----- Submission 1 (Wed May 11 14:25:42 GMT 2005) ----- Assigned ID: 383 ----- Submission 1 (Fri Jan 21 21:11:25 GMT 2005) ----- Star-Formation History of an Unmerged Fragment: the Leo A Dwarf Galaxy The Leo A dwarf irregular is the only known Local Group galaxy that on the weight of current evidence has been suggested to have experienced its first star formation within the past 2-3 billion years. As a galaxy that could have been almost purely gaseous during the epoch of giant galaxy assembly, Leo A is the best nearby candidate to be a redshift zero analogue to the major building blocks of the Milky Way. We propose to obtain deep optical images of Leo A with the ACS/WFC to achieve three main goals: 1) To establish the fractions of star-formation, by mass, that occurred prior and subsequent to the main epoch of hierarchical merging (redshift z ~ 2-4, Age ~ 10-12.5 Gigayears); 2) to measure the time variation in Leo A's star-formation rate over the past 10 Gyr, based on statistical analyses of its (V-I, I) color-magnitude diagram; and 3) to measure the radial distributions of young and old stellar populations and quantify the degree to which the optically prominent, young population is embedded in an extended, low-surface brightness sheet or halo of ancient stars. Because of the distance modulus (24.5 mag) and high degree of stellar crowding at the level of the oldest main-sequence turnoffs, the observations necessary to achieve these goals are unobtainable except with HST. The ONLY way to reliably derive the star-formation history of Leo A over its entire lifetime is with photometry to magnitudes of (B,I) = (28.6, 27.9), the level of the oldest main-sequence turnoff in Leo A. These data would confirm and extend the limited inferences obtained from WFPC2 photometry over 2 magnitudes less deep, and provide the first opportunity to measure the complete star-formation history of a potential "living fossil" analogue to the building blocks of the Milky Way. We propose to use WFPC2 in parallel to measure radial variations in the stellar populations between the galaxy's core and outskirts. Because the expected 2-gyro jitter ellipse is comparable to the pixel scale of ACS/WFC, we rely on point-spread function fitting photometry, and we require no special scheduling constraints, our proposed program would be virtually unaffected by entry into 2-gyro mode.
We will image the center region of the Leo A dwarf (DDO 69) using the F475W and F814W filters of ACS/WFC, and an outlying region of the galaxy in F555W and F814W with WFPC2 in parallel. Our exposures are designed to precisely measure the mass fraction, radial distribution, and approximate age and metallicity distributions of the most ancient stellar populations in Leo A, with ages corresponding to cosmological redshifts of z = 2-3. This requires accurate photometry down to a limiting magnitude of B ~ 28. Our limiting magnitude requirement is based on the expected magnitude and colors of the subgiant branch and main-sequence turnoff, which contain unique information about the oldest stars in the galaxy. We choose to image in the F475W and F814W bands to give the optimal combination of throughput and temperature sensitivity. Simulations using theoretical isochrones show that signal-to-noise >10 is required to resolve temperature differences of 20 percent. The ACS exposure time calculator suggests that we require roughly 19,000 seconds of exposure time (8 orbits) in each filter to achieve this goal. Leo A has been suggested to have a radial population gradient; our WFPC2 parallel exposures will accurately measure the properties of an outer field in the galaxy. For roll ranges obtained using the APT Visit Planner, WFPC2 will well-sample the Leo A halo (semimajor axis 8 arcmin). We will minimize the impact of cosmic rays and hot pixels on our data, and improve the sampling of the PSF, using prescribed dither patterns taken from the ACS instrument handbook. By filling each orbit with a single exposure, we minimize overheads and maximize sensitivity to faint stars. It is critical to obtain precise photometry for stars at the level of the oldest main-sequence turnoff, so our strategy is to integrate for as long as possible at a given individual pointing. For schedulability, we implement our observations in 2-orbit visits. Therefore the 4-point box dither pattern is implemented by use of exposure level POS-TARG constraints, confirmed using the VTT. By adopting this strategy we maximize S/N for the faint stars in Leo A while minimizing artifacts. In Phase I, we requested the LOW-SKY visit-level constraint for our observations because of the low ecliptic latitude of Leo A. However, because we expect that the earthshine will NOT be a strong contributor to the sky background for limb angles 20-40 degrees, we prefer to drop the LOW-SKY requirement, thereby gaining in efficiency by filling more of each orbit with science time. In order to ensure that we observe during periods of low zodiacal light background (the original impetus to observe during LOW-SKY), we wish instead to use the visit-level BETWEEN timing constraint to ensure that the sun-telescope-target angle is larger than 100 degrees. The APT Visit Planner shows that this does not negatively affect schedulability. Because we are using WFPC2 to obtain deep parallel exposures, Visits 2-8 are marked with the SAME ORIENT as VISIT 1 flag. Except for a 45 degree zone which would put bright foreground stars in the parallel field, the exact orient is unimportant so long as each visit is the same. Leo A has twice been observed with WFPC2 (Tolstoy et al. 1998, Schulte-Ladbeck et al. 2002) in the F435W, F555W, and F814W filters. These observations laid the groundwork for our program. The absolute magnitudes reached in the previous work are at least 2 magnitudes too faint to reach our target, the subgiant branch and main-sequence turnoffs of the oldest populations in Leo A. Deep observation with ACS is the only way to accurately measure the fraction of ancient stars in Leo A. Our WFPC2 parallels will provide imaging in the outskirts of Leo A of comparable depth to data that currently exists only for the center. STELLAR POPULATIONS 12 false 0.0 false false false false false SUPPORTED GALAXY AMORPHOUS IRREGULAR LSB STAR FORMING REGION Total mag of Leo A galaxy is V = 12.8; most stars fainter than V = 21, I = 20. J2000 09 59 26.5000 +30 44 47.00 J2000 GALAXY AMORPHOUS IRREGULAR HALO LSB Field in outer, "halo" region of Leo A dwarf irregular galaxy. most stars fainter than V=21, I=20 RA =149.858333D, DEC =+30.7463889D, R =1D ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F475W WFPC2 Imaging No F555W WFPC2 Imaging No F814W The BETWEEN timing requirement replaces the LOW-SKY request that was made in our Phase I proposal. We can more efficiently observe by using timing requirements to ensure sun-target angles greater than 100 degrees to minimize zodiacal light, because we have calculated that the earthshine within 20-40 degrees of the earth limb will not significantly impact our observations. The only orient constraint is that we wish to avoid bright stars in the parallel field; the unacceptable ORIENTs were derived using the VTT. 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