02 false 0.1856 0.2637 3 0 POS-TARG ACS-WFC-DITHER-LINE 20.7 1 true POS-TARG DITHER 8 3 point dither line with large 8" offsets, running parallel to the gap (no secondary diher) LINE false false 3 0 ACS-WFC-DITHER-LINE 2 false /private/var/folders/d6/qjdtq_qj59j5zw472sjbg00r0000gp/T/OPVisit14753577118802111652.xml The scheduling data was generated by CASM 30.3. CASM was run in Phase II mode. 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CASM was run in Phase II mode. 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