UV–Visible observations with HST in the JWST North Ecliptic Pole Time-Domain Field. IV. A Cycle 28+29 update

We report on a UV–Visible HST imaging survey of the JWST North Ecliptic Pole (NEP) Time-Domain Field (TDF). As part of a multi-cycle program (GO-15278, 16252, and 16793) we aim to secure observations with WFC3/UVIS in F275W and with ACS/WFC in F435W and F606W to m_AB ~ 27–28.5 mag (10σ). These wavelengths are inaccessible to JWST, and the resolution and depth are comparable to medium-deep observations with JWST/NIRCam. Near-contiguous 3-filter HST coverage of the central 10' diameter portion of the field was obtained during Cycle 25, and Cycle 28+29 observations will expand the HST coverage to encompass this new community field for time-domain science with JWST (Jansen & Windhorst 2018). The JWST NEP TDF, centered at (α, δ)_J2000 = (17:22:47.896, +65:49:21.54), is located within JWST’s northern Continuous Viewing Zone, spans ~14' in diameter (an area of ~154 arcmin²), is devoid of sources bright enough to saturate the sensitive NIRCam detectors, and has low Galactic foreground extinction (E(B-V) ≤ 0.028 mag). JWST GTO program 1176 will initially sample the NEP TDF during Cycle 1 at four distinct orientations (“spokes”) with JWST/NIRCam, providing 8-filter (0.8–5 μm) imaging to m ~ 28.5 mag (F200W, 10σ), and take NIRISS 1.75–2.23 μm slitless grism spectroscopy in parallel. The NIRCam imaging will almost fully overlap the area with spectroscopic coverage. This is the only region in the sky where JWST can observe a clean extragalactic deep survey field of this size at arbitrary cadence or at arbitrary orientation. This will enable a wide range of new and exciting time-domain science, including high redshift transient searches and monitoring (e.g., SNe), variability studies ranging from Active Galactic Nuclei to brown dwarf atmospheres, as well as proper motions of extreme scattered Kuiper Belt Objects and comets beyond the distance of Neptune, and of nearby Galactic brown dwarfs, low-mass stars, and ultra-cool Galactic halo stars. Ancillary data across the electromagnetic spectrum, from hard X-rays through long-wave radio, exist for the NEP TDF and surrounding area, ensuring a rich legacy of the UV–Visible HST and near-IR JWST observations. We provide an update on the HST observations already in hand and still to be executed during Cycle 29.