A JWST/NIRSpec First Census of Broad-line AGNs at z = 4–7: Detection of 10 Faint AGNs with M BH ∼ 106–108 M ⊙ and Their Host Galaxy Properties

By Y. Harikane, Yechi Zhang, K. Nakajima, M. Ouchi, Y. Isobe, Yoshiaki Ono, S. Hatano, Yi Xu, Hiroya Umeda. Astrophysical Journal, 2023

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We present the first statistical sample of faint type 1 AGNs at z > 4 identified by JWST/NIRSpec deep spectroscopy. Among the 185 galaxies at z spec = 3.8–8.9 confirmed with NIRSpec, our systematic search for broad-line emission reveals 10 type 1 AGNs at z = 4.015–6.936 whose broad component is only seen in the permitted Hα line and not in the forbidden [O iii]λ5007 line that is detected with greater significance than Hα. The broad Hα line widths of FWHM ≃ 1000–6000 km s−1 suggest that the AGNs have low-mass black holes with M BH ∼ 106–108 M ⊙, remarkably lower than those of low-luminosity quasars previously identified at z > 4 with ground-based telescopes. JWST and Hubble Space Telescope high-resolution images reveal that the majority of them show extended morphologies indicating significant contribution to the total lights from their host galaxies, except for three compact objects two of which show red spectral energy distributions, probably in a transition phase from faint AGNs to low luminosity quasars. Careful AGN-host decomposition analyses show that their host’s stellar masses are systematically lower than the local relation between the black hole mass and the stellar mass, implying a fast black hole growth consistent with predictions from theoretical simulations. A high fraction of the broad-line AGNs (∼5%), higher than z ∼ 0, indicates that the number density of such faint AGNs is higher than an extrapolation of the quasar luminosity function, implying a large population of AGNs in the early universe. Such faint AGNs contribute to cosmic reionization, while the total contribution is not large, up to ∼50% at z ∼ 6, because of their faint nature.

Subfields: Astronomy and Astrophysics, Instrumentation

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Citations in the corpus

The Rise of Faint, Red Active Galactic Nuclei at z > 4: A Sample of Little Red Dots in the JWST Extragalactic Legacy Fields (2025, unknown): “…results to come from the ﬁ rst year of JWST observations is the detection of numerous faint, broad-line active galactic nuclei ( AGN ) at z > 5 ( Y. Harikane et al. 2023; D. D. Kocevski et al. 2023; R. L. Larson et al. 2023; M. Onoue et al. 2023; J. E. Greene et al. 2024; R. Maiolino et al.…”
The Complete CEERS Early Universe Galaxy Sample: A Surprisingly Slow Evolution of the Space Density of Bright Galaxies at z ∼ 8.5–14.5 (2023, methodology): “While some early results changed owing to the uncertain characterization of the NIRCam photometric zero-point ( Boyer et al. 2022 ) , within a few months more robust samples of galaxies were in place ( e.g., Harikane et al. 2023a; Finkelstein et al. 2023; McLeod et al. 2024 ) .”
A Census of Photometrically Selected Little Red Dots at 4 < z < 9 in JWST Blank Fields (2024, background): “…both the UV and bolometric luminosity functions we note that LRDs only represent ∼ 25 % of the total type I (broad-line) AGN population as inferred by Harikane et al. (2023); Maiolino et al. (2023b), even less so compared to the most recent examination of type II AGN hosts from JADES (Scholtz et…”