Resolving the Complex Star Formation History of the Leo I Dwarf Spheroidal GalaxyDetermining 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.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.STELLAR POPULATIONS12false00.00falsefalsefalsefalsefalsefalseSUPPORTEDGALAXYDWARF SPHEROIDALThis 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.J200010 08 27.4000+12 18 27.00J2000GALAXYDWARF SPHEROIDALJ200010 08 27.4000+12 18 27.00J2000GALAXYDWARF SPHEROIDALJ200010 08 27.4000+12 18 27.00J2000ACS/WFCImagingNoNoF814WACS/WFCImagingNoNoF435WWFPC2ImagingNoF814WWFPC2ImagingNoF555WPOS-TARGDITHER0.265LINE0.187falsefalse69.050003420.67ACS-WFC-DITHER-BOX1falsePOS-TARGDITHER3.011LINEfalsefalse385.279999ACS-WFC-DITHER-LINE2false
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