2026A&A...707A.212M

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.