The rise and fall of little red dots could be driven by the environment

¹ Institute for Computational Astrophysics and Department of Astronomy and Physics, Saint Mary’s University 923 Robie Street Halifax NS B3H 3C3, Canada
² Observatorio Astronómico de Córdoba, Universidad Nacional de Córdoba Laprida 854 X5000 Córdoba, Argentina
³ Dunlap Institute for Astronomy and Astrophysics 50 St. George Street Toronto Ontario M5S 3H4, Canada
⁴ Waseda Research Institute for Science and Engineering, Faculty of Science and Engineering, Waseda University 3-4-1 Okubo Shinjuku Tokyo 169-8555, Japan
⁵ Department of Astronomy, Kyoto University Sakyo-ku Kyoto 606-8502, Japan
⁶ National Research Council of Canada, Herzberg Astronomy & Astrophysics Research Centre 5071 West Saanich Road Victoria BC V9E 2E7, Canada
⁷ Faculty of Mathematics and Physics Jadranska ulica 19 SI-1000 Ljubljana, Slovenia
⁸ Department of Physics and Astronomy, York University 4700 Keele St. Toronto Ontario M3J 1P3, Canada
⁹ Space Telescope Science Institute 3700 San Martin Drive Baltimore Maryland 21218, USA
¹⁰ INAF – Osservatorio Astronomico di Roma Via Frascati 33 Monte Porzio Catone 00078, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 7 October 2025
Accepted: 19 January 2026

Abstract

The little red dot (LRD) paradigm comprises three main unknowns that are intrinsically connected: (1) What is the nature of these sources? (2) How do they form? (3) How do they evolve? Larger spectroscopic samples and high-resolution data are needed to delve deeper into the mechanisms governing these sources. Understanding their formation and evolution requires identifying the rise and fall of the key features that characterize these systems, such as their compactness and “V”-shaped spectral energy distributions. In this work, we present a galaxy system nicknamed “The Stingray” that was identified in the Canadian NIRISS Unbiased Cluster Survey (CANUCS). This group contains three sources at z_spec = 5.12, including an active galactic nucleus (AGN), a Balmer break galaxy, and a star-forming satellite. The latter resembles a building blocks system in which interactions boost stellar mass and black hole mass growth beyond what is expected from secular processes alone. The AGN in this system exhibits features indicative of a transitional object, bridging a normal AGN and an LRD phase. These features include a blue rest-frame UV slope, compact size, and a broad Hα line (all of which are characteristic of LRDs), but a flatter rest-frame optical slope compared to that observed in LRDs. The features in this source point to the emergence or fading of an LRD, potentially triggered by environmental effects.