Following the suggestions of members of the STSci Help Desk, we have designed 4 visits of 5 orbits each. In each visit, 5 ACS-WFC-DITHER-BOX 4-point dither patterns are executed, one per orbit. We have set CR-SPLIT=NO, since we are dithering. We are restricting the orientation to be within +/-20 degrees of 0, 90, 180, and 270 degrees. If this constraint needs to be relaxed for scheduling purposes, we can still execute our program, though we would like to be notified as such, since we might need to tweak the pointing centers as a result. The optimal position angle is PA=0, 90, 180, or 270, i.e. ORIENTAT=-2.5, 87.5, 177.5, 267.5, though we can tolerate orientations within the ranges indicated in the Phase II proposal (or worse). Please be in touch if there are any problems or if any of this is unclear. ----- Submission 1 (Sun May 08 00:40:12 GMT 2005) ----- Assigned ID: 115 ----- Submission 1 (Thu Jan 20 23:45:55 GMT 2005) ----- Imaging a Proto-cluster at z=2.3: The Morphology-Density Relation at High Redshift We propose deep ACS/WFC imaging in the HS1700+643 field, which contains a significant galaxy over-density at z=2.30+/-0.015. This structure is the largest and most robustly established cluster-sized over-density at z>1.5. We have spectroscopically confirmed 100 z~2 star-forming galaxies in this field and find significant differences in their stellar populations as a function of large-scale environment. Here we propose to measure the detailed morphological properties of this sample, to determine if the universal morphology-density relationship observed in the low-redshift universe is already in place in and around a forming proto-cluster region at high redshift. As well as providing a unique window into galaxy formation through studies of the z=2.3 proto-cluster, the HS1700+643 field has an extensive wealth of complementary data to recommend it. We will combine the new ACS observations with existing multi-wavelength imaging ranging from 0.3--8.0 microns, optical and near-IR spectroscopy, and narrow band imaging of z=2.3 H-alpha emitters, to understand the evolving properties of a complete and unbiased sample of high-redshift galaxies.
We are proposing four adjacent ACS pointings in the HS1700+643 field, which will provide roughly contiguous coverage over a 6.5x6.5 arcmin-squared region. Figure 3 of our Phase I proposal indicates the locations of the proposed ACS pointings at a nominal PA=0 degrees, though we are not requiring ``ORIENT'' to be fixed. Also indicated in Figure 3 are the locations of the 100 UV-selected spectroscopically-confirmed z~2 galaxies described in this proposal. Objects at z=2.30+/-0.015 are highlighted. Though not indicated in this figure, the number of additional UV-selected z~2 photometric candidates without redshifts is a factor of four higher. In four ACS pointings, we can cover ~80 UV-selected spectroscopically-confirmed z~2 galaxies, along with an additional ~300 photometric candidates. Additionally, we will cover ~50 H-alpha narrow-band-selected objects that are candidate proto-cluster members at z=2.3, and ~50 J-K_s-selected objects that are most likely massive galaxies at z>=2. The precise set of galaxies covered by the pointings will vary as a function of ORIENT, but regardless of the orientation of the detector in each pointing, we will be able to cover roughly the same total number of objects, including all of the spectroscopically-confirmed proto-cluster galaxies. Because we are primarily interested in detailed morphology, our proposed observations emphasize depth over color information. In fact, the majority of the target galaxies already have extensive and deep color information from 0.3-8.0 micron. We are therefore proposing ACS/WFC exposures in the F814W filter for maximum instrumental throughput. The median redshift for the galaxies of primary interest is <z>=2.3, so that the F814W filter corresponds to rest-frame 2500 angstrom light. We have estimated the required exposure times as follows: our primary spectroscopic sample covers the magnitude range of AB(6900)=22.0-25.5, with an average of AB(6900)=24.3. For typical rest-frame UV colors, this corresponds to AB(8140)=22.1-25.4 with an average of AB(8140)=24.2. Using the ACS Exposure Time Calculator and assuming typical half-light radii of 0.25 arcsec, we find that in 10000 seconds we achieve S/N=30 detections of the faintest objects in our sample, with a corresponding S/N>5 per 2x2-pixel resolution element. This guarantees that any structure resolved by HST/ACS will be significantly detected for even our faintest targets. Most of the complementary sample of J-K_s-selected objects have F814W magnitudes comparable to those of the faintest of the UV-selected objects, providing further justification for the deep exposures. To put the proposed exposure times in context, we calculate that 10000 seconds in F814W is slightly deeper than the sum of the F606W and F775W data obtained by the GOODS survey. We have in fact spectroscopically confirmed a significant number (~200) of UV-selected z~2 galaxies in the GOODS-N field, with a similar range of rest-frame UV luminosities to that of the proposed HS1700+643 sample. By stacking the GOODS-N F606W and F775W exposures and inspecting a set of z~2 galaxies with a representative range of magnitudes, we determine that we will be able to measure detailed morphological properties for all the objects in our HS1700+643 sample with the proposed exposure times. We have been awarded 20 orbits, or 5 orbits for each of 4 pointings. For each pointing, we have planned a single 5-orbit visit. In each orbit of a visit, we will perform the ACS-WFC-DITHER-BOX pattern, with the default offsets and orientation. Each position of the 4-point dither pattern will have an exposure of ~600-630 seconds, yielding a total integration time of ~2400-2500 seconds per dither pattern. With 5 orbits in each pointing, and therefore 5 dither sequences, we will obtain a total integration time of ~12000-12500 seconds per pointing. The dithering should allow for cosmic ray rejection, though we will not completely cover the inter-chip gap. The declination of our field is +64 degrees, which corresponds to a visibility period of 60 minutes in 3-gyro mode. Assuming typical ACS values for guide-star acquisition, small dithers, and instrument overhead, we find ~2900 seconds available per orbit for science integration, so our proposed strategy should be feasible. 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