The Discovery of Little Red Dots in the Local Universe: Signatures of Cool Gas Envelopes

Xiaojing Lin, Xiaohui Fan, Zheng Cai, Fuyan Bian, Hanpu Liu, Fengwu Sun, Yilun Ma, Jenny E. Greene, Michael A. Strauss, Richard Green, Jianwei Lyu, Jaclyn B. Champagne, Andy D. Goulding, Kohei Inayoshi, Xiangyu Jin, Gene C. K. Leung, Mingyu Li, Weizhe Liu, Yichen Liu, Junjie Mao, Maria Anne Pudoka, Wei Leong Tee, Ben Wang, Feige Wang, Yunjing Wu, Jinyi Yang, Haowen Zhang, Yongda Zhu

First listed 2025-07-14 | Last updated 2025-12-12

Abstract

JWST observations have revealed a population of high-redshift "little red dots" (LRDs) that challenge conventional AGN models. We report the discovery of three local LRDs at $z = 0.1$-$0.2$, initially selected from the SDSS database, with follow-up optical/near-IR spectroscopy and photometry. They exhibit properties fully consistent with those of high-redshift LRDs, including broad hydrogen and helium emission lines, compact morphologies, V-shaped UV-optical SED, declining near-IR continua, and no significant variability. Two sources were targeted but not detected in X-rays with statistical significance. All three sources show blue-shifted He I absorption, while two exhibit H$α$ and Na D absorption lines. We detect full Balmer and Paschen line series in all three objects, along with abundant narrow [Fe II] emission in two. The emission line analyses suggest narrow lines originate from AGN-powered, metal-poor regions with minimal dust; broad lines come from inner regions with exceptionally high density or atypical dust properties; and [Fe II] emission arises from dense gas between broad and narrow-line regions. One of our objects, J1025+1402 (nicknamed $The~Egg$), shows extremely high equivalent width Na D, K I, and Ca II triplet absorption lines, along with other potential low-ionization absorption features, suggesting the presence of a cool ($\sim$5000 K), metal-enriched gas envelope. The optical/near-IR continua of these LRDs are also consistent with theoretical models featuring an atmosphere around black holes. The WISE-detected IR emission is consistent with weak dust emission of $T \sim 10^2-10^3$ K. We propose a conceptual model consisting of a largely thermalized cool-gas envelope surrounding the central black hole and an extended emission line region with high-density outflowing gas to explain the observed properties of these local LRDs.

Short digest

Reports three SDSS-selected local little red dots at z=0.1–0.2 with LBT/MODS and Magellan/FIRE follow-up, showing the same hallmarks as high‑z LRDs: compact morphologies, V‑shaped UV–optical SEDs with declining NIR continua, broad H/He lines, full Balmer+Paschen series, and minimal variability. All display blue‑shifted He I absorption; two show Hα and Na D absorption, and two have abundant narrow [Fe II]; J1025+1402 (The Egg) further exhibits extreme Na D, K I, and Ca II triplet absorption consistent with a cool (~5000 K), metal‑enriched envelope. Line diagnostics point to AGN‑powered, metal‑poor narrow lines with little dust, very high‑density inner regions for the broad lines (or unusual dust), and [Fe II] from dense gas between them; two targets are X‑ray non‑detections and WISE photometry implies only weak dust (T ~10^2–10^3 K). The authors propose a largely thermalized cool‑gas envelope plus an extended, high‑density outflow to unify these features, positioning local LRDs as resolvable analogs of early BH growth.

Key figures to inspect

• Figure 1: Use the spectra+photometry overlays to verify the V‑shaped UV–optical SEDs, the declining NIR continua, and the close match to JWST LRD prism templates; the Legacy Survey thumbnails confirm the compact morphologies.
• Figure 2 (J1025+1402/The Egg): Inspect the blue‑shifted He I absorption profile and the extreme Na D, K I, and Ca II triplet absorption; together with the full Balmer+Paschen series and narrow lines, this figure anchors the inference of a cool (~5000 K), metal‑enriched envelope and dense intermediate gas.
• Figure 3 (J1047+0739): Check which low‑ionization absorbers (e.g., Na D) are present or weak, the breadth of H/He emission, and any [Fe II] narrow features to contrast against The Egg and assess object‑to‑object diversity.
• Figure 4 (J1022+0841): Examine the He I blueshift, Balmer/Paschen coverage, and [Fe II] line strengths to test the proposed stratified geometry (dense inner broad‑line region vs. metal‑poor narrow‑line gas).