16506( 2) - 26-Mar-2021 17:02:49 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 16506 Version: 2 Check-in Time: 26-Mar-2021 17:02:49 STScI Edit Number: 0 Title Imaging the sites of star formation at sub-kiloparsec scales in the Epoch of Reionization ------------------------------------------------------------------------------------ Type Cycle GO 28 ------------------------------------------------------------------------------------ Investigators Contact? PI: Mr. Joris Witstok University of Cambridge Y CoI: Dr. Renske Smit Liverpool John Moores University N CoI: Dr. Roberto Maiolino University of Cambridge N CoI: Prof. Paul van der Werf Universiteit Leiden N CoI: Dr. Mauro Stefanon Universiteit Leiden N CoI: Leindert Boogaard Universiteit Leiden N CoI: Sander Schouws Sterrewacht Leiden N CoI: Prof. Ivo Labbe Swinburne University of Technology N CoI: Dr. Stefano Carniani Scuola Normale Superiore, Pisa N CoI: Dr. Jacqueline Hodge Universiteit Leiden N CoI: Prof. Pascal Oesch University of Geneva-Department of N Astronomy CoI: Dr. Nimisha Kumari Space Telescope Science Institute - ESA N CoI: Dr. Stephane De Barros Observatoire de Geneve N CoI: Dr. Rychard Bouwens Universiteit Leiden N ------------------------------------------------------------------------------------ Abstract 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. ------------------------------------------------------------------------------------ Observations Description ------------------------ We propose IR imaging with the F140W filter of WFC3 of two sources, 16506( 2) - 26-Mar-2021 17:02:49 - [ 2] 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. ------------------------------------------------------------------------------------ 16506( 2) - 26-Mar-2021 17:02:49 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 UVISTA-Z-007 GALAXY, HIGH REDSHIFT RA=09H58M46.2100S, J2000 J-band: 24.5 +/- 0.1 GALAXY, STAR FORMING DEC=+02D28'45.80" REGION Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 2 UVISTA-Z-019 GALAXY, HIGH REDSHIFT RA=10H00M29.8900S, J2000 J-band: 25.1 +/- 0.2 GALAXY, STAR FORMING DEC=+01D46'46.40" REGION Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 16506( 2) - 26-Mar-2021 17:02:49 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-IR-DITHER-BOX-MIN Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.572 Line_Spacing 0.365 Coordinate_Frame POS-TARG Pattern_Orient 18.528 Angle_Between_Sides 74.653 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 16506( 2) - 26-Mar-2021 17:02:49 - [ 5] Visit: 01 (First visit) Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 UVISTA-Z-00 WFC3/IR MULTIAC IR F140W NSAMP=13, SAMP-SEQ=SPARS50 1 DEF PATTERN 1 1-2 7 CUM (2411.751 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 UVISTA-Z-00 WFC3/IR MULTIAC IR F140W NSAMP=14, SAMP-SEQ=SPARS50 1 DEF 7 CUM (2611.753 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ UVISTA-Z-007 1 WFC3/IR MULTIAC IR F140W 1 none none none 1 N/A CUM UVISTA-Z-007 1 WFC3/IR MULTIAC IR F140W 2 none none none 1 N/A CUM UVISTA-Z-007 1 WFC3/IR MULTIAC IR F140W 3 none none none 2 N/A CUM UVISTA-Z-007 1 WFC3/IR MULTIAC IR F140W 4 none none none 2 N/A CUM UVISTA-Z-007 2 WFC3/IR MULTIAC IR F140W 1 none none none 1 N/A CUM UVISTA-Z-007 2 WFC3/IR MULTIAC IR F140W 2 none none none 1 N/A CUM UVISTA-Z-007 2 WFC3/IR MULTIAC IR F140W 3 none none none 2 N/A CUM UVISTA-Z-007 2 WFC3/IR MULTIAC IR F140W 4 none none none 2 N/A CUM 16506( 2) - 26-Mar-2021 17:02:49 - [ 6] Visit: 02 (Second visit) Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 UVISTA-Z-01 WFC3/IR MULTIAC IR F140W NSAMP=14, SAMP-SEQ=SPARS50 1 DEF PATTERN 1 1-2 9 CUM (2611.753 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 UVISTA-Z-01 WFC3/IR MULTIAC IR F140W NSAMP=13, SAMP-SEQ=SPARS50 1 DEF 9 CUM (2411.751 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ UVISTA-Z-019 1 WFC3/IR MULTIAC IR F140W 1 none none none 1 N/A CUM UVISTA-Z-019 1 WFC3/IR MULTIAC IR F140W 2 none none none 1 N/A CUM UVISTA-Z-019 1 WFC3/IR MULTIAC IR F140W 3 none none none 2 N/A CUM UVISTA-Z-019 1 WFC3/IR MULTIAC IR F140W 4 none none none 2 N/A CUM UVISTA-Z-019 2 WFC3/IR MULTIAC IR F140W 1 none none none 1 N/A CUM UVISTA-Z-019 2 WFC3/IR MULTIAC IR F140W 2 none none none 1 N/A CUM UVISTA-Z-019 2 WFC3/IR MULTIAC IR F140W 3 none none none 2 N/A CUM UVISTA-Z-019 2 WFC3/IR MULTIAC IR F140W 4 none none none 2 N/A CUM 16506( 2) - 26-Mar-2021 17:02:49 - [ 7] Summary Form for Proposal 16506 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures IR ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/IR ------------------------------------------------------------------------------------------------------------------------------------ Opmodes MULTIACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters NSAMP=13, NSAMP=14, SAMP-SEQ=SPARS50 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 1-2 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F140W ------------------------------------------------------------------------------------------------------------------------------------ Target Names UVISTA-Z-007, UVISTA-Z-019 ------------------------------------------------------------------------------------------------------------------------------------