Updated visit requirements to include timing request (based on suggestion from PC on June 25th) ----- Attempting Submission 2 (Fri Jul 13 11:01:06 GMT 2012) ----- Unveiling the structure of the farthest galaxy protocluster: WFC3 imaging of a z~8 galaxy overdensity Our pure-parallel WFC3/BoRG survey identified the highest redshift (z~8) protocluster candidate, presenting the unique opportunity for follow-up observations that will unveil the nature and properties of star formation at the earliest stages of galaxy cluster assembly. We found a rare overdensity of 5 Y-dropout galaxies, with m_AB=26-27, all brighter than m_*(z=8) and strongly clustered in a region of about 1 arcmin diameter (~2 Mpc/h comoving). The existing data establish at 99.9% confidence that the overdensity is physical and not a line-of-sight effect, but only show a glimpse of the protocluster structure and properties. Not only is the protocluster center near the edge of the imaged field but theoretical modeling predicts the presence of dozens of fainter galaxies. We propose follow-up multi-band WFC3 observations centered on the protocluster and reaching m_AB=27.5 to image the only known galaxy overdensity at z>7, missing in legacy surveys such as CANDELS because of cosmic variance. From the new dataset we will detect 5-10 new protocluster members, tighten redshift uncertainty to +/-0.1, and double the S/N of current detections to characterize their stellar population properties. The new data, combined with, and compared to detailed numerical and theoretical modeling will characterize the earliest stages of galaxy and cluster formation, shedding light on the processes that drove cosmic reionization in overdense environments. Because of a one order of magnitude boost in the number density of galaxies with m_AB<m_* in the protocluster region, the proposed observations will create the best legacy pointing for follow-up spectroscopy at z~8 with multi-object instruments. The observations consist of 3 5-orbit visits, each with 1 orbit in F606W, F098M, F105W, F125W, and F160W, in that order to help with persistence. Visit 4 is 1 orbit of F606W and 2 orbits of F098M. Each orbit is comprised of 4 exposures. For the IR exposures, 4 15-NSAMP SPARS50 exposures don't fill the complete orbit, so a combination of SPARS50 and SPARS100 was used to optimally use all of the available visibility. The UVIS dither is a 4-point line dither to cover the chip gap. The IR dither is a 4-point minimum box with optimal half-pixel sampling in both x and y. It is the same as the WFC3-IR-DITHER-BOX-MIN pattern, but implemented as POSTARGS, which is necessary since the 4 exposures don't all have the same NSAMP/SAMP-SEQ. Timing requirements are requested to coordinate ground-based follow-up observations of targets selected by these HST observations using the MOSFIRE spectrograph on Keck. COSMOLOGY 12 false 0.0 0.0 false false false false SUPPORTED CLUSTER OF GALAXIES HIGH REDSHIFT CLUSTER J2000 J2000 WFC3/UVIS Imaging No No F606W WFC3/IR Imaging No No F098M WFC3/IR Imaging No No F105W WFC3/IR Imaging No No F125W WFC3/IR Imaging No No F160W POS-TARG MOSAIC DITHER 4 2.414 WFC3-UVIS-DITHER-BOX false 81.785 true 0.112 0.173 4 85.759 POS-TARG WFC3-UVIS-GAP-LINE 23.884 1 false Same orient constraint links visits (01 02 03 04) The scheduling data was generated by CASM 20.2. 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