A JWST/NIRSpec First Census of Broad-Line AGNs at z=4-7: Detection of 10 Faint AGNs with M_BH~10^6-10^8 M_sun and Their Host Galaxy Properties

TL;DR

This study targets faint broad-line AGNs at z>4 with JWST/NIRSpec to probe low-mass black holes and their host galaxies. By systematically fitting rest-frame optical emission lines and enforcing robust broad-line criteria, the authors identify 10 type-1 AGNs at z=4.015–6.936 with BH masses ~10^6–10^8 M☉, often residing in galaxies with extended hosts and relatively low stellar masses. The results suggest rapid black hole growth and a higher number density of faint AGNs than extrapolated from the quasar luminosity function, with implications for galaxy-SMBH coevolution and a modest but non-negligible contribution to cosmic reionization. Overall, the work provides the first statistical view of faint, low-mass BHs and their hosts at z>4, enabled by JWST's spectroscopic capabilities.

Abstract

We present a first statistical sample of faint type-1 AGNs at $z>4$ identified by JWST/NIRSpec deep spectroscopy. Among the 185 galaxies at $z_\mathrm{spec}=3.8-8.9$ 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 H$α$ line and not in the forbidden [OIII]$λ$5007 line that is detected with greater significance than H$α$. The broad H$α$ line widths of $\mathrm{FWHM}\simeq1000-6000\ \mathrm{km\ s^{-1}}$ suggest that the AGNs have low-mass black holes with $M_\mathrm{BH}\sim10^6-10^8\ M_\odot$, remarkably lower than those of low-luminosity quasars previously identified at $z>4$ with ground-based telescopes. JWST and HST 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 SEDs, 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 ($\sim5\%$), higher than $z\sim0$, indicates that a number density of such faint AGNs is higher than an extrapolation of the quasar luminosity function, implying a large population of AGNs including type 1 and type 2 in the early universe. Such faint AGNs contribute to cosmic reionization, while the total contribution is not large, up to $\sim50\%$ at $z\sim6$, because of their faint nature.

Figures (3)

• Figure 1: NIRSpec spectra of CEERS_01244, GLASS_160133, GLASS_150029, and CEERS_00746. For each object, the left and middle panels show spectra around H$\beta$+[ Oiii]$\lambda\lambda$4959,5007 and H$\alpha$+[ Nii]$\lambda\lambda$6548,6584, respectively. The 2D and 1D spectra are shown in the top and bottom panels, respectively. The red dashed line with the shaded region shows the best-fit broad-line component ($\mathrm{FWHM}>1000\ \mathrm{km\ s^{-1}}$) and other red dashed lines show the best-fit narrow components ($\mathrm{FWHM}<500\ \mathrm{km\ s^{-1}}$). For GLASS_160133 and GLASS_150029, we also show the outflow components with $\mathrm{FWHM}\lesssim500\ \mathrm{km\ s^{-1}}$. The right panels show the spectra around H$\alpha$+[ Nii]$\lambda\lambda$6548,6584 with the logarithmic scale. The broad-line components only seen in H$\alpha$, which are detected with a higher signal-to-noise ratio than [Oiii]$\lambda$5007, indicates that these objects are type-1 AGNs.
• Figure 2: Same as Figure \ref{['fig_spec_line_1']} but for CEERS_01665, CEERS_00672, CEERS_02782, and CEERS_00397.
• Figure 3: Same as Figure \ref{['fig_spec_line_1']} but for CEERS_00717 and CEERS_01236.