Discovery of a Little Red Dot candidate at $z\gtrsim10$ in COSMOS-Web based on MIRI-NIRCam selection

Takumi S. Tanaka, Hollis B. Akins, Yuichi Harikane, John D. Silverman, Caitlin M. Casey, Kohei Inayoshi, Jan-Torge Schindler, Kazuhiro Shimasaku, Dale D. Kocevski, Masafusa Onoue, Andreas L. Faisst, Brant Robertson, Vasily Kokorev, Marko Shuntov, Anton M. Koekemoer, Maximilien Franco, Eiichi Egami, Daizhong Liu, Anthony J. Taylor, Jeyhan S. Kartaltepe, Sarah E. Bosman, Jaclyn B. Champagne, Koki Kakiichi, Santosh Harish, Zijian Zhang, Sophie L. Newman, Darshan Kakkad, Qinyue Fei, Seiji Fujimoto, Mingyu Li, Steven L. Finkelstein, Zi Jian Li, Erini Lambrides, Laura Sommovigo, Jorge A. Zavala, Kei Ito, Zhaoxuan Liu, Ezequiel Treister, Manuel Aravena, Ghassem Gozaliasl, Haowen Zhang, Hossein Hatamnia, Hiroya Umeda, Akio K. Inoue, Jinyi Yang, Makoto Ando, Junya Arita, Xuheng Ding, Suin Matsui, Yuki Shibanuma, Georgios Magdis, Ming-Yang Zhuang, Xiaohui Fan, Zihao Li, Weizhe Liu, Jianwei Lyu, Jason Rhodes, Sune Toft, Feige Wang, Siwei Zou, Rafael C. Arango-Toro, A. J. Battisti, Steven Gillman, Ali Ahmad Khostovan, Arianna S. Long, Bahram Mobasher, David B. Sanders

First listed 2025-07-31 | Last updated 2025-10-21

Abstract

JWST has revealed a new high-redshift population called little red dots (LRDs). Since LRDs may be in the early phase of black hole growth, identifying them in the early universe is crucial for understanding the formation of the first supermassive black holes. However, no robust LRD candidates have been identified at $z>10$, because commonly-used NIRCam photometry covers wavelengths up to $\sim5\,{\rm μm}$ and is insufficient to capture the characteristic V-shaped spectral energy distributions (SEDs) of LRDs. In this study, we present the first search for $z\gtrsim10$ LRD candidates using both NIRCam and MIRI imaging from COSMOS-Web, which provides the largest joint NIRCam-MIRI coverage to date ($0.20\,{\rm deg^2}$). Taking advantage of MIRI/F770W to remove contaminants, we identify one robust candidate, CW-LRD-z10 at $z_{\rm phot}=10.5^{+0.7}_{-0.6}$ with $M_{\rm UV}=-19.9^{+0.1}_{-0.2}\,{\rm mag}$. CW-LRD-z10 exhibits a compact morphology, a distinct V-shaped SED, and a non-detection in F115W, all consistent with being an LRD at $z\sim10$. Based on this discovery, we place the first constraint on the number density of LRDs at $z\sim10$ with $M_{\rm UV}\sim-20$ of $1.2^{+2.7}_{-1.0}\times10^{-6}\,{\rm Mpc^{-3}\,mag^{-1}}$, suggesting that the fraction of LRDs among the overall galaxy population increases with redshift, reaching $\sim3\%$ at $z\sim10$. Although deep spectroscopy is necessary to confirm the redshift and the nature of CW-LRD-z10, our results imply that LRDs may be a common population at $z>10$, playing a key role in the first supermassive black hole formation.

Short digest

Using COSMOS-Web’s joint NIRCam–MIRI imaging, the authors conduct the first targeted search for little red dots beyond z≈10, leveraging F770W to isolate the V‑shaped SED signature and cull Balmer‑break contaminants. They uncover one robust source, CW-LRD-z10, with zphot=10.5(+0.7/−0.6), MUV=−19.9(+0.1/−0.2), compact morphology, a clear V‑shaped SED, and an F115W non‑detection consistent with an LRD at z∼10. From this single detection they infer a number density at MUV≈−20 of 1.2(+2.7/−1.0)×10⁻⁶ Mpc⁻³ mag⁻¹, implying the LRD fraction rises with redshift to ≈3% at z∼10. If confirmed, this points to LRDs as a common early population potentially tied to the first SMBH growth; deep spectroscopy is still required to secure the redshift and nature of CW-LRD-z10.

Key figures to inspect

• Figure 1: Inspect how the MIRI F770W point falls relative to NIRCam bands to form the V‑shaped SED, and compare against the modeled stellar Balmer‑break galaxy—this shows why F770W is decisive for separating true LRDs from contaminants at z>10.
• Figure 2: Check the NIRCam–MIRI color–color planes, the red selection box, and the position of CW-LRD-z10 (star symbol with error bars and a lower‑limit arrow); compare against dusty SFG, Balmer‑break, and seed‑BH model tracks to see which loci are excluded once MIRI is included.
• Figure 3: Review the HST+JWST cutouts for compactness and non‑detections (F606W, F814W, F115W) and the observed SED panel where F770W anchors the red side—confirming the dropout and the characteristic V‑shape of CW-LRD-z10.
• Figure 4: Examine BAGPIPES fits and p(z): the LRD template provides the best fit near z≈10.5, while galaxy alternatives either favor lower redshift or yield poorer χ²—quantifying the preference for the LRD interpretation.