0 Assigned ID: 1130 ----- Attempting Submission 1 (Fri Sep 27 13:43:33 GMT 2019) ----- HST Phase I Proposal 1130 (McQ_mid_HST27_VXMP_v1.apt) successfully submitted. Receipt: # 1130-1 ----- Attempting Submission 2 (Sat Sep 28 01:39:51 GMT 2019) ----- 0 Extremely Metal Poor Galaxies: Understanding the Boundaries of Galaxy Evoution After decades of fruitless searches, we are finally starting to find star-foming galaxies with exceptionally low gas-phase oxygen abundances in the nearby universe. Such galaxies have properties at the limit of what we expect from galaxy evoution models, providing unique boundary conditions for low-mass galaxies in the present-day universe. Termed 'extremely metal-poor' or XMP galaxies, with abundances <5% solar, these galaxies nearly ubiquitously disagree with expectations of the well-established luminosity-metallicity relation (LZR) of dwarf galaxies. There are two exceptions. The first is the nearby galaxy Leo P (D=1.6 Mpc), whose low abundance is consistent with inefficient star formation and metal-loss via galactic winds. The second is the newly discovered XMP galaxy J1005+3722. Tantalizingly close at 2.7 Mpc, the galaxy has a robust, direct-method oxygen abundance of 12+log(O/H) = 7.25 and, similar to Leo P, is consistent with the LZR. We request HST ACS imaging of the stellar populations in J1005+3722 to derive the star formation and chemical enrichment history of the galaxy and determine whether (i) the galaxy has experienced constant or delayed star formation, and (ii) the low oxygen abundance is consistent with expectations from secular processes (i.e., low star formation rates and metal expulsion). J1005+3722 is in the expected abundance regime of galaxies at high-redshift that will be identified by JWST, but forever out-of-reach for detailed study. It is through the analysis possible on galaxies like J1005+3722 in the local universe, where the observations are best, that we build our foundational knowledge of metal-poor galaxies and help prepare for JWST. The goal of this proposal is to measure the distance, search for variable stars, and reconstruct and interpret the star formation history (SFH) and chemical enrichment history of the newly discovered XMP dwarf galaxy J1005+3722 from a deep color-magnitude diagram (CMD). Orbit allocation = 13 orbits. Most visits span 2-orbits with the ACS with the final vist spanning 3-orbits; each orbit is split between one F606W and one F814W exposure (with no CR-splits). Small ditherings are performed between exposures in order to remove hot pixels and to smooth the detector response. We don't intend to cover the interchip gap. Ditherings are performed using POS-TARG, and following the basic UDF dither pattern, shifted around by 2-3 pix. The observations will complete a total of 4 uniquely positioned UDF dither patterns plus an additional set of F606W and F814W images with a POS-TARG offset of 0.05, 0.05. The first visit is requested within a given range of orientations. This range has been chosen to minimize the impact of a bright foreground star on the photometry of the galaxy and to ensure the galaxy is not impacted by the chip gap. Subsequent visits are requested to be obtained with the same ORIENT as the first visit in order to maximize the sky area covered at full depth. We request the visits to be executed sequentially within a maximum time span of 3 days, ideally avoiding 12 hour intervals in the observations as this matches the cadence of the expected variable star light curves. The request sequence of data will ensure optimal sampling of the light curves of short period variable stars such as RR Lyrae, which will provide an independent constraint on the SFH of the galaxy, the main science goal of our progam. PI McQuinn is an expert on resolved stellar populations, star formation histories, and extremely metal-poor galaxies. She was primarily responsible for the analysis of the galaxy Leo P, on which the proposed analysis is based. Dr. Berg has deep experience studying galaxy metallicities and the luminosity-metallicity scaling relation. Dr. Telford brings experience in resolved stars and modeling the metal distribution in galaxies. Dr. Dolphin, author of the photometry software DOLPHOT and the CMD-fitting code MATCH that we will use on the data, brings significant technical expertise and deep knowledge of stellar populations. Stellar Populations Galaxies and the IGM 0 false false false false true false false SUPPORTED GALAXY AMORPHOUS IRREGULAR STAR FORMING REGION NO J2000 J2000 ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W /private/var/folders/46/4jpf0wpx40jbz16h87362w240001d2/T/OPVisit11757013132946940197.xml Timing link suitability includes constraints ([Sequence (01 02 03 04 05 06) within 3.0 days] [02 After 01 by 0.2 to 3.0 days]) Hst Same orient constraint links visits (01 02 03 04 05 06) The scheduling data was generated by CASM 27.2. CASM was run in Phase II mode. We request a range of orientations in this first visit and the same orientation in the subsequent visits. Visits are also requested to be executed sequentially within a 3 day period. 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