13745( 6) - 18-Sep-2014 20:08:10 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 13745 Version: 6 Check-in Time: 18-Sep-2014 20:08:10 STScI Edit Number: 4 Title Resolving the Tip of the Red Giant Branch of Two New Candidate Local Group Dwarf Galaxies ------------------------------------------------------------------------------------ Type Cycle GO 22 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Erik Tollerud Yale University Y CoI: Prof. Marla C. Geha Yale University N CoI: Prof. Mary E. Putman Columbia University in the City of New N York CoI: Jana Grcevich American Museum of Natural History N CoI: Dr. Daniel Stern Jet Propulsion Laboratory N ------------------------------------------------------------------------------------ Abstract We propose to use ACS/WFC to observe two faint dwarf galaxies recently discovered via their HI emission. Based on a blind HI search of 40 HI clumps from 7500 square degrees of the GALFA-HI survey, these two candidates are the only objects with optical counterparts. They show HI and Halpha emission consistent with nearby galaxies, and have blue stars that are barely resolved in ground-based optical imaging with good seeing. These resolved stars are consistent with the galaxies being at Local Group distances. If they are in the Local Group, these galaxies are both less luminous and more compact than the recently-discovered Leo P, also found first with HI observations. They may then also be the faintest known star-forming galaxies. The ground-based imaging leaves large distance uncertainty, however, because the tip of the red giant branch cannot be resolved. We propose one orbit per galaxy of ACS/WFC imaging in F606W and F814W to measure accurate TRGB distances and determine if they truly are Local Group galaxies. If so, these galaxies provide tests on both the efficacy of Lambda CDM in predicting the properties of dwarf galaxies in low density environments, and the lowest-luminosity data points on models of galaxy star formation. ------------------------------------------------------------------------------------ Observations Description ------------------------ The observations for this program are aimed at detecting RGB stars in the two target galaxies, to apply the Tip of the Red Giant Branch method to determine their distance. There are two targets, HI22 and HI23, which are to be observed for one orbit each with ACS/WFC. Each orbit is to be split roughly evenly between F606W and F814W (slightly favoring more exposure time for F606W). Each target should be centered on the WHC1 chip because they both fit on a single chip (and this avoids some part of the galaxies being on the chip gap). They are also small enough that their long axes fit on the chip, so there is no need for a special orientation. Two exposures are to be done for each filter per target, with a dither between them. This serves to fill the chip gap, avoid hot pixels/CCD defects, and reject cosmic rays. This ends up providing about 1100 s per filter per exposure, which the ETC indicates should get down to around 27 mags at S/N~10. 13745( 6) - 18-Sep-2014 20:08:10 - [ 2] ------------------------------------------------------------------------------------ 13745( 6) - 18-Sep-2014 20:08:10 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 HI22 GALAXY, AMORPHOUS RA=00H14M46.0300S, J2000 r~17.5 IRREGULAR, DWARF DEC=+10D48'47.01" COMPACT, EMISSION LINE NEBULA Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 2 HI23 GALAXY, AMORPHOUS RA=01H19M11.7110S, J2000 r~17.3 IRREGULAR, DWARF DEC=+11D07'18.22" COMPACT, EMISSION LINE NEBULA Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 13745( 6) - 18-Sep-2014 20:08:10 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-LINE Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 3.011 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 85.28 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 13745( 6) - 18-Sep-2014 20:08:10 - [ 5] Visit: 01 Visit Requirements: ORIENT 325D TO 120 D; ORIENT 185D TO 202 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 HI22 ACS/WFC ACCUM WFC1 F606W 1 546 S PATTERN 1 1 (1092 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 HI22 ACS/WFC ACCUM WFC1 F814W 1 520 S PATTERN 1 2 (1040 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 | ------------------------------------------------------------------------------------------------------------------------------------ HI22 1 ACS/WFC ACCUM WFC1 F606W 1 none none none 1 N/A HI22 1 ACS/WFC ACCUM WFC1 F606W 2 none none none 1 N/A HI22 2 ACS/WFC ACCUM WFC1 F814W 1 none none none 1 N/A HI22 2 ACS/WFC ACCUM WFC1 F814W 2 none none none 1 N/A 13745( 6) - 18-Sep-2014 20:08:10 - [ 6] Visit: 02 Visit Requirements: ORIENT 103D TO 106 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 HI23 ACS/WFC ACCUM WFC1 F606W 1 536 S POS TARG 20, 15; PATTERN (1072 S) 1 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 HI23 ACS/WFC ACCUM WFC1 F814W 1 510 S POS TARG 20, 15; PATTERN (1020 S) 1 2 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ HI23 1 ACS/WFC ACCUM WFC1 F606W 1 none none none 1 N/A HI23 1 ACS/WFC ACCUM WFC1 F606W 2 none none none 1 N/A HI23 2 ACS/WFC ACCUM WFC1 F814W 1 none none none 1 N/A HI23 2 ACS/WFC ACCUM WFC1 F814W 2 none none none 1 N/A 13745( 6) - 18-Sep-2014 20:08:10 - [ 7] Summary Form for Proposal 13745 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFC1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 103D TO 106 D, ORIENT 185D TO 202 D, ORIENT 325D TO 120 D, PATTERN 1 1, PATTERN 1 2, POS TARG 20, 15 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names HI22, HI23 ------------------------------------------------------------------------------------------------------------------------------------