Hidden Little Monsters: Spectroscopic Identification of Low-Mass, Broad-Line AGN at $z>5$ with CEERS

TL;DR

This work reports the spectroscopic discovery of two low-luminosity broad-line AGN at z>5 (CEERS 1670 and CEERS 3210) using JWST NIRSpec in the CEERS survey. By fitting broad Hα emission, the authors estimate BH masses around 10^7 M⊙, placing these systems below the luminosity of known z>5 quasars and near seed-mass scales, with one source likely obscured or in transition. Emission-line diagnostics indicate moderately low metallicity AGN activity, while host-galaxy mass constraints suggest a relatively small stellar component for CEERS 1670 and a potentially larger, obscured host for CEERS 3210. The findings demonstrate JWST’s capability to probe BH growth and BH–galaxy coevolution at the earliest cosmic epochs and provide updated constraints on the z~5 AGN luminosity function.

Abstract

We report on the discovery of two low-luminosity, broad-line AGN at $z>5$ identified using JWST NIRSpec spectroscopy from the CEERS Survey. We detect broad H$α$ emission from both sources, with FWHM of $2038\pm286$ and $1807\pm207$ km s$^{-1}$, resulting in black hole (BH) masses that are 1-2 dex below that of existing samples of luminous quasars at $z>5$. The first source, CEERS 1670 at $z=5.242$, is 2-3 dex fainter than known quasars at similar redshifts and was previously identified as a candidate low-luminosity AGN based on its rest-frame optical SED. We measure a BH mass of $M_{\rm BH}=1.3\pm0.4\times 10^{7}~M_{\odot}$, confirming that this AGN is powered by the least-massive BH known in the universe at the end of cosmic reionization. The second source, CEERS 3210 at $z=5.624$, is inferred to be a heavily obscured, broad-line AGN caught in a transition phase between a dust-obscured starburst and an unobscured quasar. We estimate its BH mass to be $M_{\rm BH}\simeq 0.9-4.7 \times 10^{7}~M_{\odot}$, depending on the level of dust obscuration assumed. We derive host stellar masses, $M_\star$, allowing us to place constraints on the BH-galaxy mass relationship in the lowest mass range yet probed in the early universe. The $M_{\rm BH}/M_\star$ ratio for CEERS 1670, in particular, is consistent with or higher than the empirical relationship seen in massive galaxies at $z=0$. We examine the emission-line ratios of both sources and find that their location on the BPT and OHNO diagrams is consistent with model predictions for low-metallicity AGN with $Z/Z_\odot \simeq 0.2-0.4$. The spectroscopic identification of low-luminosity, broad-line AGN at $z>5$ with $M_{\rm BH}\simeq 10^{7}~M_{\odot}$ demonstrates the capability of JWST to push BH masses closer to the range predicted for the BH seed population and provides a unique opportunity to study the early stages of BH-galaxy assembly.

Figures (8)

• Figure 1: JWST NIRCam images of our broad-line AGN sample at $z>5$ taken in the short-wavelength (F150W and F200W) and long-wavelength (F277W, F356W, and F444W) filters. The RGB images are composed of images in the F150W, F277W, and F444W filters. All images are $2^{\prime\prime}\times~2^{\prime\prime}$ in size. The alignment of the NIRSpec microshutter aperture relative to each source is shown in red overtop the F444W image.
• Figure 2: NIRSpec spectra of sources CEERS 1670 and CEERS 3210 taken in the G395M grating with $R\sim1000$. The 2D spectra are shown above with extraction windows highlighted in red. Grey regions in both the 1D and 2D spectra indicate regions masked due to artifacts identified via visual inspection. The location of several prominent emission lines are noted.
• Figure 3: The SEDs of the two low-luminosity AGN (CEERS 1670 and CEERS 3210) obtained with the JSWT NIRSpec and NIRCam. Left panel (a): the continuum spectral shape is explained by the composite quasar spectrum of VB_2001 scaled to match the photometry of CEERS 1670 (blue), and is fitted well with a single power law with an index of $_ =-1.14$ (dashed). The galaxy SED model with $M_) \simeq 6.0\times 10^9~M_\odot$ is overlaid (red), where the stellar continuum in the F356W filter becomes comparable to the observed F356W flux density. This gives a robust upper bound of the underlying stellar population. Right panel (b): the source has a blue continuum spectrum with a UV slope of $_ < -3.0$ at $_{\rm obs}\simeq 1-2~$m and a very steep continuum spectrum ($_ \simeq 2.0$). The redder part can be explained either by a heavily obscured quasar (cyan) or a dusty starburst galaxy (red). As a possible explanation of the blue excess in the spectrum, the unobscured broad-line AGN contribution is added to the dusty starburst galaxy (blue). In the dusty galaxy model, the stellar mass is set to $M_) \simeq 6\times 10^{10}~M_\odot$ (see the text in Section \ref{['sec:SED3210']}).
• Figure 4: The rest-frame spectra (black histograms) and associated uncertainty (gray error bars) of both sources in regions with emission-line features. Red lines show the best-fit Gaussians for narrow emission lines and the blue line shows the best-fit broad component for H$$, which have a FWHM of $2060\pm286$ and $1802\pm204$ km s$^{-1}$ for CEERS 1670 and CEERS 3210, respectively.
• Figure 5: Left Panel (a): The BPT emission-line diagnostic diagram. The gray contours denote the distribution of local star-forming galaxies and AGN as measured by the SDSS survey sdss. Black diamonds denote stacked line ratios of CEERS galaxies at $z \sim 5.6$, $z \sim 4.5$, and $z \sim 3.3$Sanders_2023. The black long and short-dashed lines denote the $z=0$ and $z=2.3$ boundary between the star-forming and AGN regions of the diagram defined by Kauffmann_2003 and Kewley_2013a, respectively. Right Panel (b): the OHNO diagnostic diagram. Black squares denote line ratios of SMACS ERO galaxies at $5.3<z<8.5$trump23 and gray contours denote the distribution of $z \sim 0$ SDSS galaxies. The dashed line denotes the boundary between star-forming and AGN regions as defined in Backhaus22. Colored curves in both panels show MAPPINGS V photoionization models Kewley_2019. The three color-coded sets of curves and points along those curves correspond to different ionization parameters and metallicities, as indicated by the legends, with three curves for each color corresponding to different gas pressures as described in the text. Both of our $z \sim 5$ AGN have narrow-line ratios that are consistent with low metallicity and high ionization, with little difference from the emission-line ratios observed for other populations of high-redshift galaxies.