15426( 2) - 12-Dec-2017 17:11:55 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 15426 Version: 2 Check-in Time: 12-Dec-2017 17:11:55 STScI Edit Number: 0 Title Characterizing the first Ultra-Compact Dwarf with a resolved extended halo ------------------------------------------------------------------------------------ Type Cycle GO 25 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Denija Crnojevic Texas Tech University Y CoI: Prof. David J. Sand University of Arizona N CoI: Dr. Nelson Caldwell Smithsonian Institution Astrophysical N Observatory CoI: Prof. Anil C. Seth University of Utah N CoI: Prof. Jay Strader Michigan State University N CoI: Dr. Karina Voggel University of Utah N ------------------------------------------------------------------------------------ Abstract We request HST/WFC3 imaging to obtain a deep color-magnitude diagram of CenA-MM-UCD1: this peculiar ultra-compact dwarf (UCD) is the first reported UCD with a partially resolved extended halo, and it was serendipitously discovered within our Magellan/Megacam survey of the closest elliptical CenA. The origin of UCD is still uncertain, and mounting observational evidence suggests that these objects could be tidally stripped nuclei of more massive galaxies. CenA-MM-UCD1 represents the missing link in the evolutionary sequence between a nuclear star cluster (pre-stripping phase) and a ``normal'' UCD (post-stripping phase): characterizing the nature of CenA-MM-UCD1 thus represents a milestone for the understanding of the origin of UCDs. The exquisite resolution of the proposed follow-up HST imaging is essential to derive an accurate stellar mass profile for CenA-MM-UCD1, investigate age/metallicity spreads in its populations with a significantly improved accuracy with respect to the ground-based discovery data, and search for possible faint signs of ongoing tidal disruption. ------------------------------------------------------------------------------------ Observations Description ------------------------ The observations are going to be performed with WFC3/UVIS (primary) and ACS/WFC (parallel). We request imaging in the F606W and F814W filters (broad V and I) for both instruments to observe our peculiar UCD target: these observations will enable an accurate characterization of its stellar content (central surface brightness profile, TRGB distance, photometric metallicities, star formation history, structural parameters) and will resolve a significantly larger number of stars 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: we will thus be able to estimate more precisely the number of background/foreground contaminants (both Milky Way populations and Centaurus A halo stars) present in our WFC3 field. Our goal is to obtain a SNR of ~10 at ~1.5 mag below the TRGB, i.e., at F814W~25.5 and F606W~26.5, in order to derive a precise distance measurement, star formation history, color variations, structural properties and investigate 15426( 2) - 12-Dec-2017 17:11:55 - [ 2] possible spatial asymmetries for our target. With the requested SNR, we will obtain a photometric uncertainty as low as ~0.1 mag at the TRGB magnitude. Our goal requires ~2400 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 with a standard three-dither line pattern to remove detector defects and improve the PSF sampling; we additionally insert a 50 sec exposure in each filter to avoid saturation at the center of our target. We combine the two orbits into one visit in order to maintain the same orientation and thus target the same stars in both filters; our loose ORIENT constraints are necessary to avoid ghosts/spikes at the target's position caused by bright stars within our WFC3 field-of-view. ------------------------------------------------------------------------------------ 15426( 2) - 12-Dec-2017 17:11:55 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 CENA-MM-UCD1 UNIDENTIFIED, NUCLEUS, RA=13H18M10.8830S, J2000 V = 17 UNDESIGNATED DEC=-44D09'26.03" Reference Frame: ICRS Extended: NO Comments: Total apparent magnitude ------------------------------------------------------------------------------------------------------------------------------------ 15426( 2) - 12-Dec-2017 17:11:55 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-LINE-3PT Pattern_Purpose DITHER Number_Of_Points 3 Point_Spacing 0.135 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 46.84 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 15426( 2) - 12-Dec-2017 17:11:55 - [ 5] Visit: 01 (CENA-MM-UCD1) Visit Requirements: ORIENT 60D TO 85 D; ORIENT 172D TO 197 D; ORIENT 240D TO 297 D; ORIENT 352D TO 17 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 CENA-MM-UCD WFC3/UVI ACCUM UVIS1 F814W FLASH=11 1 50 S 1 S see below (50 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 CENA-MM-UCD WFC3/UVI ACCUM UVIS1 F814W 1 800 S PATTERN 1 2-3; PAR 3 1 S see below WITH 2 (2514 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 ANY ACS/WFC ACCUM WFC F814W 1 700 S see below (2228 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 CENA-MM-UCD WFC3/UVI ACCUM UVIS1 F606W FLASH=11 1 50 S 1 S see below (50 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 CENA-MM-UCD WFC3/UVI ACCUM UVIS1 F606W 1 800 S PATTERN 1 5-6; PAR 6 1 S see below WITH 5 (2625 S) ------------------------------------------------------------------------------------------------------------------------------------ 6 ANY ACS/WFC ACCUM WFC F606W 1 700 S see below (2379 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 | ------------------------------------------------------------------------------------------------------------------------------------ CENA-MM-UCD1 1 WFC3/UVI ACCUM UVIS1 F814W none none none none 1 50.0 S S CENA-MM-UCD1 2 WFC3/UVI ACCUM UVIS1 F814W 1 none none none 1 828.0 S S CENA-MM-UCD1 2 WFC3/UVI ACCUM UVIS1 F814W 2 none none none 1 858.0 S S CENA-MM-UCD1 2 WFC3/UVI ACCUM UVIS1 F814W 3 none none none 1 828.0 S S ANY 3 ACS/WFC ACCUM WFC F814W 1 none none none 1 710.0 S ANY 3 ACS/WFC ACCUM WFC F814W 2 none none none 1 820.0 S ANY 3 ACS/WFC ACCUM WFC F814W 3 none none none 1 698.0 S 15426( 2) - 12-Dec-2017 17:11:55 - [ 6] 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 | ------------------------------------------------------------------------------------------------------------------------------------ CENA-MM-UCD1 4 WFC3/UVI ACCUM UVIS1 F606W none none none none 2 50.0 S S CENA-MM-UCD1 5 WFC3/UVI ACCUM UVIS1 F606W 1 none none none 2 855.0 S S CENA-MM-UCD1 5 WFC3/UVI ACCUM UVIS1 F606W 2 none none none 2 915.0 S S CENA-MM-UCD1 5 WFC3/UVI ACCUM UVIS1 F606W 3 none none none 2 855.0 S S ANY 6 ACS/WFC ACCUM WFC F606W 1 none none none 2 770.0 S ANY 6 ACS/WFC ACCUM WFC F606W 2 none none none 2 880.0 S ANY 6 ACS/WFC ACCUM WFC F606W 3 none none none 2 729.0 S 15426( 2) - 12-Dec-2017 17:11:55 - [ 7] Summary Form for Proposal 15426 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS1, WFC ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC, WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=11 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 172D TO 197 D, ORIENT 240D TO 297 D, ORIENT 352D TO 17 D, ORIENT 60D TO 85 D, PAR 3 WITH 2, PAR 6 WITH 5, PATTERN 1 2-3, PATTERN 1 5-6 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names ANY, CENA-MM-UCD1 ------------------------------------------------------------------------------------------------------------------------------------