RUBIES: A spectroscopic census of little red dots

All point sources with v-shaped continua have broad lines

¹ Max-Planck-Institut für Astronomie, Königstuhl 17 D-69117, Heidelberg, Germany
² Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University, 1800 Sherman Ave, Evanston, IL 60201, USA
³ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08544, USA
⁴ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Melbourne, VIC 3122, Australia
⁵ Cosmic Dawn Center (DAWN), Copenhagen, Denmark
⁶ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, Copenhagen, Denmark
⁷ Department of Physics and Astronomy and PITT PACC, University of Pittsburgh, Pittsburgh, PA 15260, USA
⁸ Leiden Observatory, Leiden University, PO Box 9513 NL-2300 RA Leiden, The Netherlands
⁹ Department of Astronomy & Astrophysics; Institute for Computational & Data Sciences; Institute for Gravitation and the Cosmos; The Pennsylvania State University, University Park, PA 16802, USA
¹⁰ Department of Astronomy, University of Wisconsin-Madison, Madison, WI 53706, USA
¹¹ Institute of Science and Technology Austria (ISTA), Am Campus 1 3400 Klosterneuburg, Austria
¹² MIT Kavli Institute for Astrophysics and Space Research, 70 Vassar Street, Cambridge, MA 02139, USA
¹³ Department of Astronomy, University of Geneva, Chemin Pegasi 51 1290 Versoix, Switzerland
¹⁴ Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
¹⁵ NSF National Optical-Infrared Astronomy Research Laboratory, 950 North Cherry Avenue, Tucson, AZ 85719, USA

^⋆ Corresponding author: hviding@mpia.de

Received: 4 June 2025
Accepted: 29 July 2025

Abstract

The physical nature of little red dots (LRDs), a population of compact red galaxies revealed by JWST, remains unclear. Photometric samples were constructed from varying selection criteria with limited spectroscopic follow-up available to test intrinsic spectral shapes and the prevalence of broad emission lines. We used the RUBIES survey, a large spectroscopic program with wide color-morphology coverage and homogeneous data quality, to systematically analyze the emission-line kinematics, spectral shapes, and morphologies of ∼1500 galaxies at z > 3.1. We identified broad Balmer lines via a novel fitting approach that simultaneously models NIRSpec/PRISM and G395M spectra, yielding 80 broad-line sources with 28 (35%) at z > 6. A large subpopulation naturally emerged from the broad Balmer line sources, with 36 exhibiting v-shaped UV-to-optical continua and a dominant point source component in the rest-optical; we define these as spectroscopic LRDs, constituting the largest such sample to date. Strikingly, the spectroscopic LRD population is largely recovered when either a broad line or rest-optical point source is required in combination with a v-shaped continuum, suggesting an inherent link between these three defining characteristics. We compared the spectroscopic LRD sample to published photometric searches. Although these selections have high accuracy, 80%−95% down to F444W < 26.5, only 50%−80% of the RUBIES LRDs were photometrically identified, depending on the selection criteria used. The remainder were missed due to a mixture of faint rest-UV photometry, comparatively blue rest-optical colors, or highly uncertain photometric redshifts. Our findings highlight that well-selected spectroscopic campaigns are essential for robust LRD identification, while photometric criteria require refinement to capture the full population.