TREASUREHUNT: Hubble's UV-Visible treasury imaging of the JWST NEP Time-Domain Field

We propose to complete the near-contiguous UV-Visible imaging of the JWST NEP Time-Domain Field, the best field within the JWST Continuous Viewing Zones to conduct deep and wide extragalactic JWST surveys at arbitrary cadence or orientation. This recently developed, ~14' diameter time-domain field near the North Ecliptic Pole (1) is devoid of sources brighter than m_AB ~ 16 mag at 2-4 micron that would deeply saturate JWST/NIRCam, (2) has the lowest possible zodiacal foreground, (3) has low Galactic foreground extinction, and (4) has a wealth of deep ground- and space-based ancillary data from X-ray through radio. Deep UV-Visible HST imaging to m_AB ~ 28 mag exists for the inner ~9' diameter portion of this field. Completing the coverage of the NEP Time-Domain Field to the same depth, in combination with extant data, allows the community to:

(1) Establish a baseline UV-Visible detection image at ~0.07" FWHM at wavelengths inaccessible to JWST, and map transients and moving objects in areas of 2- and 3-epoch overlap; (2) Identify rest-frame UV-bright objects at z < 6 that would contaminate JWST ultra-high-z galaxy samples; (3) Identify galaxies with steep intrinsic UV-Visible SEDs indicative of (weak)AGN, assess the escape of LyC photons at 2 < z < 3, and place limits on that escape and IGM porosity at z < ~3.5; and (4) Study mass assembly and evolution of all significantly resolved UV-bright galaxies at z < 6 on a 'pixel-by- pixel' basis.

In order to maximize the scientific return of early-release JWST science, it is critical to image this field in the UV-Visible while HST is still in its prime and before commencement of JWST observations in Fall 2021.