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

TL;DR

This work leverages the first 10% COSMOS-3D JWST/NIRCam grism data to identify 13 broad-line AGNs at $z>5$ (12 with broad H$\alpha$ at $z\approx5-6$ and 1 with broad H$\beta$ at $z=7.646$). It combines line-profile modeling, multiwavelength SEDs, and MIRI constraints to explore the continuum shapes, Balmer absorption, and near-IR colors, finding a mix of LRD-like and quasar-like SEDs and a notable subset with Balmer absorption associated with dense circumnuclear gas. The paper derives the H$\alpha$ luminosity function at $z\approx5-6$ and, by incorporating $z\sim8$ H$\beta$ emitters, presents the first H$\beta$ LF at $z>7$, showing that broad-line AGNs can outnumber UV-selected quasars at high luminosities and that Balmer-absorber systems preferentially exhibit red optical continua. Together with a discussion of UV variability and the prospective yield of the full COSMOS-3D dataset, these results imply a substantial, perhaps previously underestimated, population of UV-faint broad-line AGNs at $z>7$, with important implications for SMBH seeding and growth models. The COSMOS-3D survey demonstrates the power of JWST grism spectroscopy in delivering a nearly complete census of bright broad-line AGNs and their Balmer-line properties at cosmic dawn.

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 $z>5$.

Figures (14)

• Figure 1: Long-wavelength magnitude (LW Magnitude) as a function of redshift for the broad-line AGN sample in this work, compared to literature samples Matthee2024Maiolino2024Greene2024Lin2024Zhang2025Sun2025Fudamoto2025. Filled markers represent broad-line AGNs selected from NIRCam/WFSS, and open markers denote those identified with NIRSpec. Note that the sample from Greene2024 has not been corrected for lensing magnification. LW magnitude refers to either F356W for $z<5$ samples or F444W for $z>5$ samples.
• Figure 1: Similar to Figure \ref{['fig:quasar_like_SED']} and \ref{['fig:lrd_like_SED_MIRI']}, but show the SEDs of the remaining nine broad-line AGNs. For objects with $\beta_{\rm opt} > 0$, the red dashed line represents the best-fit optical continuum, with the Akins2024 LRD template shown as a red solid line for reference and the VandenBerk2001 quasar template as a blue dashed line for comparison. For objects with $\beta_{\rm opt} < 0$, we indicate the optical continuum using the blue dashed line, and highlight the VandenBerk2001 quasar template for reference. For ID19661, whose $\beta_{\rm opt}$ is consistent with zero within $1\sigma$, both the Akins2024 LRD template and the VandenBerk2001 quasar template are shown as solid lines.
• Figure 1: Left: Cumulative LF measured using Lynden-Bell's $C^{-1}$ method, uncorrected for completeness. Middle: The comparison between the $C^{-1}$ method-measured total H$\alpha$ LF and 1/$V_{\rm vmax}$ method-measured total H$\alpha$ LF. Right: Comparison between the $C^{-1}$ method-measured broad H$\alpha$ LF and 1/$V_{\rm vmax}$ method-measured broad H$\alpha$ LF.
• Figure 2: The broad-line AGNs in this work are plotted in the F277W - F444W vs. F444W color space. The gray dots represent the photometrically selected LRDs from Akins2024 within the current COSMOS-3D footprint, but with photometry measurements taken from our catalog (§\ref{['sec:image_reduction']}). The dashed lines indicate the selection criteria for these LRDs. The broad-line AGNs in this work are selected based on the presence of broad emission lines and, therefore, do not necessarily satisfy the LRD color selection criteria.
• Figure 3: Eight broad-H$\alpha$ emitters in the first 10% COSMOS-3D footprint. For each source, the upper-left panel displays a 2$^{\prime\prime}$$\times$2$^{\prime\prime}$ RGB thumbnail composed of JWST/NIRCam F444W, F277W, and F115W images. The top panel shows the 2D grism spectrum and the bottom panel presents the optimally extracted 1D spectrum (black) along with the corresponding error spectrum (gray-shaded region). The best-fit line profiles are shown as a solid red line. If the profiles include two Gaussian components, the narrow component is depicted as a blue dashed line and the broad component as an orange dashed line. All models have been convolved with the LSFs. The FWHMs of the broad emission lines are labeled for each object.