The goal of Hubble Space Telescope Cycle 27–29 Archival Legacy project “SKYSURF” is to measure the panchromatic sky surface brightness and source catalogs from all archival HST ACS and WFC3 images since the launch of these instruments by the Space Shuttle—more than 57,000 images in total since 2002. All SKYSURF images together will measure the panchromatic Zodiacal brightness, the Diffuse Galactic Light, and the Extragalactic Background Light. SKYSURF will significantly constrain the various amounts of diffuse light in the universe with major ramifications for cosmic star formation and planet formation.<br/><br/>Several sky background measurement algorithms are capable of measuring the background levels of images in the SKYSURF database. To test the fidelity of these sky background measurement algorithms, images with known sky background and noise levels were necessary to determine quantitatively how far a sky measurement algorithm strays from the true value. For this purpose, I developed an algorithm that could create simulated images for filter F125W of the WFC3/IR instrument on the Hubble Space Telescope (HST). Filter F125W was selected because the Extragalactic Background Light is brightest in this wavelength band; moreover, the COBE Zodiacal light measurement is also at 1.25 microns. The simulated images created contain stars, galaxies, cosmic rays, and light gradients. We discuss here how these simulated images were made and the different kinds of simulated images that were produced.

The reionization of the Universe is thought to have completed by redshift z~5.5. To probe this era, galaxy observations in the Subaru Deep Field (SDF) have identified more than 100 galaxies at z~6, many spectroscopically confirmed through follow-up observations. Using available optical/IR data, we model with CIGALE the spectral energy distributions (SEDs) of 43 SDF galaxies, including newly acquired data from the UKIRT WFCAM K-band for seven previously studied objects. In particular, modeling deep IR photometry is sensitive to the galaxy's Lyman continuum (LyC) escape fraction (fesc). We find the median implied fesc value as ~0.4+/-0.1 (mean error). Significant uncertainties in data and fitting result in a large range of fesc for individual objects, but analysis suggests that fesc is likely high enough for galaxies to finish reionization by z~6. More importantly, we find trends between the CIGALE UV slope b, fesc, and dust extinction E(B-V): for a given E(B-V), b appear steeper by ~0.4 than at z=0. Lower fesc values appear to be associated with bluer b and lower E(B-V), but only weakly. This suggests that LyC could have escaped through holes with sufficiently wide opening angles surrounding the ISM from outflows of supernovae and/or weak AGN to escape, but resulting in a large range of implied fesc values depending on the orientation of each galaxy. The current HST, Spitzer and ground-based photometric and model errors for the 43 galaxies are large, so IR spectroscopic observations with the James Webb Space Telescope are needed to better constrain this possibility.