Bridging Quasars and Little Red Dots: Insights into Broad-Line AGNs at $z=5-8$ from the First JWST COSMOS-3D Dataset

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

First listed 2025-04-10 | Last updated 2025-12-08

Abstract

We report the discovery of 13 broad-line AGNs at $z = 5 - 8$ from the first 10% data of the JWST Cycle 3 Treasury Program COSMOS-3D. These AGNs are identified by their broad H$α$ or H$β$ emission lines through the NIRCam grism wide-field 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 $z > 7.5$ broad-line AGNs yet known. Among the 13 AGNs, 10 objects have reddened optical continua with slopes $β_{\rm opt}>0$. The remaining three objects have their overall SEDs that resemble those of UV-luminous quasars at similar redshifts, but their $β_{\rm opt}$, though negative, are not as blue as those of unobscured quasars. We also obtain MIRI photometry at 7.7-18 $μ$m for two AGNs and place strong constraints on their rest-frame near-IR SED. 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 functions (LFs) for the broad H$α$ emitters at $z \approx 5-6$ and find a potential redshift evolution when compared with that of the $z \approx 4-5$ sample. We also derive the H$β$ LF at $z\sim8$ for AGNs and galaxies by combining our sample with those from the literature. The broad H$β$ emitters in this work suggest a number density two 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 $z>5$.

Short digest

From the first 10% (≈127 arcmin²) of COSMOS-3D NIRCam grism WFSS, the team identifies 13 broad-line AGNs at z=5–8 via Hα/Hβ, including a z=7.646 Hβ source at F444W=23.6, among the brightest z>7.5 BL AGN. Most (10/13) show reddened optical continua with β_opt>0, while three have quasar-like SEDs that are less blue than unobscured QSOs; two objects have 7.7–18 μm MIRI points constraining rest-NIR, and no significant F115W UV variability is seen over Δt_rest≈60 days. The Hα luminosity functions at z≈5–6 hint at evolution relative to z≈4–5. An Hβ LF at z∼8 implies number densities ~100× above UV-selected quasar LFs, pointing to a populous bridge between UV-bright quasars and LRD-like BL AGN.

Key figures to inspect

• Figure 1: Use the LW magnitude–redshift plane to locate the z=7.646 Hβ emitter (F444W=23.6) and compare COSMOS-3D WFSS-selected points to NIRSpec literature samples; note that Greene et al. (2024) are uncorrected for lensing, affecting the bright end.
• Figure 2: Inspect where the 13 spectroscopic BL AGNs fall relative to the Akins et al. LRD color cuts in F277W–F444W vs F444W; identify which objects land outside the dashed selection and how this maps onto the reported β_opt>0 reddened continua.
• Figure 3: For the eight broad-Hα emitters, examine the 2D+1D spectra and the Gaussian decompositions to read off FWHM and the presence/absence of narrow components; check line shapes and continuum visibility source by source.
• Figure 4: Focus on ID18221 and ID27974 with detected continua to gauge continuum slopes and Balmer-line widths directly; these illustrate how continuum detection informs the β_opt and SED classification within the sample.