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Over the past decades, rest-frame ultraviolet (UV) observations have provided large samples of UV luminous galaxies at redshift (z) greater than 6 (refs. 1–3), during the so-called epoch of reionization. While a few of these UV-identified galaxies revealed substantial dust reservoirs4–7, very heavily dust-obscured sources at these early times have remained elusive. They are limited to a rare population of extreme starburst galaxies8–12and companions of rare quasars13,14. These studies conclude that the contribution of dust-obscured galaxies to the cosmic star formation rate density at z> 6 is sub-dominant. Recent ALMA and Spitzer observations have identified a more abundant, less extreme population of obscured galaxies at z= 3−6 (refs. 15,16). However, this population has not been confirmed in the reionization epoch so far. Here, we report the discovery of two dust-obscured star-forming galaxies at z= 6.6813 ± 0.0005 and z= 7.3521 ± 0.0005. These objects are not detected in existing rest-frame UV data and were discovered only through their far-infrared [C ii] lines and dust continuum emission as companions to typical UV-luminous galaxies at the same redshift. The two galaxies exhibit lower infrared luminosities and star-formation rates than extreme starbursts, in line with typical star-forming galaxies at z≈ 7. This population of heavily dust-obscured galaxies appears to contribute 10–25% to the z> 6 cosmic star formation rate density.

The James Webb Space Telescope (JWST) will be an exquisite new near-infrared observatory with imaging and multi-object spectroscopy through ESA’s NIRspec instrument with its unique Micro-Shutter Array (MSA), allowing for slits to be positioned on astronomical targets by opening specific 0′′.2-wide micro shutter doors. To ensure proper Target Acquisition (TA), the on-sky position of the MSA needs to be verified before spectroscopic observations start. An onboard centroiding program registers the position of pre-identified guide stars in a TA image, a short pre-spectroscopy exposure without dispersion (image mode) through the MSA with all shutters open. The outstanding issue is the availability of Galactic stars in the right luminosity range for TA relative to typical high redshift targets. We explore this here using the stars and z∼8candidate galaxies identified in the source extractor catalogs of Brightest of Reionizing Galaxies survey (BoRG[z8]), a pure-parallel program with Hubble Space Telescope Wide-Field Camera 3. We find that (a) a single WFC3 field contains enough Galactic stars to satisfy the NIRspec astrometry requirement (20 milli-arcseconds), provided its and the NIRspec TA’s are mlim>24.5AB in WFC3 F125W, (b) a single WFC3 image can therefore serve as the pre-image if need be, (c) a WFC3 mosaic and accompanying TA image satisfy the astrometry requirement at ∼23AB mag in WFC3 F125W, (d) no specific Galactic latitude requires deeper TA imaging due to a lack of Galactic stars, and (e) a depth of ∼24AB mag in WFC3 F125Wis needed if a guide star in the same MSA quadrant as a target is required. We take the example of a BoRG identified z∼8candidate galaxy and require a Galactic star within 20′′of it. In this case, a depth of 25.5 AB in F125Wis required (with ∼97% confidence).

We present a multi-wavelength study of galaxy populations in the core of the massive, X-ray luminous cluster XMMU J2235 at z= 1.39, based on high quality VLT and HST photometry at optical and near-infrared wavelengths.