17131( 3) - 18-Oct-2022 13:02:49 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 17131 Version: 3 Check-in Time: 18-Oct-2022 13:02:49 STScI Edit Number: 0 Title DDO68 C: the actual appearance of a ghost satellite dwarf ------------------------------------------------------------------------------------ Type Cycle GO 30 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Francesca Annibali Y CoI: Dr. Michele Bellazzini N CoI: Dr. Michele Cignoni Universita di Pisa N CoI: Dr. Matteo Correnti INAF - Osservatorio Astronomico di Roma N CoI: Dr. Elena Sacchi Leibniz-Institut für Astrophysik Potsdam N (AIP) CoI: Dr. Alessandra Aloisi Space Telescope Science Institute N CoI: Dr. Raffaele Pascale INAF-Osservatorio di Astrofisica e Scienza N dello Spazio CoI: Dr. Leslie Hunt INAF - Osservatorio Astrofisico di Arcetri N CoI: Prof. John Michael Cannon Macalester College N CoI: Dr. Monica Tosi N ------------------------------------------------------------------------------------ Abstract 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. ------------------------------------------------------------------------------------ Observations Description ------------------------ We will observe the faint galaxy DDO68C wiht the ACS WFC for ~3.5 orbits in 17131( 3) - 18-Oct-2022 13:02:49 - [ 2] 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. ------------------------------------------------------------------------------------ 17131( 3) - 18-Oct-2022 13:02:49 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 DDO68-C GALAXY, AMORPHOUS RA=09H56M32.6024S, J2000 V = 16 IRREGULAR DEC=+29D00'30.81" Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 17131( 3) - 18-Oct-2022 13:02:49 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-LINE ACS-WFC-DITHER-BOX Pattern_Purpose DITHER DITHER Number_Of_Points 3 4 Point_Spacing 8 0.2637 Line_Spacing 0.1856 Coordinate_Frame POS-TARG POS-TARG Pattern_Orient 0 20.7 Angle_Between_Sides 69.02 Center_Pattern NO NO Pattern_Comments: 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) ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-LINE Pattern_Purpose DITHER Number_Of_Points 3 Point_Spacing 8 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 0 Angle_Between_Sides Center_Pattern NO Pattern_Comments: 3 point dither line with large 8" offsets, running parallel to the gap (no secondary diher) ------------------------------------------------------------------------------------------------------------------------------------ 17131( 3) - 18-Oct-2022 13:02:49 - [ 5] Visit: 01 (DDO68C_vis01_V) Visit Requirements: ORIENT 265D TO 275 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 DDO68-C ACS/WFC ACCUM WFC F606W 1 550 S PATTERN 1 1 see below (6406 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 | ------------------------------------------------------------------------------------------------------------------------------------ DDO68-C 1 ACS/WFC ACCUM WFC F606W 1 1 none none 1 549.0 S DDO68-C 1 ACS/WFC ACCUM WFC F606W 1 2 none none 1 549.0 S DDO68-C 1 ACS/WFC ACCUM WFC F606W 1 3 none none 1 549.0 S DDO68-C 1 ACS/WFC ACCUM WFC F606W 1 4 none none 1 439.0 S DDO68-C 1 ACS/WFC ACCUM WFC F606W 2 1 none none 2 N/A DDO68-C 1 ACS/WFC ACCUM WFC F606W 2 2 none none 2 N/A DDO68-C 1 ACS/WFC ACCUM WFC F606W 2 3 none none 2 N/A DDO68-C 1 ACS/WFC ACCUM WFC F606W 2 4 none none 2 510.0 S DDO68-C 1 ACS/WFC ACCUM WFC F606W 3 1 none none 3 N/A DDO68-C 1 ACS/WFC ACCUM WFC F606W 3 2 none none 3 N/A DDO68-C 1 ACS/WFC ACCUM WFC F606W 3 3 none none 3 N/A DDO68-C 1 ACS/WFC ACCUM WFC F606W 3 4 none none 3 510.0 S 17131( 3) - 18-Oct-2022 13:02:49 - [ 6] Visit: 02 (DDO68C_vis02_I) Visit Requirements: SAME ORIENT AS 01 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 DDO68-C ACS/WFC ACCUM WFC F814W 1 350 S PATTERN 1 1 see below (4676 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 DDO68-C ACS/WFC ACCUM WFC F606W 1 350 S PATTERN 2 2 see below (1149 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 | ------------------------------------------------------------------------------------------------------------------------------------ DDO68-C 1 ACS/WFC ACCUM WFC F814W 1 1 none none 1 384.0 S DDO68-C 1 ACS/WFC ACCUM WFC F814W 1 2 none none 1 384.0 S DDO68-C 1 ACS/WFC ACCUM WFC F814W 1 3 none none 1 384.0 S DDO68-C 1 ACS/WFC ACCUM WFC F814W 1 4 none none 1 384.0 S DDO68-C 1 ACS/WFC ACCUM WFC F814W 2 1 none none 1 384.0 S DDO68-C 1 ACS/WFC ACCUM WFC F814W 2 2 none none 2 398.0 S DDO68-C 1 ACS/WFC ACCUM WFC F814W 2 3 none none 2 398.0 S DDO68-C 1 ACS/WFC ACCUM WFC F814W 2 4 none none 2 398.0 S DDO68-C 1 ACS/WFC ACCUM WFC F814W 3 1 none none 2 398.0 S DDO68-C 1 ACS/WFC ACCUM WFC F814W 3 2 none none 2 398.0 S DDO68-C 1 ACS/WFC ACCUM WFC F814W 3 3 none none 3 383.0 S DDO68-C 1 ACS/WFC ACCUM WFC F814W 3 4 none none 3 383.0 S DDO68-C 2 ACS/WFC ACCUM WFC F606W 1 none none none 3 383.0 S DDO68-C 2 ACS/WFC ACCUM WFC F606W 2 none none none 3 383.0 S DDO68-C 2 ACS/WFC ACCUM WFC F606W 3 none none none 3 383.0 S 17131( 3) - 18-Oct-2022 13:02:49 - [ 7] Summary Form for Proposal 17131 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFC ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 265D TO 275 D, PATTERN 1 1, PATTERN 2 2, SAME ORIENT AS 01 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names DDO68-C ------------------------------------------------------------------------------------------------------------------------------------