15332( 3) - 23-Feb-2018 13:11:25 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 15332 Version: 3 Check-in Time: 23-Feb-2018 13:11:25 STScI Edit Number: 0 Title Unveiling the extreme nature of the hyper faint galaxy Virgo I ------------------------------------------------------------------------------------ Type Cycle GO 25 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Denija Crnojevic Texas Tech University Y CoI: Prof. David J. Sand University of Arizona N CoI: Prof. Thomas J. Maccarone Texas Tech University N ------------------------------------------------------------------------------------ Abstract We request HST/ACS imaging to obtain a deep color-magnitude diagram of the newly discovered candidate Milky Way satellite Virgo I. With an estimated absolute magnitude of only M_V~-0.8 and a Galactocentric radius of ~90 kpc, Virgo I is one of the faintest and most distant dwarfs ever observed, and could be identified as a prototype ``hyper'' faint galaxy. The detailed characterization of the smallest inhabited dark matter subhalos is crucial to guide hierarchical galaxy formation models, and in particular to constrain reionization, the nature of the dark matter particle, etc. With the advent of deep wide-field, ground-based surveys, the potential of uncovering these lowest-mass galaxies is quickly turning into reality, as demonstrated by the discovery in the past two years of tens of new Local Group members in the ultra-faint regime (M_V>-8). Virgo I represents a new record in galaxy physical properties, and urges us to be prepared for the likely emergence of an entirely new class of such objects in the era of future wide-field surveys (e.g., LSST). Only high resolution HST observations can enable us to confirm the nature of Virgo I, providing significantly more accurate estimates for its distance and structural properties, when compared to the discovery Subaru/HyperSuprimeCam imaging. Our proposed dataset will constitute a fundamental step in the upcoming hunt for galaxies with similarly extreme properties. ------------------------------------------------------------------------------------ Observations Description ------------------------ The observations are going to be performed with ACS/WFC (primary) and WFC3/UVIS (parallel). We request imaging in F606W and F814W filters (broad V and I) for both instruments, in order to derive an accurate MSTO distance for our target, resolve a significantly larger number of stars and derive improved structural parameters with respect to the ground-based discovery data, while optimizing the exposure times. The parallel pointing will allow us to observe a field region next to our target, which is extended and completely fills the ACS field of view: we will thus be able to estimate more precisely the number of background/foreground contaminants present in our ACS field. Our goal is to obtain a SNR of ~100 at ~0.5 mag below the MSTO, i.e. at F814W~23 and F606W~23.5, in order to derive a precise distance measurement and structural properties for our target. With the requested SNR, we will obtain a photometric uncertainty as low as ~0.02 mag at the MSTO, and we will be able to improve the discovery measurements by a factor of ~10. Our goal requires ~2400 15332( 3) - 23-Feb-2018 13:11:25 - [ 2] sec in the F606W filter and ~2300 sec in the F814W filter, which nicely fit into one orbit per filter after taking into account overheads, and subdividing the exposures into three dithered observations to remove detector defects and improve the PSF sampling. We combine the two orbits into one visit in order to maintain the same orientation and thus target the same stars in both filters. ------------------------------------------------------------------------------------ 15332( 3) - 23-Feb-2018 13:11:25 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 VIRGOI GALAXY, DWARF RA=12H00M12.0000S, J2000 V = 23 SPHEROIDAL, DEC=-00D40'51.60" UNDESIGNATED Reference Frame: ICRS Extended: NO Comments: MSTO apparent magnitude. ------------------------------------------------------------------------------------------------------------------------------------ 15332( 3) - 23-Feb-2018 13:11:25 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-LINE Pattern_Purpose DITHER Number_Of_Points 3 Point_Spacing 3.034 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 85.29 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 15332( 3) - 23-Feb-2018 13:11:25 - [ 5] Visit: 01 (Virgo I) Visit Requirements: ORIENT 330D TO 145 D; ORIENT 165D TO 318 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 VIRGOI ACS/WFC ACCUM WFC F814W 1 1200 S PATTERN 1 1-2; PAR 2 see below WITH 1 (2324 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 ANY WFC3/UVI ACCUM UVIS1 F814W 1 1200 S S see below (2560 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 VIRGOI ACS/WFC ACCUM WFC F606W 1 900 S PATTERN 1 3-4; PAR 4 see below WITH 3 (2428 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 ANY WFC3/UVI ACCUM UVIS1 F606W 1 900 S S see below (2689 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 | ------------------------------------------------------------------------------------------------------------------------------------ VIRGOI 1 ACS/WFC ACCUM WFC F814W 1 none none none 1 790.0 S VIRGOI 1 ACS/WFC ACCUM WFC F814W 2 none none none 1 830.0 S VIRGOI 1 ACS/WFC ACCUM WFC F814W 3 none none none 1 704.0 S ANY 2 WFC3/UVI ACCUM UVIS1 F814W 1 none none none 1 865.0 S S ANY 2 WFC3/UVI ACCUM UVIS1 F814W 2 none none none 1 865.0 S S ANY 2 WFC3/UVI ACCUM UVIS1 F814W 3 none none none 1 830.0 S S VIRGOI 3 ACS/WFC ACCUM WFC F606W 1 none none none 2 860.0 S VIRGOI 3 ACS/WFC ACCUM WFC F606W 2 none none none 2 875.0 S VIRGOI 3 ACS/WFC ACCUM WFC F606W 3 none none none 2 693.0 S ANY 4 WFC3/UVI ACCUM UVIS1 F606W 1 none none none 2 940.0 S S ANY 4 WFC3/UVI ACCUM UVIS1 F606W 2 none none none 2 930.0 S S ANY 4 WFC3/UVI ACCUM UVIS1 F606W 3 none none none 2 819.0 S S 15332( 3) - 23-Feb-2018 13:11:25 - [ 6] Summary Form for Proposal 15332 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS1, WFC ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC, WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 165D TO 318 D, ORIENT 330D TO 145 D, PAR 2 WITH 1, PAR 4 WITH 3, PATTERN 1 1-2, PATTERN 1 3-4 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names ANY, VIRGOI ------------------------------------------------------------------------------------------------------------------------------------