17798( 3) - 15-Aug-2024 12:58:34 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 17798 Version: 3 Check-in Time: 15-Aug-2024 12:58:34 STScI Edit Number: 0 Title Two new isolated, faint dwarf galaxies beyond the Local Group ------------------------------------------------------------------------------------ Type Cycle GO 32 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Burcin Mutlu-Pakdil Dartmouth College Y CoI: Prof. David J. Sand University of Arizona N CoI: Dr. Denija Crnojevic University of Tampa N CoI: Prof. Dennis Zaritsky University of Arizona N CoI: Dr. Michael Gordon Jones University of Arizona N CoI: Dr. Paul Bennet Space Telescope Science Institute N CoI: Dr. Ananthan Karunakaran University of Toronto N CoI: Dr. Catherine Fielder University of Arizona N CoI: Dr. Kristine Spekkens Royal Military College of Canada N CoI: Dr. Laura Congreve Hunter Dartmouth College N CoI: Dr. Amandine Doliva-Dolinksy University of Tampa N CoI: Richard Donnerstein University of Arizona N ------------------------------------------------------------------------------------ Abstract The Local Group satellite dwarf galaxies are often used as benchmarks for testing galaxy formation and evolution theories on the smallest scales because they are the lowest-mass galaxies for which detailed observations are available. However, most of these dwarfs have been affected by the Local Group environment. If we want to understand the physical mechanisms driving the evolution of low-mass galaxies, we must study pristine isolated dwarf galaxies beyond the Local Group as a control sample. Unfortunately, identifying such galaxies in the field is extremely challenging due to their extreme faintness. To date, Tucana B is the only known isolated, ultra-faint dwarf galaxy that is independent of group dynamics but close enough at 1.4 Mpc to be studied with resolved stars. With only one system, it is challenging to draw broad conclusions about the lowest-mass galaxies evolving in isolation. We request two orbits of HST/ACS imaging (F606W+F814W) to follow up two new nearby isolated faint dwarf galaxies (Cetus B and Hydrus A) discovered in ground-based surveys. The galaxies are likely just beyond the edge of the Local Group, with apparent morphologies and colors strikingly similar to Tucana B. HST imaging is essential to the determination of high-precision distances, measurement of basic physical properties, and measurement of star formation histories of these two newly discovered faint dwarfs. The requested observations will put these two galaxies into context with other dwarf galaxies in and beyond the Local Group, providing a rare opportunity to assess the impact of environment on low-mass systems. ------------------------------------------------------------------------------------ Observations Description ------------------------ The observations are going to be performed with ACS/WFC (primary) and WFC3/UVIS (parallel) to match previous observations of faint dwarfs beyond the Local 17798( 3) - 15-Aug-2024 12:58:34 - [ 2] Group (e.g. Weisz et al. 2014, Hargis et al. 2020, Danieli et al. 2017, Bennet et al. 2019, Crnojevic et al. 2019, Mutlu-Pakdil et al 2022). We request imaging in F606W and F814W filters (broad V and I) for both instruments, in order to compute accurate TRGB distances for our targets and characterize their stellar content, while optimizing the exposure times. The parallel pointings will allow us to observe a field region next to our targets. We will thus be able to estimate more precisely the number of background/foreground contaminants present in our ACS fields. Given the small size of each target (<1 arcmin), one pointing is sufficient to cover the entire spatial extent of each galaxy. Each dwarf will be placed on one of the ACS/WFC chips, and small dithers between exposures will remove detector defects and improve the PSF sampling. Given that the possible distance range for our targets is ~1-3 Mpc, we use existing HST ACS data of Local Volume dwarfs at <3 Mpc (NGC253 dwarfs [GO-14259, GO-15938], CenA dwarfs [GO-13856], MADCASH dwarfs [GO-15228]) to guide our observational request and ensure that the depth is sufficient for the science goals of this proposal. The archival ACS data for similar limiting magnitudes were obtained with 2 CR-SPLIT exposures of 1200 sec in F606W and 900 sec in F814W. The required exposure times nicely fit into one orbit per object after taking into account realistic overheads. With these exposures, the achieved SNR given by the ACS ETC is ~10 at 1 mag below the TRGB for both bands (assuming a distance of 3~Mpc), with a limiting magnitude of ~27 in F814W, thus fulfilling our requirements. This translates into only a total of three orbits for our three dwarfs. We have checked that the charge transfer efficiency losses of ACS do not significantly affect our observations (at most 10% loss of flux). Our visits are 1-orbit, and we have no ORIENT requirements. Nor does this program have time constraints (other than the baseline excitement of the science!). ------------------------------------------------------------------------------------ 17798( 3) - 15-Aug-2024 12:58:34 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 CETUSB GALAXY, DWARF RA=00H40M34.8884S, J2000 V = 21 +/- 0.1 SPHEROIDAL DEC=-20D33'21.88" Reference Frame: ICRS Extended: YES ------------------------------------------------------------------------------------------------------------------------------------ 2 HYDRUSA GALAXY, DWARF RA=03H44M46.5718S, J2000 V = 21 +/- 0.1 SPHEROIDAL DEC=-70D21'17.73" Reference Frame: ICRS Extended: YES ------------------------------------------------------------------------------------------------------------------------------------ 17798( 3) - 15-Aug-2024 12:58:34 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-LINE Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 0.364 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 47.23 Angle_Between_Sides Center_Pattern NO Pattern_Comments: Two-point dither. No need to fill chip gap. ------------------------------------------------------------------------------------------------------------------------------------ 17798( 3) - 15-Aug-2024 12:58:34 - [ 5] Visit: 01 (CetusB) 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 CETUSB ACS/WFC ACCUM WFC1 F606W 1 400 S PATTERN 1 1 see below (948 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 CETUSB ACS/WFC ACCUM WFC1 F814W 1 400 S PATTERN 1 2 see below (948 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 | ------------------------------------------------------------------------------------------------------------------------------------ CETUSB 1 ACS/WFC ACCUM WFC1 F606W 1 none none none 1 474.0 S CETUSB 1 ACS/WFC ACCUM WFC1 F606W 2 none none none 1 474.0 S CETUSB 2 ACS/WFC ACCUM WFC1 F814W 1 none none none 1 474.0 S CETUSB 2 ACS/WFC ACCUM WFC1 F814W 2 none none none 1 474.0 S 17798( 3) - 15-Aug-2024 12:58:34 - [ 6] Visit: 02 (HydrusA) 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 HYDRUSA ACS/WFC ACCUM WFC2 F606W 1 400 S PATTERN 1 1 see below (1026 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 HYDRUSA ACS/WFC ACCUM WFC2 F814W 1 400 S PATTERN 1 2 see below (1026 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 | ------------------------------------------------------------------------------------------------------------------------------------ HYDRUSA 1 ACS/WFC ACCUM WFC2 F606W 1 none none none 1 513.0 S HYDRUSA 1 ACS/WFC ACCUM WFC2 F606W 2 none none none 1 513.0 S HYDRUSA 2 ACS/WFC ACCUM WFC2 F814W 1 none none none 1 513.0 S HYDRUSA 2 ACS/WFC ACCUM WFC2 F814W 2 none none none 1 513.0 S 17798( 3) - 15-Aug-2024 12:58:34 - [ 7] Summary Form for Proposal 17798 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFC1, WFC2 ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 1, PATTERN 1 2 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names CETUSB, HYDRUSA ------------------------------------------------------------------------------------------------------------------------------------