A remarkable ruby: Absorption in dense gas, rather than evolved stars, drives the extreme Balmer break of a little red dot at z = 3.5

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
² MIT Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Ave., Cambridge, MA 02139, USA
³ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Melbourne VIC 3122, Australia
⁴ Department of Astronomy & Astrophysics, The Pennsylvania State University, University Park, PA 16802, USA
⁵ Institute for Computational & Data Sciences, The Pennsylvania State University, University Park, PA, 16802, USA
⁶ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
⁷ Institute of Science and Technology Austria (ISTA), Am Campus 1, 3400 Klosterneuburg, Austria
⁸ Department of Astronomy & Astrophysics, University of Chicago, 5640 S Ellis Avenue, Chicago, IL, 60637, USA
⁹ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ, 08544, USA
¹⁰ Astronomy Department, Yale University, 219 Prospect St, New Haven, CT 06511, USA
¹¹ Department of Physics and Astronomy and PITT PACC, University of Pittsburgh, Pittsburgh, PA 15260, USA
¹² Leiden Observatory, Leiden University, PO Box 9513 NL-2300 RA Leiden, The Netherlands
¹³ Cosmic Dawn Center (DAWN), Copenhagen, Denmark
¹⁴ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, Copenhagen, Denmark
¹⁵ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
¹⁶ Institute of Physics, Laboratory for Galaxy Evolution, Ecole Polytechnique Fédérale de Lausanne, Observatoire de Sauverny, Chemin Pegasi 51, 1290 Versoix, Switzerland
¹⁷ Department of Astronomy, University of Wisconsin-Madison, 475 N. Charter St., Madison, WI 53706, USA
¹⁸ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University, 1800 Sherman Ave, Evanston, IL 60201, USA
¹⁹ Department for Astrophysical and Planetary Science, University of Colorado, Boulder, CO 80309, USA
²⁰ Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
²¹ Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir km 4, Torrejón de Ardoz, E-28850 Madrid, Spain
²² Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
²³ NSF’s National Optical-Infrared Astronomy Research Laboratory, 950 North Cherry Avenue, Tucson, AZ, 85719, USA

^⋆ Corresponding author: degraaff@mpia.de

Received: 21 March 2025
Accepted: 30 June 2025

Abstract

The origin of the rest-optical emission of compact, red, high-redshift sources known as little red dots (LRDs) poses a major puzzle. If interpreted as starlight, it would imply that LRDs constitute the densest stellar systems in the Universe. However, alternative models suggest active galactic nuclei (AGN) may instead power the rest-optical continuum. Here, we present JWST/NIRSpec, NIRCam, and MIRI observations from the RUBIES and PRIMER programs of The Cliff: a bright LRD at z = 3.55 with an exceptional Balmer break, twice as strong as that of any high-redshift source previously observed. The spectra also reveal broad hydrogen (Hα FWHM ∼ 1500 km s⁻¹) and He I emission, but no significant metal lines. We demonstrate that massive evolved stellar populations cannot explain the observed spectrum, even when considering unusually steep and strong dust attenuation or reasonable variations in the initial mass function. Moreover, the formally best-fit stellar mass and compact size (M_* ∼ 10^10.5 M_⊙,  r_e ∼ 40 pc) would imply densities at which near-monthly stellar collisions might lead to significant X-ray emission. We argue that the Balmer break, emission lines, and Hα absorption line are instead most plausibly explained by a black hole star (BH^*) scenario, in which dense gas surrounds a powerful ionising source. In contrast to recently proposed BH^* models of dust-reddened AGN, we show that spectral fits in the rest UV to near-infrared favour an intrinsically redder continuum over strong dust reddening. This may point to a super-Eddington accreting massive black hole or, possibly, the presence of (super)massive stars in a nuclear star cluster. The Cliff is the clearest evidence to date that at least some LRDs are not ultra-dense massive galaxies, and are instead powered by a central ionising source embedded in dense, absorbing gas.