The Star Formation History of Leo PThe nearby (D = 1.7+/-0.3 Mpc), very low luminosity (M_V = -9.3+/-0.4mag), gas-rich star forming galaxy Leo P was discovered by its HI 21cm emission in the Arecibo ALFALFA survey. Follow-up optical spectroscopy of its single HII region revealed an oxygen abundance of 12+log(O/H)=7.16+/-0.04, making it the lowest metallicity star forming galaxy in the Local Volume (D < 5 Mpc) and commensurate with the metallicities of emission line galaxies I Zw 18 and DDO 68. Thus, Leo P presents us with a unique opportunity to understand the evolution of extremely metal deficient (XMD) galaxies. Specifically, an HST color magnitude diagram which reaches below the red clump stars will allow us to reconstruct the lifetime star formation history with reasonable time resolution.
The star formation history will answer two vital questions: (1) Did Leo P experience suppressed star formation during its early evolution like another isolated dwarf galaxy Leo A? and (2) What fraction of all newly created metals has Leo P been able to retain during its lifetime?The goal of this proposal is to reconstruct and interpret the star formation history (SFH) and chemical enrichment history of the newly discovered XMD dwarf galaxy Leo~P from a deep (I~27; V~28) color-magnitude diagram (CMD). Orbit allocation = 17 orbits.
Most visits span 2-orbits with the ACS; each orbit is split between one F475W 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 F475W 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 both to minimize the impact of a bright foreground star on the photometry of the galaxy and to maximize the areal coverage of both the main star-forming body and stellar halo of the galaxy. 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. This 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.RESOLVED STELLAR POPULATIONSDefaultfalsefalsefalsefalsefalseSUPPORTEDGALAXYAMORPHOUS IRREGULARHI102144.7+180438AGC208583J2000J2000ACS/WFCImagingNoNoF814WACS/WFCImagingNoNoF475WTiming link suitability includes constraints ([Sequence (01 02 03 04 05 06 07 08) 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 07 08)The scheduling data was generated by CASM 21.2.CASM was run in Phase II mode.C:\Users\Bill Januszewski\AppData\Local\Temp\OPVisit8252556550258623059.xmlWe 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.(Tic-Visit
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