10520( 10) - 02/02/07 14:04 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 10520 Version: 10 Check-in Time: 02-Feb-2007 19:03:35 STScI Edit Number: 3 Title Resolving the Complex Star Formation History of the Leo I Dwarf Spheroidal Galaxy ------------------------------------------------------------------------------------ Type Cycle Parallel Pointing Tolerance GO 14 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Tammy A. Smecker-Hane University of California - Irvine CoI: Dr. Andrew A. Cole Kapteyn Astronomical Institute N CoI: Prof. John S. Gallagher III University of Wisconsin - Madison N CoI: Dr. James Bullock University of California - Irvine N ------------------------------------------------------------------------------------ Abstract Determining the star formation histories (SFHs) and chemical evolution of nearby galaxies gives us powerful constrains on the physical processes that regulate galaxy evolution. The SFHs can be measured most accurately by comparing the observed densities of stars in color-magnitude diagrams (CMDs) to predictions from stellar evolutionary models. WFPC2 imaging of the Leo I dSph shows it is unique because its stellar population is relatively young. Approximately 68% of its stars formed between 1 and 7 Gyr ago and only 12% of its stars formed >~ 10 Gyr ago. We propose to vastly improve the derived SFH of Leo I by exploiting ACS/WFC's higher quantum efficiency at bluer wavelengths, higher spatial resolution, and larger field-of- view. The figure of merit for our proposed observations, defined as the age resolution times the number of stars detected, will be a factor of 12 higher than existing WFPC2 observations. To surmount the degeneracy of age and metallicity in the CMD, we have independently measured the metallicity distribution of its stars using spectroscopy. Simultaneously modeling the metallicity distribution and CMD, we will firmly constrain the evolution of the Leo I dSph, a unique example of an isolated dwarf galaxy that has not been influenced by interactions with the Milky Way or M31. ------------------------------------------------------------------------------------ 10520( 10) - 02/02/07 14:04 - [ 2] Observations Description ------------------------ We will take a series of images in F435W (B filter) and F814W (I filter) with ACS WFC in order to do accurate stellar photometry for stars in the Leo I dSph galaxy. Our goal is to get 5 sigma detections in the combined set of images about 2 magnitudes below the oldest main-sequence turnoff in Leo I at stellar magnitudes of B=28.6 mag and I = 27.5 mag. This gives us photometric errors of sigma_B = 0.04 mag and sigma_I = 0.03 mag at the oldest main-sequence turnoff, which occurs at B = 26.6 mag and I = 25.5 mag. We request Earth Shadow for the observations because of the faintness of these stars. The field center is chosen so that the corner of the ACS WFC will be at the center of the galaxy so that the field will span the largest galactocentric radius in the galaxy to average over any existing spacial gradient in the stellar population. We do not specify a fixed orientation angle, because the galaxy is much larger than the ACS field of view (core radius = 3.3 arcmin, tidal radius = 12.6 arc min). However all subsequent Visits must have the same orientation as the initial Visit, Visit 01. We will acquire a total of nine 1700 sec images in F435W (B filter). Two sets of 4-point WFC-Box-Dither patterns will be performed and one additional image taken. Pos targs will be used for the later two's initial starting positions. We will acquire six 1700s images in F814W (I filter) using two sets of 3 point WFC-Line-Dither patterns with a pos targ for the second set's initial starting point. We will also acquire three 440 sec images in F814W (I filter) with a 3 point WFC-Line-Dither pattern. The shorter images are necessary because saturation occurs for the stars on the top 1.5 magnitudes of the red giant branch in the 1700s F814W images. We also will obtain coordinated parallel observations with WFPC2. When we are observing with F435W with ACS we will observe with F555W (V filter) with WFPC2, and when we are observing with F814W with ACS we will observe with F814W with WFPC2. Because WFPC2's blue sensitivity is much less than ACS, we need to use a redder filter to reach approximately the same depth in the color-magnitude diagrams. Again, because the galaxy is large (tidal radius = 12.8 arcmin), WFPC2 will fall at some point inside the galaxy no matter what the orientation angle. We will acquire nine 1600 sec images in F555W, six 1600 sec images in F814W and three 400 sec images in F814W in the parallel WFPC2 field. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 10520( 10) - 02/02/07 14:04 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 LEO-I GALAXY, DWARF RA=10H08M22.5000S +/- 0.1", J2000 V = 28.0 SPHEROIDAL DEC=+12D18'6.00" +/- 0.1" Coordinate Source: GUIDE_STAR_CATALOG Comments: This position was chosen to probe the maximum range of galactocentric radii by putting the center of the galaxy near the edge of the ACS frame. This also keeps some bright stars from possibly falling into the WFPC2 field in parallel exposures. 2 LEO-I-B GALAXY, DWARF RA=10H08M24.5000S +/- 0.1", J2000 V = 28.0 SPHEROIDAL DEC=+12D18'5.00" +/- 0.1" Coordinate Source: GUIDE_STAR_CATALOG 3 LEO-I-C GALAXY, DWARF RA=10H08M20.3000S +/- 0.1", J2000 V = 28.0 SPHEROIDAL DEC=+12D18'14.00" +/- 0.1" Coordinate Source: GUIDE_STAR_CATALOG 10520( 10) - 02/02/07 14:04 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.265 Line_Spacing 0.187 Coordinate_Frame POS-TARG Pattern_Orient 20.67 Angle_Between_Sides 69.05 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-LINE Pattern_Purpose DITHER Number_Of_Points 3 Point_Spacing 3.011 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 85.28 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 10520( 10) - 02/02/07 14:04 - [ 5] Visit: 01 Visit Priority: 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| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 LEO-I ACS/WFC ACCUM WFCENTER F435W CR-SPLIT=NO 1 1700 S SHADOW; PATTERN 1 1-2; PAR 2 WITH 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 ANY WFPC2 IMAGE WFALL F555W CR-SPLIT=NO 1 1600 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 | ------------------------------------------------------------------------------------------------------------------------------------ LEO-I 1 ACS/WFC ACCUM WFCENTER F435W 1 none none none 1 N/A LEO-I 1 ACS/WFC ACCUM WFCENTER F435W 2 none none none 2 N/A LEO-I 1 ACS/WFC ACCUM WFCENTER F435W 3 none none none 3 N/A LEO-I 1 ACS/WFC ACCUM WFCENTER F435W 4 none none none 4 N/A ANY 2 WFPC2 IMAGE WFALL F555W 1 none none none 1 N/A ANY 2 WFPC2 IMAGE WFALL F555W 2 none none none 2 N/A ANY 2 WFPC2 IMAGE WFALL F555W 3 none none none 3 N/A ANY 2 WFPC2 IMAGE WFALL F555W 4 none none none 4 N/A 10520( 10) - 02/02/07 14:04 - [ 6] Visit: 02 Visit Priority: 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| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 LEO-I ACS/WFC ACCUM WFCENTER F435W CR-SPLIT=NO 1 1700 S POS TARG 0.25935, 0.01235; SHADOW; PATTERN 1 1-2; PAR 2 WITH 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 LEO-I WFPC2 IMAGE WFALL F555W CR-SPLIT=NO 1 1600 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 | ------------------------------------------------------------------------------------------------------------------------------------ LEO-I 1 ACS/WFC ACCUM WFCENTER F435W 1 none none none 1 N/A LEO-I 1 ACS/WFC ACCUM WFCENTER F435W 2 none none none 2 N/A LEO-I 1 ACS/WFC ACCUM WFCENTER F435W 3 none none none 3 N/A LEO-I 1 ACS/WFC ACCUM WFCENTER F435W 4 none none none 4 N/A LEO-I 2 WFPC2 IMAGE WFALL F555W 1 none none none 1 N/A LEO-I 2 WFPC2 IMAGE WFALL F555W 2 none none none 2 N/A LEO-I 2 WFPC2 IMAGE WFALL F555W 3 none none none 3 N/A LEO-I 2 WFPC2 IMAGE WFALL F555W 4 none none none 4 N/A 10520( 10) - 02/02/07 14:04 - [ 7] Visit: 03 Visit Priority: 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| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 LEO-I ACS/WFC ACCUM WFCENTER F435W CR-SPLIT=NO 1 1700 S POS TARG 0.3705, 0.1235; SHADOW; PAR 2 WITH 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 LEO-I WFPC2 IMAGE WFALL F555W CR-SPLIT=NO 1 1600 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 | ------------------------------------------------------------------------------------------------------------------------------------ LEO-I 1 ACS/WFC ACCUM WFCENTER F435W none none none none 1 N/A LEO-I 2 WFPC2 IMAGE WFALL F555W none none none none 1 N/A 10520( 10) - 02/02/07 14:04 - [ 8] Visit: 04 Visit Priority: 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| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 LEO-I ACS/WFC ACCUM WFCENTER F814W CR-SPLIT=NO 1 1700 S SHADOW; PATTERN 2 1-2; PAR 2 WITH 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 LEO-I WFPC2 IMAGE WFALL F814W CR-SPLIT=NO 1 1600 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 | ------------------------------------------------------------------------------------------------------------------------------------ LEO-I 1 ACS/WFC ACCUM WFCENTER F814W 1 none none none 1 N/A LEO-I 1 ACS/WFC ACCUM WFCENTER F814W 2 none none none 2 N/A LEO-I 1 ACS/WFC ACCUM WFCENTER F814W 3 none none none 3 N/A LEO-I 2 WFPC2 IMAGE WFALL F814W 1 none none none 1 N/A LEO-I 2 WFPC2 IMAGE WFALL F814W 2 none none none 2 N/A LEO-I 2 WFPC2 IMAGE WFALL F814W 3 none none none 3 N/A 10520( 10) - 02/02/07 14:04 - [ 9] Visit: 05 Visit Priority: 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| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 LEO-I ACS/WFC ACCUM WFCENTER F814W CR-SPLIT=NO 1 1700 S POS TARG 0.25935, 0.01235; SHADOW; PATTERN 2 1-2; PAR 2 WITH 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 LEO-I WFPC2 IMAGE WFALL F814W CR-SPLIT=NO 1 1600 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 | ------------------------------------------------------------------------------------------------------------------------------------ LEO-I 1 ACS/WFC ACCUM WFCENTER F814W 1 none none none 1 N/A LEO-I 1 ACS/WFC ACCUM WFCENTER F814W 2 none none none 2 N/A LEO-I 1 ACS/WFC ACCUM WFCENTER F814W 3 none none none 3 N/A LEO-I 2 WFPC2 IMAGE WFALL F814W 1 none none none 1 N/A LEO-I 2 WFPC2 IMAGE WFALL F814W 2 none none none 2 N/A LEO-I 2 WFPC2 IMAGE WFALL F814W 3 none none none 3 N/A 10520( 10) - 02/02/07 14:04 - [ 10] Visit: 06 Visit Priority: 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| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 LEO-I ACS/WFC ACCUM WFCENTER F814W CR-SPLIT=NO 1 440 S SHADOW; PATTERN 2 1 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ LEO-I 1 ACS/WFC ACCUM WFCENTER F814W 1 none none none 1 N/A LEO-I 1 ACS/WFC ACCUM WFCENTER F814W 2 none none none 1 N/A LEO-I 1 ACS/WFC ACCUM WFCENTER F814W 3 none none none 1 N/A 10520( 10) - 02/02/07 14:04 - [ 11] Visit: 51 Visit Priority: Visit Requirements: ORIENT 314.0D TO 314.0 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| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 LEO-I ACS/WFC ACCUM WFCENTER F435W CR-SPLIT=NO 1 1700 S POS TARG 0.248, 0.094; SHADOW; PAR 2 WITH 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 ANY WFPC2 IMAGE WFALL F555W CR-SPLIT=NO 1 1600 S ------------------------------------------------------------------------------------------------------------------------------------ 3 LEO-I ACS/WFC ACCUM WFCENTER F435W CR-SPLIT=NO 1 1700 S POS TARG 0.124, 0.233; SHADOW; PAR 4 WITH 3 ------------------------------------------------------------------------------------------------------------------------------------ 4 ANY WFPC2 IMAGE WFALL F555W CR-SPLIT=NO 1 1600 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 | ------------------------------------------------------------------------------------------------------------------------------------ LEO-I 1 ACS/WFC ACCUM WFCENTER F435W none none none none 1 N/A ANY 2 WFPC2 IMAGE WFALL F555W none none none none 1 N/A LEO-I 3 ACS/WFC ACCUM WFCENTER F435W none none none none 2 N/A ANY 4 WFPC2 IMAGE WFALL F555W none none none none 2 N/A 10520( 10) - 02/02/07 14:04 - [ 12] Visit: 52 Visit Priority: Visit Requirements: ORIENT 278.0D TO 278.0 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| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 LEO-I-B WFPC2 IMAGE WFALL F450W CR-SPLIT=NO 1 1700 S SHADOW ------------------------------------------------------------------------------------------------------------------------------------ 2 LEO-I-B WFPC2 IMAGE WFALL F450W CR-SPLIT=NO 1 1700 S POS TARG 2.5, 2.5; SHADOW ------------------------------------------------------------------------------------------------------------------------------------ 3 LEO-I-B WFPC2 IMAGE WFALL F450W CR-SPLIT=NO 1 1700 S POS TARG 5.25, 5.25; SHADOW ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ LEO-I-B 1 WFPC2 IMAGE WFALL F450W none none none none 1 N/A LEO-I-B 2 WFPC2 IMAGE WFALL F450W none none none none 2 N/A LEO-I-B 3 WFPC2 IMAGE WFALL F450W none none none none 3 N/A 10520( 10) - 02/02/07 14:04 - [ 13] Visit: 53 Visit Priority: Visit Requirements: ORIENT 278.0D TO 278.0 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| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 LEO-I-C WFPC2 IMAGE WFALL F450W CR-SPLIT=NO 1 1700 S SHADOW ------------------------------------------------------------------------------------------------------------------------------------ 2 LEO-I-C WFPC2 IMAGE WFALL F450W CR-SPLIT=NO 1 1700 S POS TARG 2.5, 2.5; SHADOW ------------------------------------------------------------------------------------------------------------------------------------ 3 LEO-I-C WFPC2 IMAGE WFALL F450W CR-SPLIT=NO 1 1700 S POS TARG 5.25, 5.25; SHADOW ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ LEO-I-C 1 WFPC2 IMAGE WFALL F450W none none none none 1 N/A LEO-I-C 2 WFPC2 IMAGE WFALL F450W none none none none 2 N/A LEO-I-C 3 WFPC2 IMAGE WFALL F450W none none none none 3 N/A 10520( 10) - 02/02/07 14:04 - [ 14] Summary Form for Proposal 10520 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFCENTER WFALL ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements SHADOW; PATTERN 1 1-2; PAR 2 WITH 1; SAME ORIENT AS 01; POS TARG 0.25935, 0.01235; POS TARG 0.3705, 0.1235; PATTERN 2 1-2; PATTERN 2 1; ORIENT 314.0D TO 314.0 D; POS TARG 0.248, 0.094; POS TARG 0.124, 0.233; PAR 4 WITH 3; ORIENT 278.0D TO 278.0 D; POS TARG 2.5, 2.5; POS TARG 5.25, 5.25; ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F435W F555W F814W F450W ------------------------------------------------------------------------------------------------------------------------------------ Target Names LEO-I ANY LEO-I-B LEO-I-C ------------------------------------------------------------------------------------------------------------------------------------