HST: Hot or Cold? Improving Constraints on the Thermal Foreground of HST

Project SKYSURF is an HST Archival Legacy program that aims to provide an accurate characterization of the Zodiacal sky background, as well as both the discrete and the diffuse extragalactic background. As part of this effort, we found that thermal emission from HST itself represents a non-negligible and time-variable source of foreground emission in the redder WFC3IR filters that currently is not well constrained. This thermal emission contributes ~15% to the F160W background, and variations within HST’s orbit may bring it up to 18% of the background, similar to the expected level of the diffuse extragalactic background. Here, we report that by modeling the near-IR spectral energy distribution of the background in HST observations, we are able to precisely constrain the level of this emission, and identify any variability. Our improved characterization of the relevant sources of HST backgrounds will improve the ability to perform accurate surface photometry of extended, diffuse targets, and will reduce uncertainties in the inferred level of the diffuse Extragalactic Background Light.