The BlueDOG at Cosmic Noon: A Possible Analog to Little Red Dots?

Seongjae Kim, Woong-Seob Jeong, Minjin Kim, Hyunsung D. Jun, Yujin Yang, Takao Nakagawa

First listed 2025-08-27 | Last updated 2025-08-27

Abstract

We discovered a hyperluminous dust-obscured galaxy with mysterious blue-excess emission (BlueDOG) in rest-frame UV of its spectral energy distribution (SED) from a multi-wavelength 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 $M_{*}$/$M_\odot=12.3$) 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 supermassive black hole (SMBH) estimated using the extinction-corrected broad H$\rmα$ emission line yields log $M_{\rm BH}$/$M_\odot$=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.

Short digest

Spectro-photometry of the hyperluminous BlueDOG ADFS-KMTDOG-102 (z=2.604) in ADF-S shows an extremely massive, heavily obscured system (log M*=12.3) with SED shape akin to JWST little red dots. Gemini-S GMOS+FLAMINGOS-2 spectra indicate AGN-dominated UV line ratios, a double-peaked, ~2″-extended Lyα, and an extinction-corrected broad Hα giving an enormous black hole (log MBH=10.2). The source thus links DOG-like hyperluminous systems to LRD-like SEDs at cosmic noon, suggesting rapid black-hole growth in a mature stellar host. The blue-excess UV continuum remains unresolved—consistent with either recent star formation or AGN-scattered light, with the Lyα escape fraction sitting near LAE relations.

Key figures to inspect

• Fig. 1: Slit placements over the KMTNet color image—verify that the GMOS slit crosses the nucleus and the off-nuclear blue-excess knot targeted in the B band.
• Fig. 2 (left/center): Inspect the double-peaked, spatially extended (~2″) Lyα and note the skyline-contaminated C IV; together they establish AGN-powered UV lines and extended resonant scattering.
• Fig. 2 (right): Broad+narrow Hα decomposition used for the MBH estimate; check the [S II] masked region and continuum handling near the FLAMINGOS-2 band edge.
• Fig. 3: Best-fit SED components (stellar, AGN with polar dust, and SF-heated dust) demonstrating old-star dominance and the LRD-like SED shape.
• Fig. 4: Lyα escape fraction vs E(B−V) placing the object near LAE trends; compare the SFR-derived and AGN-derived cases to gauge the plausibility of scattered-light vs young-star origins.