2025ApJ...992...70K

We discovered a hyperluminous dust-obscured galaxy with mysterious blue-excess emission (BlueDOG) in the rest-frame UV of its spectral energy distribution (SED) from a multiwavelength survey in the AKARI Deep Field - South (ADF-S). We present the results of SED analysis with multiwavelength photometric data and spectroscopic analysis, observed with Gemini-S/GMOS, FLAMINGOS-2, to explore the origin of blue-excess emission of a hyperluminous BlueDOG, ADFS-KMTDOG-102, at z = 2.6. The SED analysis shows that this BlueDOG is a highly massive system (log M_*/M_⊙ = 12.3) with substantial extinction. Additionally, the proportion of the old stellar population exceeds that of the young stellar population, which suggests stellar evolution cumulated from the early Universe. The mass of the supermassive black hole estimated using the extinction-corrected broad Hα emission line yields log M_BH/M_⊙ = 10.2. We discuss the similarity between the BlueDOG and “Little Red Dots” (LRDs), recently discovered with the James Webb Space Telescope, showing SED shapes remarkably similar to those of LRDs. The UV emission line ratios indicate that the emission lines are primarily powered by the central active galactic nuclei (AGN). In contrast, the origin of the blue-excess UV continuum remains ambiguous, since both recent star formation and AGN-induced scattered light are viable explanations, based on the results from the SED fitting and scattered light modeling.