We report on a UV–Visible HST imaging survey of the JWST North Ecliptic Pole (NEP) Time-Domain Field (TDF). Using nine CVZ and pseudo-CVZ opportunities, we secured observations with WFC3/UVIS in F275W and with ACS/WFC in F435W and F606W to mAB ∼ 28 mag. Our HST survey is designed to provide near-contiguous 3-filter coverage of the central r∼<5 ′ of this new community field for time-domain science with JWST (Jansen & Windhorst 2018). The JWST NEP TDF at (RA,Dec)J2000 = (17:22:47.896, +65:49:21.54) is located within JWST’s northern Continuous Viewing Zone, will span ∼14 in diameter, is devoid of sources bright enough to saturate the sensitive NIRCam detectors, has low Galactic foreground extinction, and will be roughly circular in shape. JWST GTO program 1176 will initially sample the NEP TDF during Cycle 1 at four distinct orientations (“spokes”) with JWST/NIRCam, and take NIRISS slitless grism spectroscopy in parallel such that it overlaps the coverage of an alternate NIRCam orientation. 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 crucially enable a wide range of new and exciting time-domain science, including high redshift transient searches and monitoring (e.g., SNe), variability studies from Active Galactic Nuclei to brown dwarf atmospheres, as well as proper motions of extreme scattered Kuiper Belt and comets beyond the distance of Neptune, and of nearby Galactic brown dwarfs, low-mass stars, and ultracool white dwarfs. Ancillary data across the electromagnetic spectrum will exist for the NEP TDF and surrounding area when JWST science operations commence in 2021, ensuring a rich legacy of the UV–Visible HST observations. This includes deep X-ray observations; ground-based UgrizYJHK imaging, narrow-band spectrophotometry, and spectroscopy; (sub)mm observations; and both shortand long-wave radio observations. Fig. 1 — Measured astrometric offsets between the HST and Gaia DR2 astrometric reference frame as defined by 364 detected sources in common for [a] the initial pipeline-reduced drizzled and CTIcorrected mosaics, as reported by the WCS keyword values in the FITS headers, and [b] the same mosaics after correcting for systematic mean rotation and offsets resulting from the uncertainties in the absolute positions of the guide stars used in a particular visit. (Adapted from Jansen, Grogin, et al. 2020, in prep.). [c] Measured astrometric residuals with respect to the Gaia DR2 astrometric reference frame from a preliminary analysis of 24,013 objects observed in Sloan-g with both LBT/LBC and Subary/HSC within the ∼23×25 area surrounding the JWST NEP Time-Domain Field. (Adapted from V. Jones 2019). ✶✻ ✶✽ ✷✵ ✷✷ ✷✹ ✷✻ ✷✽ ❋✷✼✺❲ ✶✻ ✶✽ ✷✵ ✷✷ ✷✹ ✷✻ ✷✽ ❋✹✸✺❲ ✵✳✵✵ ✵✳✷✺ ✵✳✺✵ ✵✳✼✺ ✶✳✵✵ ✶✳✷✺ ✶✳✺✵ ✶✻ ✶✽ ✷✵ ✷✷ ✷✹ ✷✻ ✷✽ ❋✻✵✻❲ ✵ ✳✷ ✵✳✹ ✵✳✻ ✵ ✽ ✶✳✵ ❋❲ ▼ ❬✥✥❪ ✵✳✵ ✵ ✷ ✵✳✹ ✵ ✻ ✵✳✽ ✶ ✵ ♠ ✁ ✂ ✄☎ ✆ ✝ ✶✵ ✶✵✷ ✶✵✸ ✶✵✹ ✶✵✺ ✶✵✻ ❋✁✼✂❲ ✶✵ ✶✵✷ ✶✵✸ ✶✵✹ ✶✵✺ ✶✵✻ ❋✄☎✂❲ ✶✆ ✶✽ ✁✵ ✁✁ ✁✄ ✁✆ ✁✽ ✶✵ ✶✵✷ ✶✵✸ ✶✵✹ ✶✵✺ ✶✵✻ ❋✆✵✆❲ ✵✳✵ ✵✳✁ ✳✄ ✵✳✆ ✵✳✽ ✶✳✵ ♠✝✞ ❬✟✠✡ ✵✳✵ ✵✳✁ ✵✳✄ ✵ ✆ ✵✳✽ ✵ ◆ ☛☞ ✌ ✍✎ ✏ ✑ ✒ ✮ ✥ ✓ ❞ ✔ ✒ ✥ ✕ ❪ Fig. 2 — [left] Star-Galaxy separation, and [right] differential number counts. Point sources were identified and the surface brightness limit assessed in plots of source magnitude vs. fwhm for each of the three HST filters. Differential number counts of stars (red) and galaxies (black) are shown in units of number per 0.5 mag per deg for the ACS/WFC filters, and in units of number per mag per deg for the WFC3/UVIS F275W filter. The galaxy number counts follow a power law to mAB≃ 26–28 mag, before turning over due to incompleteness, while the stellar counts follow a much shallower trend. (Figures from C. White 2019).

What are the faintest distant galaxies we can see with the Hubble Space Telescope (HST) now, before the launch of the James Webb Space Telescope? This is the challenge taken up by the Frontier Fields, a Director's discretionary time campaign with HST and the Spitzer Space Telescope to see deeper into the universe than ever before. The Frontier Fields combines the power of HST and Spitzer with the natural gravitational telescopes of massive high-magnification clusters of galaxies to produce the deepest observations of clusters and their lensed galaxies ever obtained. Six clusters-Abell 2744, MACSJ0416.1-2403, MACSJ0717.5+3745, MACSJ1149.5+2223, Abell S1063, and Abell 370-have been targeted by the HST ACS/WFC and WFC3/IR cameras with coordinated parallel fields for over 840 HST orbits. The parallel fields are the second-deepest observations thus far by HST with 5s point-source depths of similar to 29th ABmag. Galaxies behind the clusters experience typical magnification factors of a few, with small regions magnified by factors of 10-100. Therefore, the Frontier Field cluster HST images achieve intrinsic depths of similar to 30-33 mag over very small volumes. Spitzer has obtained over 1000 hr of Director's discretionary imaging of the Frontier Field cluster and parallels in IRAC 3.6 and 4.5 mu m bands to 5 sigma point-source depths of similar to 26.5, 26.0 ABmag. We demonstrate the exceptional sensitivity of the HST Frontier Field images to faint high-redshift galaxies, and review the initial results related to the primary science goals.

The Frontier Fields are a director's discretionary time campaign with HST and the Spitzer Space Telescope to see deeper into the universe than ever before. The Frontier Fields combine the power of HST and Spitzer with the natural gravitational telescopes of massive high-magnification clusters of galaxies to produce the deepest observations of clusters and their lensed galaxies ever obtained. Six clusters - Abell 2744, MACSJ0416.1-2403, MACSJ0717.5+3745, MACSJ1149.5+2223, Abell S1063, and Abell 370 - were selected based on their lensing strength, sky darkness, Galactic extinction, parallel field suitability, accessibility to ground-based facilities, HST, Spitzer and JWST observability, and pre-existing ancillary data. These clusters have been targeted by the HST ACS/WFC and WFC3/IR with coordinated parallels of adjacent blank fields for over 840 HST orbits. The Spitzer Space Telescope has dedicated > 1000 hours of director's discretionary time to obtain IRAC 3.6 and 4.5 micron imaging to ~26.5, 26.0 ABmag 5-sigma point-source depths in the six cluster and six parallel Frontier Fields. The Frontier Field parallel fields are the second-deepest observations thus far by HST with ~29th ABmag 5-sigma point source depths in seven optical - near-infrared bandpasses. Galaxies behind the Frontier Field cluster lenses experience typical magnification factors of a few, with small regions near the critical curves magnified by factors 10-100. Therefore, the Frontier Field cluster HST images achieve intrinsic depths of ~30-33 magnitudes over very small volumes. Early studies of the Frontier Fields have probed galaxies fainter than any seen before during the epoch of reionization 6 < z < 10, mapped out the cluster dark matter to unprecedented resolution, and followed lensed transient events.

In 2006 March, the Hubble Heritage Team obtained a large, four-filter ( B, V, I, and Ha), 6 point mosaic data set of the prototypical starburst galaxy NGC 3034 ( M82) with the Advanced Camera for Surveys on board the Hubble Space Telescope (HST). The resulting color composite Heritage image was released in 2006 April to celebrate HST's 16th anniversary. Cycle 15 HST proposers were encouraged to submit General Observer and Archival Research proposals to complement and/or analyze this unique data set. Since our M82 mosaics represent a significant investment of expert processing beyond the standard archival products, we also released our drizzle-combined FITS data as a High- Level Science Product via the Multimission Archive at STScI in 2006 December. This paper documents the key aspects of the observing program and image processing: calibration, image registration and combination ( drizzling), and the rejection of cosmic rays and detector artifacts.

This paper presents the Hubble Ultra Deep Field (HUDF), a one million second exposure of an 11 square minute-of-arc region in the southern sky with the Advanced Camera for Surveys on the Hubble Space Telescope using Director’s Discretionary Time. The exposure time was divided among four filters, F435W (B435), F606W (V606), F775W (i775), and F850LP (z850), to give approximately uniform limiting magnitudes mAB ∼ 29 for point sources. The image contains at least 10,000 objects presented here as a catalog, the vast majority of which are galaxies. Visual inspection of the images shows few if any galaxies at redshifts greater than ∼ 4 that resemble present day spiral or elliptical galaxies. The image reinforces the conclusion from the original Hubble Deep Field that galaxies evolved strongly during the first few billion years in the infancy of the universe. Using the Lyman break dropout method to derive samples of galaxies at redshifts between 4 and 7, it is possible to study the apparent evolution of the galaxy luminosity function and number density. Examination of the catalog for dropout sources yields 504 B435-dropouts, 204 V606-dropouts, and 54 i775-dropouts. The i775-dropouts are most likely galaxies at redshifts between 6 and 7. Using these samples that are at different redshifts but derived from the same data, we find no evidence for a change in the characteristic luminosity of galaxies but some evidence for a decrease in their number densities between redshifts of 4 and 7. Assessing the factors needed to derive the luminosity function from the data suggests there is considerable uncertainty in parameters from samples discovered with different instruments and derived using independent assumptions about the source populations. This assessment calls into question some of the strong conclusions of recently published work on distant galaxies. The ultraviolet luminosity density of these samples is dominated by galaxies fainter than the characteristic luminosity, and the HUDF reveals considerably more luminosity than shallower surveys. The apparent ultraviolet luminosity density of galaxies appears to decrease from redshifts of a few to redshifts greater than 6, although this decrease may be the result of faint-end incompleteness in the most distant samples. The highest redshift samples show that star formation was already vigorous at the earliest epochs that galaxies have been observed, less than one billion years after the Big Bang. Subject headings: astronomical data bases: miscellaneous — cosmology: early universe — galaxies: evolution — galaxies: high-redshift Space Telescope Science Institute Johns Hopkins University McDonald Observatory, University of Texas European Space Agency European Southern Observatory Space Telescope European Coordinating Facility US Naval Observatory, Flagstaff Station University of Texas Max-Planck-Institut für Astronomie

Deep, multiband observations of high Galactic latitude fields are an essential tool for studying topics ranging from Galactic structure to extragalactic background radiation. The Hubble Deep Field (HDF-N) observations obtained in 1995 December established a standard for such narrow, deep surveys. The field has been extensively analyzed by a variety of groups and has been widely studied with imaging and spectroscopy over wavelengths ranging from 10-3 to 2 × 105 μm. We describe here a second deep field campaign (HDF-S), this time in the southern hemisphere, undertaken by the Hubble Space Telescope (HST) in 1998 October in a program very similar to the northern Hubble Deep Field. Imaging and spectroscopy of three adjacent fields in the southern continuous viewing zone were obtained simultaneously for 150 orbits, and a mosaic of flanking fields was imaged for 27 additional orbits. Two important features of the HDF-S distinguish it from the HDF-N: the campaign included parallel observations by the three main HST instruments—WFPC2, STIS, and NICMOS—and the HDF-S location was selected to place a bright z = 2.24 quasar in the STIS field of view. The HDF-S observations consist of WFPC2 images in filters close to U, B, V, and I, a deep STIS image of the field surrounding the quasar, spectroscopy of the quasar with STIS from 1150 to 3560 Å, and deep imaging of an adjacent field with NICMOS camera 3 at 1.1, 1.6, and 2.2 μm. All of the HDF-S data were fully reduced and made publicly available within 2 months of the observations, and we describe here the selection of the fields and the observing strategy that was employed. Detailed descriptions of the data and the reduction techniques for each field, together with the corresponding source catalogs, appear in separate papers.