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

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 H α or H β emission lines through NIRCam grism slitless spectroscopy. One object at z = 7.646 with broad H β emission has an F444W magnitude of 23.6 mag, making it one of the brightest z &gt; 7.5 broad-line AGNs yet known. Among the 13 AGNs, 10 objects exhibit reddened optical continua with slopes β opt &gt; 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 μ m) 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 H α luminosity function (LF) at z ≈ 5–6 and find potential redshift evolution compared to z ≈ 4–5. We also derive the H β LF at z ∼ 8 by combining our sample with those from the literature. The broad H β 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 &gt; 5.