The observing setup is similar to the program described in Bowler et al. (2017; GO 13793). We have set up visits of two subsequent orbits for each target as discussed with the contact scientist, allowing a slightly longer overall exposure time (setting NSAMP=14 rather than 13 in one of the orbits) and therefore more efficient use of each orbit. 1162669092 ----- Attempting Submission 1 (Fri Mar 26 16:44:25 GMT 2021) ----- Imaging the sites of star formation at sub-kiloparsec scales in the Epoch of Reionization We propose to acquire HST/WFC3 imaging of two galaxies at z~7 recently detected by ALMA in [CII] 158 micron and [OIII] 88 micron emission at high angular resolution. The [OIII] emission, corresponding to diffuse, highly-ionized gas, and [CII] emission, tracing neutral gas, allow some of the first explorations of the interstellar medium in the Epoch of Reionization (EoR). Combined, they suggest we are witnessing intense star formation which locally shows extreme [OIII]/[CII] luminosity ratios that are indicative of 'leaking' ionizing radiation and hence may be of great importance to Reionization. Other regions in these galaxies, however, might be more metal-enriched and obscured by dust, as indicated by ALMA measurements of the dust continuum. To conclusively shed light on the key processes affecting star formation in the EoR, we propose to obtain deep HST images of the rest-frame UV (both a ~5x improvement in limiting flux and angular resolution over existing, ground-based imaging). We will measure accurate sizes of star-forming clumps on sub-kiloparsec scales, study possible ongoing mergers, and compare the UV morphology with spatially resolved maps of dust continuum and physical parameters such as the ionization parameter extracted from the [CII] and [OIII] observations. These comparisons will give insight into the physical conditions of both UV-bright star-forming clumps and dust-obscured star-forming regions. Combined with a larger sample including three other galaxies, this data will paint a physically motivated picture of star formation in the EoR in time for science exploration with JWST. We propose IR imaging with the F140W filter of WFC3 of two sources, UVISTA-Z-007 and UVISTA-Z-019, both with spectroscopically confirmed redshifts of z = 6.75, for a total of 4 orbits. HST imaging is required for an accurate comparison between the spatial extent of the rest-frame UV emission with that of [OIII] 88 micron, imaged by ALMA at a resolution of 0.4"-0.5" (existing ground-based IR imaging only reaches an angular resolution of 0.7" at best). In order to properly resolve the spatial substructure of the rest-frame UV - that is, obtain a robust size measurement and have sufficient sensitivity for fainter galaxy fragments that could indicate dust-obscured star formation or a merger - we require a signal-to-noise ratio of 5 for the faint UV structures with typical surface brightness of 25 mag/arcsec^2 (see e.g. Bowler et al. 2017 for galaxies at z ~ 7 with similar brightness). With an estimated orbital visibility of 52 minutes (3120 s) per orbit for both targets, or about 41.5 minutes (2490 s) after subtracting overhead time for 4 individual science exposures per target, we have inferred each target will require 2 orbits of observing time to reach this signal-to-noise threshold. We divide each orbit into 4 exposures, which will allow to perform a box dithering pattern to improve PSF sampling, and to remove bad pixels, cosmic ray impacts, and detector artefacts. We use the SPARS50 sampling sequence with NSAMP=13 and NSAMP=14 in two subsequent orbits for both targets (similar to Bowler et al. 2017). In the event HST will operate in reduced-gyro mode, these observations will be impacted by having a shorter exposure time and therefore signal to noise. As described on the documentation of HST Cycle 28 Mid-Cycle Time Submission, the review of the team expertise will be done internally by the Science Policies Group for Mid-Cycle GO programs in Cycle 28 and there is no need to submit a separate Team Expertise and Background document. Galaxies 6 false false false false false false false SUPPORTED GALAXY HIGH REDSHIFT GALAXY STAR FORMING REGION J2000 J2000 J-band: 24.5 +/- 0.1 GALAXY HIGH REDSHIFT GALAXY STAR FORMING REGION J2000 J2000 J-band: 25.1 +/- 0.2 WFC3/IR Imaging No No F140W WFC3/IR Imaging No No F140W POS-TARG DITHER 0.572 LINE 0.365 false false 74.653 4 18.528 WFC3-IR-DITHER-BOX-MIN 1 false /Users/Joris/Documents/Research/Proposals/HST/UVISTA_imaging/APT/Phase-II/OP-cache/UVISTA_imaging-v01.xml The scheduling data was generated by CASM 29.0. CASM was run in Phase II mode. 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