14716( 2) - 21-Jul-2016 16:08:37 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 14716 Version: 2 Check-in Time: 21-Jul-2016 16:08:37 STScI Edit Number: 0 Title DDO 68: A flea with smaller fleas that on him prey ------------------------------------------------------------------------------------ Type Cycle GO 24 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Francesca Annibali INAF, Osservatorio Astronomico di Bologna Y CoI: Dr. Monica Tosi INAF, Osservatorio Astronomico di Bologna N CoI: Dr. Michele Bellazzini INAF, Osservatorio Astronomico di Bologna N CoI: Dr. Felice Cusano INAF, Osservatorio Astronomico di Bologna N CoI: Elena Sacchi Universita di Bologna N CoI: Dr. Michele Cignoni Universita di Pisa N CoI: Dr. Carlo Nipoti Universita di Bologna N CoI: Prof. Luca Ciotti Universita di Bologna N CoI: Prof. Daniela Calzetti University of Massachusetts - Amherst N CoI: Dr. Janice C. Lee Space Telescope Science Institute N CoI: Dr. Alessandra Aloisi Space Telescope Science Institute N ------------------------------------------------------------------------------------ Abstract With the Large Binocular Cameras on the LBT, we have recently discovered a stellar stream apparently connected to DDO 68, one of the most metal-poor and isolated star-forming dwarf galaxies, with a stellar mass of only 10^8 solar masses. Here we propose HST/WFC3 follow-up imaging of the stream to accurately measure, map and characterize its individual stars, in order to: a) constrain its precise distance through the red giant branch tip, and confirm its physical association with DDO 68; and b) infer the evolution of its stellar population. At DDO 68' s distance of about 12 Mpc, only HST can resolve the stream into individual stars. We have previously obtained ACS imaging of DDO 68. Those data, which only capture a small portion of the stream, provide tentative evidence that it is indeed at the distance of the galaxy. We also used the previous ACS data to derive DDO 68' s star formation history, and combined the results with new N-body simulations which reproduce both DDO 68' s distorted morphology and the position of the stream. This analysis suggests that DDO 68 has cannibalized a ten times smaller companion. Our proposed observations will image the full extent of the stream, and are critical for not only understanding the true nature of this structure, but may also enable us to trace it further, beyond the extent apparent in the LBT data. DDO 68 will be the least massive dwarf galaxy with direct evidence for accretion of a satellite thus far if confirmed with the proposed observations. This study will be extremely important to test the self-similarity of the hierarchical galaxy formation process at all scales. ------------------------------------------------------------------------------------ Observations Description ------------------------ At the DDO 68's distance of ~12.6 Mpc, corresponding to a distance modulus of (m-M)_0~30.51 (Sacchi et al 2016), the tip of the Red Giant Branch (RGB) is detected at I ~ 26.4. The same distance is assumed for our targeted stellar stream apparently connected to DDO 68. In order to derive a robust 14716( 2) - 21-Jul-2016 16:08:37 - [ 2] determination of the stream's distance and stellar population properties, we require to detect an RGB star with a S/N~5 at ~1 mag below the RGB tip. This corresponds to a red giant star with I=27.4 and V=28.7 in Johnson-Cousins (conservatively assuming that the reddest RGB stars have V-I~1.3 at the metallicity of DDO 68). The WFC3 ETC provides an exposure time of ~12, 000 sec in F606W and ~12, 000 sec in F814W to reach a S/N~5 within a circular aperture of 4 pixel radius (0.016", ~2xFWHM). For each filter, the total exposure time will be split into two separate visits lasting respectively 2 and 3 orbits, for a total of 10 orbits for the entire program. Each visit will be organized into 8 sub-exposures executed with a primary dither pattern WFC3_UVIS_GAP_LINE to fill the gap between the chips, plus a secondary pattern WFC3_UVIS_DITHER_BOX to improve the PSF sampling (and get rid of cosmic rays, and hot/bad pixels). Within the longer 3 orbit visits, we added two short ~300 s exposures, executed with a WFC3_UVIS_DITHER_LINE pattern (with a point spacing ~ 4.8 ", or ~ 120 pixels) in order to get photometry of bright objects saturated in the long exposures. The targeted stellar stream has a projected size of ~20" x 80" on the LBT images. We require a specific orient for our observations in order to place the whole stream within one of the two WFC3 chips, avoiding the gap between the two detectors. The selected orient also maximizes the overlap with the portion of the LEGUS UV data lacking coverage in V and I. We require that the 4 visits will be all executed with the same position and same orient. We also require that all 10 orbits are taken within 2 weeks of each other. Carbon stars are an important component of color-magnitude diagrams, and many of these stars are long-period variables. Having all observations made within a two week window will reduce the effect of the long-term variability of the carbon stars on their inferred positions in the color-magnitude diagrams. ------------------------------------------------------------------------------------ 14716( 2) - 21-Jul-2016 16:08:37 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 UGC-5340 GALAXY, TIDAL TAIL RA=09H56M53.6700S, J2000 V = 20 DEC=+28D49'42.30" Reference Frame: ICRS Comments: This object was generated by the targetselector and retrieved from the SIMBAD database. ------------------------------------------------------------------------------------------------------------------------------------ 14716( 2) - 21-Jul-2016 16:08:37 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-GAP-LINE WFC3-UVIS-DITHER-BOX Pattern_Purpose OTHER DITHER Number_Of_Points 2 4 Point_Spacing 2.414 0.173 Line_Spacing 0.112 Coordinate_Frame POS-TARG POS-TARG Pattern_Orient 85.759 23.884 Angle_Between_Sides 81.785 Center_Pattern YES NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-LINE Pattern_Purpose OTHER Number_Of_Points 2 Point_Spacing 4.828 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 46.84 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 14716( 2) - 21-Jul-2016 16:08:37 - [ 5] Visit: 01 Visit Requirements: ORIENT 276D TO 276 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 UGC-5340 WFC3/UVI ACCUM UVIS F606W 1 340 S (680 PATTERN 2 1 S S) ------------------------------------------------------------------------------------------------------------------------------------ 2 UGC-5340 WFC3/UVI ACCUM UVIS F606W 1 910 S PATTERN 1 2 S (7280 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 | ------------------------------------------------------------------------------------------------------------------------------------ UGC-5340 1 WFC3/UVI ACCUM UVIS F606W 1 none none none 1 N/A S UGC-5340 1 WFC3/UVI ACCUM UVIS F606W 2 none none none 1 N/A S UGC-5340 2 WFC3/UVI ACCUM UVIS F606W 1 1 none none 1 N/A S UGC-5340 2 WFC3/UVI ACCUM UVIS F606W 1 2 none none 1 N/A S UGC-5340 2 WFC3/UVI ACCUM UVIS F606W 1 3 none none 2 N/A S UGC-5340 2 WFC3/UVI ACCUM UVIS F606W 1 4 none none 2 N/A S UGC-5340 2 WFC3/UVI ACCUM UVIS F606W 2 1 none none 2 N/A S UGC-5340 2 WFC3/UVI ACCUM UVIS F606W 2 2 none none 3 N/A S UGC-5340 2 WFC3/UVI ACCUM UVIS F606W 2 3 none none 3 N/A S UGC-5340 2 WFC3/UVI ACCUM UVIS F606W 2 4 none none 3 N/A S 14716( 2) - 21-Jul-2016 16:08:37 - [ 6] Visit: 02 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 UGC-5340 WFC3/UVI ACCUM UVIS F814W FLASH=4 1 340 S (680 PATTERN 2 1 S S) ------------------------------------------------------------------------------------------------------------------------------------ 2 UGC-5340 WFC3/UVI ACCUM UVIS F814W 1 900 S PATTERN 1 2 S (7284 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 | ------------------------------------------------------------------------------------------------------------------------------------ UGC-5340 1 WFC3/UVI ACCUM UVIS F814W 1 none none none 1 N/A S UGC-5340 1 WFC3/UVI ACCUM UVIS F814W 2 none none none 1 N/A S UGC-5340 2 WFC3/UVI ACCUM UVIS F814W 1 1 none none 1 N/A S UGC-5340 2 WFC3/UVI ACCUM UVIS F814W 1 2 none none 1 N/A S UGC-5340 2 WFC3/UVI ACCUM UVIS F814W 1 3 none none 2 914.0 S S UGC-5340 2 WFC3/UVI ACCUM UVIS F814W 1 4 none none 2 914.0 S S UGC-5340 2 WFC3/UVI ACCUM UVIS F814W 2 1 none none 2 914.0 S S UGC-5340 2 WFC3/UVI ACCUM UVIS F814W 2 2 none none 3 914.0 S S UGC-5340 2 WFC3/UVI ACCUM UVIS F814W 2 3 none none 3 914.0 S S UGC-5340 2 WFC3/UVI ACCUM UVIS F814W 2 4 none none 3 914.0 S S 14716( 2) - 21-Jul-2016 16:08:37 - [ 7] Visit: 03 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 UGC-5340 WFC3/UVI ACCUM UVIS F606W 1 600 S PATTERN 1 1 S see below (5116 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 | ------------------------------------------------------------------------------------------------------------------------------------ UGC-5340 1 WFC3/UVI ACCUM UVIS F606W 1 1 none none 1 626.0 S S UGC-5340 1 WFC3/UVI ACCUM UVIS F606W 1 2 none none 1 626.0 S S UGC-5340 1 WFC3/UVI ACCUM UVIS F606W 1 3 none none 1 626.0 S S UGC-5340 1 WFC3/UVI ACCUM UVIS F606W 1 4 none none 1 626.0 S S UGC-5340 1 WFC3/UVI ACCUM UVIS F606W 2 1 none none 2 653.0 S S UGC-5340 1 WFC3/UVI ACCUM UVIS F606W 2 2 none none 2 653.0 S S UGC-5340 1 WFC3/UVI ACCUM UVIS F606W 2 3 none none 2 653.0 S S UGC-5340 1 WFC3/UVI ACCUM UVIS F606W 2 4 none none 2 653.0 S S 14716( 2) - 21-Jul-2016 16:08:37 - [ 8] Visit: 04 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 UGC-5340 WFC3/UVI ACCUM UVIS F814W 1 600 S PATTERN 1 1 S see below (5116 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 | ------------------------------------------------------------------------------------------------------------------------------------ UGC-5340 1 WFC3/UVI ACCUM UVIS F814W 1 1 none none 1 626.0 S S UGC-5340 1 WFC3/UVI ACCUM UVIS F814W 1 2 none none 1 626.0 S S UGC-5340 1 WFC3/UVI ACCUM UVIS F814W 1 3 none none 1 626.0 S S UGC-5340 1 WFC3/UVI ACCUM UVIS F814W 1 4 none none 1 626.0 S S UGC-5340 1 WFC3/UVI ACCUM UVIS F814W 2 1 none none 2 653.0 S S UGC-5340 1 WFC3/UVI ACCUM UVIS F814W 2 2 none none 2 653.0 S S UGC-5340 1 WFC3/UVI ACCUM UVIS F814W 2 3 none none 2 653.0 S S UGC-5340 1 WFC3/UVI ACCUM UVIS F814W 2 4 none none 2 653.0 S S 14716( 2) - 21-Jul-2016 16:08:37 - [ 9] Summary Form for Proposal 14716 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=4 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 276D TO 276 D, PATTERN 1 1, PATTERN 1 2, PATTERN 2 1, SAME ORIENT AS 01 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names UGC-5340 ------------------------------------------------------------------------------------------------------------------------------------