Bridging Quasars and Little Red Dots: Insights into Broad-line Active Galactic Nuclei at z = 5─8 from the First JWST COSMOS-3D Dataset

January 2026 • 2026ApJ...996...93L

Authors • Lin, Xiaojing • Fan, Xiaohui • Wang, Feige • Sun, Fengwu • Champagne, Jaclyn B. • Egami, Eiichi • Kakiichi, Koki • Lyu, Jianwei • Tee, Wei Leong • Yang, Jinyi • Bian, Fuyan • Bosman, Sarah E. I. • Cai, Zheng • Casey, Caitlin M. • Decarli, Roberto • Faisst, Andreas L. • Finkelstein, Steven L. • Fujimoto, Seiji • Harish, Santosh • Ilbert, Olivier • Inoue, Akio K. • Jin, Xiangyu • Kartaltepe, Jeyhan S. • Kocevski, Dale D. • Li, Mingyu • Liu, Weizhe • Liu, Yichen • Schindler, Jan-Torge • Shuntov, Marko • Tanaka, Takumi S. • Vestergaard, Marianne • Wu, Yunjing • Zhang, Haowen • Zhang, Zijian

Abstract • We report the discovery of 13 broad-line active galactic nuclei (AGNs) at z = 5─8 from the first 10% of JWST Cycle 3 Treasury Program COSMOS-3D. These AGNs are identified by their broad Hα or Hβ emission lines through NIRCam grism slitless spectroscopy. One object at z = 7.646 with broad Hβ emission has an F444W magnitude of 23.6 mag, making it one of the brightest z > 7.5 broad-line AGNs yet known. Among the 13 AGNs, 10 objects exhibit reddened optical continua with slopes β_opt > 0. The remaining three resemble UV-luminous quasars at similar redshift but with β_opt less blue than those of typical unobscured quasars. We also obtain MIRI photometry (7.7─18 μm) for two AGNs and place strong constraints on their rest-frame near-IR spectral energy distribution. We find no significant variability in the rest-frame UV by comparing the COSMOS-3D and COSMOS-Web F115W images taken apart by 60 days in the rest-frame. We compute the Hα luminosity function (LF) at z ≍ 5─6 and find potential redshift evolution compared to z ≍ 4─5. We also derive the Hβ LF at z ∼ 8 by combining our sample with those from the literature. The broad Hβ emitters in this work suggest a number density 2 orders of magnitude higher than that predicted by the quasar LF based on rest-frame UV-selected samples. As a preview, our work showcases the ability of the COSMOS-3D grism survey to provide a complete view of the properties, growth, and evolution of bright broad-line AGNs at z > 5.

Links

• PDF https://iopscience.iop.org/article/10.3847/1538-4357/ae1b9b/pdf
• ELECTR https://doi.org/10.3847/1538-4357/ae1b9b
• PREPRINT http://arxiv.org/abs/2504.08039