0 Assigned ID: 216 ----- Attempting Submission 1 (Thu Apr 04 16:43:34 GMT 2019) ----- HST Phase I Proposal 216 (Sunburst_Arc_LyC_v2.apt) successfully submitted. Receipt: # 216-1 ----- Attempting Submission 2 (Fri Apr 05 19:56:57 GMT 2019) ----- HST Phase I Proposal 216 (Sunburst_Arc_LyC_v2.apt) successfully submitted. Receipt: # 216-2 FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser This is adding two orbits that have been accepted through a HOPR request. 2001899808 ----- Attempting Submission 1 (Thu Jul 25 20:03:30 GMT 2019) ----- HST Phase II Proposal 15966 (Sunburst_Arc_LyC_v2.apt) successfully submitted. Receipt: # 15966-1 ----- Attempting Submission 2 (Thu Aug 08 20:36:33 GMT 2019) ----- ----- Attempting Submission 3 (Tue Oct 05 16:28:22 GMT 2021) ----- These errors are related to FLASH values and we are keeping these values consistent with previous versions of the observations It is a remarkable fact that the shape and strength of the ionizing continuum of the most massive stars has NEVER been spectroscopically mapped, at ANY redshift. Locally, the sparsity of such stars and the Hydrogen opacity precludes observations, and by the time redshift gives us access to these rest wavelengths in distant galaxies they are normally too faint, even for HST. Here we propose to acquire, for the first time, spectroscopy of the ionizing continuum in a star-forming galaxy down to a restframe of 560A (or photon energies of 24 eV) by observing THE brightest known gravitationally lensed galaxy, the Sunburst Arc. Our proposed observations - WFC3 grism spectroscopy - will directly probe the strength and spectral shape of the ionizing continuum in this galaxy. This will be the first-ever opportunity to directly compare the measured ionizing continuum against that inferred by the non-ionizing continuum and nebular emission lines. With this unprecedented observation, we will directly observe the strength and shape of the ionizing continuum, test whether the observed ionizing continuum is capable of producing the previously observed nebular emission lines, inform the next generation of theoretical models of stellar populations, and determine the impact of binary evolution on the production of ionizing photons. These observations will leave a lasting legacy for Hubble by mapping the unseen source of most extragalactic observations: the ionizing continuum of massive stars. Here we propose WFC3 UVIS G280 grism observations to obtain low-resolution spectra of the ionizing continuum from the Sunburst Arc, which as a bright gravitationally lensed redshift 2.4 galaxy. To acheive this will break the 27 orbits up into 6 visits with a maximum of 5 orbits per visit. We define the Sunburst-Arc RA/Dec as the coordinates of the brightest LyC emitting region and then place that region at the center of Chip 1 using the POS TARG Y of -50 for each exposure. This is to boost the QE and flux calibration of the brightest LyC region. Each visit will start with two undispersed 70s F300X images to set the location of the star-forming region. The proposed 140s of total integration time for the undispersed image is expected to produce a SNR of 5 for each visit. We have set the undispersed aperture of the F300X images to the G280-REF. We will use post-flash for the F300X images to achieve a background of 12e-. We will then follow with 2700-3200s of integration using the G280 grating. We will not dither within a single visit, rather each visit follows the WFC3-UVIS-Dither-Box pattern, as outlined in the COS/WFC3 handbook. We use previous HST IR G141 observations (HST PID: 15101) to set a strict orientation angle of 27.37. The F275W images (HST PID: 15418) suggest that the foreground cluster will be much fainter in the G280 grating than the IR G141, but we restrict the strong orientation constraints to ensure that redder wavelengths from the foreground cluster do not contaminate the traces of the Sunburst Arc. Our team is uniquely qualified to reduce, interpret and delivery timely analysis of the ionizing continuum of the Sunburst Arc. First, our team has an intimate knowledge of the target. We have discovered (Co-I Dahle: Dahle et al. 2016) and characterized the restframe UV continuum (Co-I Rivera-Thorsen: Rivera-Thorsen et al. 2017, 2019) of the object. We have obtained restframe UV spectra of ten images using the Magellan Telescope with the MagE as well as the MIKE instrument (Co-I Rigby et al. 2018; Co-I Bayliss et al. in preparation). Our team (through Co-I Dahle) received HST restframe ionizing continuum observations (Proposal ID: 15418, PI: Dahle) and HST WFC3 grism observations which cover the restframe optical (Proposal ID: 15101, PI: Dahle). These grism observations will guide us in planning the orientations of the proposed grism observations. Finally, Co-I Bayliss has a Joint Chandra/HST broad-band imaging and 40ks of X-ray imaging of the Sunburst Arc (Proposal ID: 15377, PI: Bayliss). The proposed observations would allow our team to study the Sunburst Arc from restframe 600-5000A and characterize all of the ionizing sources, including stellar and non-stellar. Consequently, our team has intimate knowledge of the proposed object, all of the necessary ancillary data to make the comparison between the ionizing continuum and the UV and optical emission lines, and extensive observational experience with the galaxy. Second, our team consists of the forefront experts in stellar population synthesis modeling. This includes single-star models (Co-I Leitherer for Starburst99) as well as binary evolution (Co-I Eldridge for BPASS). Further, we have substantial expertise in using these models to fit the stellar continuum to observed spectra (PI: Chisholm et al. 2015, 2016, 2019). We will leverage this expertise to fully exhaust the possible models of the stellar ionizing continuum in an unbiased, agnostic, and all-inclusive manner to guide, refine, and develop future stellar models of the ionizing continuum. Co-I Sharon has recently used extensive 8 band HST imaging of the Sunburst Arc to produce extremely robust and precise lens models of the system. This modelling substantially improves the physical interpretation of the individual physical regions and the ionizing radiation produced therein. Finally, Co-I Prochaska was part of the first non-commissioning observations using the G280 grism (O'Meara et al. 2011) and helped develop the custom code to extract and reduce the G280 grism data. Co-I Prochaska maintains this data reduction pipeline (DRP) as part of the XIDL software package that is publicly available. Our team is uniquely positioned to observe, reduce, and interpret these new and exciting results, and we will leverage our vast observational and theoretical expertise to decipher what lurks below the Lyman-Limit. Galaxies and the IGM Stellar Physics 6 false false true false false false false false SUPPORTED GALAXY GRAVITATIONAL LENS STARBURST J2000 J2000 R-band: 17.8 WFC3/UVIS Spectroscopic No No G280 1565276390756 The scheduling data was generated by CASM 27.2. CASM was run in Phase II mode. The orientation requirements are determined by building a model based upon an empirical estimate of the trace of bright objects from previous G280 observations of QSOs. We then applied that empirical trace to each target and each foreground object in the proposed field using previous F606W observations. We then calculated the number of foreground traces that intersect the blue portions of the brightest targets. We then stepped through each possible orientation angle by rotating the HST orientation by .01 degrees and calculated how many foreground objects intersected the faint EUV of the background galaxy. We confirmed that the predicted orientation angle matched that from the APT. 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