0 Assigned ID: 427 ----- Attempting Submission 1 (Fri Apr 08 09:26:39 GMT 2016) ----- HST Phase I Proposal 427 (SPT0311_GO24.apt) successfully submitted. Receipt: # 427-1 ----- Attempting Submission 2 (Fri Apr 08 23:30:37 GMT 2016) ----- HST Phase I Proposal 427 (SPT0311_GO24.apt) successfully submitted. Receipt: # 427-2 ----- Attempting Submission 3 (Fri Apr 08 23:59:22 GMT 2016) ----- 0 ----- Attempting Submission 1 (Thu Jul 14 19:26:47 GMT 2016) ----- We request deep multi-band imaging of a unique dusty galaxy in the Epoch of Reionization (EoR), selected via its millimeter-wavelength dust emission in the 2500-square-degree South Pole Telescope survey. Spectroscopically confirmed to lie at z=6.900, this galaxy has a large dust mass and is likely one of the most rapidly star-forming objects in the EoR. Using Gemini-S, we have identified z-band emission from this object that could be UV continuum emission at z=6.9 or from a foreground lens. Interpretation of this object, and a complete understanding of its meaning for the census of star formation in the EoR, requires that we establish the presence or absence of gravitational lensing. The dust mass observed in this source is also unexpectedly large for its era, and measurements of the assembled stellar population, through the UV-continuum slope and rest-frame optical color, will help characterize the stellar mass and dust properties in this very early galaxy, the most spectacular galaxy yet discovered by the SPT. We propose five-band imaging with HST /ACS, HST /WFC3, and Spitzer /IRAC of a unique dusty galaxy at z = 6.9. These observations are designed to confirm or refute the lensing hypothesis, provide a photometric redshift estimate for the putative lens galaxy, and to search for rest-UV and rest-optical emission from the background galaxy. Our target depths are informed by our deep Gemini imaging, as well as a handful of lower redshift analogues with HST imaging (HFLS3: Cooray et al. 2014; SPT DSFGs: Ma et al. 2015), and are outlined in Table 1. Five orbits are required. Why HST? - SPT0311-58 is a very dusty, very distant object, that is likely lensed by a 24th magnitude galaxy. The most comparable object, a dusty maximum starburst galaxy at z = 6.34 (HFLS3; Riechers et al. 2013), was only separable from two nearby 24/25th magnitude galaxies with HST and only after subtraction of the foreground objects. No sufficiently bright guide stars for adaptive optics lie near the source. If the source is lensed, the unmatched depth and sharpness of HST imaging will be required to distinguish any rest-frame UV emission from the target galaxy, and to deconvolve source/lens in the Spitzer imaging. High-resolution ALMA submillimeter continuum observations are also scheduled for April/May as part of a survey of our targets that was approved for ALMA Cycle 3 (our redshift identification came from this program), which will greatly aid in the interpretation of the HST and Spitzer imaging. Filters and Depths - Assuming that the Gemini-detected object is in the foreground, its i-z>0.6 color is consistent with a z~1 early-type galaxy (Fukugita et al. 1995). For interpretation of the background object we must photometrically determine the redshift for this object and estimate its mass. In the event that the lensing geometry is not easily constrained from the ALMA data, the combination of a well-determined position and the mass/redshift of the lens will allow a probabilistic magnification estimate, similar to HFLS3 (Cooray et al. 2014). Our observations are designed to provide a reliable photometric redshift for a very wide range of possible lens redshifts. A single orbit divided between two ACS filters, F606W and F775W (i), will cover the Balmer break for 0.2 < z < 1.1. For higher redshifts there will be two filters on either side of the break out to z = 1.75, above which the break passes through WFC3/F125W. The zAB = 24.4 Gemini measurement shows that the half-orbit ACS depths of 27.2 and 26.5 (5sig, per ACS ETC) will be adequate. To target the z = 6.9 dusty source, we use deep imaging with WFC3/IR and Spitzer. The two ACS filters correspond to the blue side of the Lyman break at this redshift, so we expect no emission. We request two IR filters to enable a measurement of the UV spectral slope, and to provide color information that may help separate foreground and background sources. We will be able to infer dust properties and column densities through the infrared- excess to UV-slope (IRX-beta) relation (Reddy et al. 2010; Casey et al. 2014b), which relates the amount of dust absorption measured in the UV to the amount of infrared re-emission. We select the longest-wavelength bands possible (F125W and F160W), spanning rest wavelengths of 140-210 nm. While pairing F160W with either F105W or F110W would provide a longer wavelength baseline to measure the UV slope, both of these filters would include any LyA (Lambda_obs = 960 nm) flux, complicating the interpretation of the rest-UV slope. Cooray et al. (2014) found that after subtracting out two foreground objects, residual emission from HFLS3 was visible in both F125W and F160W at mAB = 27.0. As this is the best example we presently have of a dusty galaxy near the EoR, we use these values to guide our integration times. Two-orbit integrations in both filters will achieve depths of mAB = 27.4 and 27.1 (5sig, per WFC3 ETC), which would be adequate to detect similar residual stellar continuum. The combination of both bands will allow us to push even deeper, though without color information. Spitzer/IRAC observations will probe the rest-frame optical (450, 570 nm) emission. These data will likely provide the best available handle on the stellar content of the galaxy before JWST. Depths are determined from SED models fit to the FIR and optical data, as well as from the Cooray et al. (2014) SED for HFLS3. Two IRAC channels will provide color information that will improve our SED modeling of the stellar mass and stellar population ages. We note also that Smit et al. (2015) suggested IRAC 3.6/4.5um color could be contaminated by extreme line emission at similar redshifts, but fortunately the OIII and OII lines fall between and outside the filters for this object. Although our current imaging provides no information about the degree to which the foreground and background objects overlap, in Bothwell et al. (2013) and Ma et al. (2015) we have disentangled lens and background source emission using HST imaging or jointly between HST and Spitzer. It is possible that the observed z-band emission arises from the dusty source rather than a lens (see Figure 3 for a possible SED). This unlikely case will be readily identifiable by the lack of detections of a lens galaxy or any other emission in F606W and F775W, both of which are beyond the Lyman break. Intergalactic Medium and Cosmology Galaxies 6 false false false false false true SUPPORTED GALAXY GRAVITATIONAL LENS J2000 J2000 ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F775W WFC3/IR Imaging No No F125W WFC3/IR Imaging No No F160W POS-TARG DITHER 0.149 LINE false false 2 34.25 ACS-WFC-DITHER-LINE 1 false POS-TARG DITHER 0.572 LINE 0.365 false false 74.653 4 18.528 WFC3-IR-DITHER-BOX-MIN 2 false 1468522254723 The scheduling data was generated by CASM 24.2. CASM was run in Phase II mode. 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