RUBIES: A complete census of the bright and red distant Universe with JWST/NIRSpec

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
² Cosmic Dawn Center (DAWN), Copenhagen, Denmark
³ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, Copenhagen, Denmark
⁴ Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁵ Department of Astronomy, University of Wisconsin-Madison, 475 N. Charter St., Madison, WI 53706, USA
⁶ Department of Physics and Astronomy and PITT PACC, University of Pittsburgh, Pittsburgh, PA 15260, USA
⁷ Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, The Netherlands
⁸ Department of Astronomy & Astrophysics, The Pennsylvania State University, University Park, PA 16802, USA
⁹ Institute for Computational & Data Sciences, The Pennsylvania State University, University Park, PA 16802, USA
¹⁰ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
¹¹ Department of Astronomy, The University of Texas at Austin, Austin, TX, USA
¹² Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08544, USA
¹³ Institute of Physics, Laboratory for Galaxy Evolution, Ecole Polytechnique Federale de Lausanne, Observatoire de Sauverny, Chemin Pegasi 51, 1290 Versoix, Switzerland
¹⁴ Department of Astronomy & Astrophysics, University of Chicago, 5640 S Ellis Avenue, Chicago, IL 60637, USA
¹⁵ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Melbourne, VIC 3122, Australia
¹⁶ Institute of Science and Technology Austria (ISTA), Am Campus 1, 3400 Klosterneuburg, Austria
¹⁷ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University, 1800 Sherman Ave, Evanston, IL 60201, USA
¹⁸ MIT Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Ave., Cambridge, MA 02139, USA
¹⁹ Department for Astrophysical & Planetary Science, University of Colorado, Boulder, CO 80309, USA
²⁰ Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
²¹ NSF’s National Optical-Infrared Astronomy Research Laboratory, 950 North Cherry Avenue, Tucson, AZ 85719, USA

^⋆ Corresponding author; degraaff@mpia.de

Received: 9 September 2024
Accepted: 17 March 2025

Abstract

We present the Red Unknowns: Bright Infrared Extragalactic Survey (RUBIES) providing JWST/NIRSpec spectroscopy of red sources selected across ∼150 arcmin² from public JWST/NIRCam imaging in the UDS and EGS fields. The novel observing strategy of RUBIES offers a well-quantified selection function. The survey has been optimised to reach high (>70%) spectroscopic completeness for bright and red (F150W−F444W>2) sources that are very rare. To place these rare sources in context, we simultaneously observed a reference sample of the 2<z<7 galaxy population, sampling sources at a rate that is inversely proportional to their number density in the 3D parameter space of F444W magnitude, F150W−F444W colour, and photometric redshift. In total, RUBIES observed ∼3000 targets across 1<z_phot<10 with both the PRISM and G395M dispersers and ∼1500 targets at z_phot>3 using only the G395M disperser. The RUBIES data reveal a highly diverse population of red sources that span a broad redshift range (z_spec∼1−9), with photometric redshift scatter and an outlier fraction that are three times higher than for similarly bright sources that are less red. This diversity is not apparent from the photometric spectral energy distributions (SEDs). Only spectroscopy reveals that the SEDs encompass a mixture of galaxies with dust-obscured star formation, extreme line emission, a lack of star formation indicating early quenching, and luminous active galactic nuclei. As a first demonstration of our broader selection function we compared the stellar masses and rest-frame U−V colours of the red sources and our reference sample. We find that the red sources are typically more massive (M_*∼10^10−11.5 M_⊙) across all redshifts. However, we also find that the most massive systems span a wide range in U−V colour. We describe our data reduction procedure and data quality, and we publicly release the reduced RUBIES data and vetted spectroscopic redshifts of the first half of the survey through the DAWN JWST Archive.