The missing FeII bump in faint JWST AGN: possible evidence for metal-poor broad-line regions at early cosmic times

TL;DR

This work examines the weakness of the optical Fe II bump in faint JWST-detected AGN at $z\sim6$–7, postulating metal-poor broad-line regions as the primary cause. By assembling a 26-source high-$z$ sample with JWST and constructing well-matched low-$z$ control samples, the authors quantify $R_{\rm Fe}$ and demonstrate a pronounced deficit in the faint population, while luminous high-$z$ quasars resemble local AGN in Fe II strength. Detailed spectral fitting with Cloudy Fe II templates, non-parametric checks, and spectral stacking underpin a consistent picture in which subsolar BLR metallicities ($Z/Z_{\odot} \sim 0.1$–0.5) reproduce the observed $R_{\rm Fe}$, supported by limited direct oxygen abundance measurements. The results imply an early chemical maturation in luminous quasars but lagged enrichment in faint JWST AGN, with local metal-poor dwarf galaxies providing useful analogues; these findings have implications for BLR evolution and the early chemical history of the Universe, and call for larger, pan-chromatic surveys with upcoming facilities.

Abstract

Recent JWST observations have revealed a large population of intermediate/low-luminosity AGN at early times with peculiar properties, different from local AGN or luminous quasars. To better understand the physical conditions in the BLRs of these early AGN, we used the optical FeII (4434--4684 Å) and the broad $\rm H β$ emission, and the ratio between their equivalent widths $R_{Fe}$, as a probe on a purposefully assembled sample. Specifically, we gathered a sample of 26 high redshift ($\langle z \rangle$=6.4) AGN, observed by JWST, with broad $\rm Hβ$ detection both in the high and low luminosity regimes (respectively 14 faint AGN and 12 quasars), to investigate their optical FeII emission properties. In addition, we carefully selected control samples at lower $z$. We found that the population of faint AGN ($\rm \log(L_{H β} / (erg \, s^{-1}))\lesssim 44$) exhibits a significantly lower FeII emission than their local counterparts ($R_{Fe}<$0.24 versus $R_{Fe}\simeq$0.85 in the control sample), while the quasars at the epoch of reionisation observed by JWST present a FeII emission profile that closely resembles that observed at $z<3$. We argue that the weakness of the FeII bump in the faint JWST AGN might be due to the reduced metallicity of their broad line region ($\lesssim 0.5~Z_{\odot}$), while luminous quasars have already reached chemical maturity ($\sim Z{_\odot}$ or higher). Lastly, we highlight an intriguing similarity between the spectral properties of the high redshift population of faint AGN with those harboured in local metal poor dwarf galaxies.

Figures (16)

• Figure 1: The $\log(\rm FWHM_{H\beta,br})-\log(\rm L_{H\beta,br})$ parameter space for our broad line AGN (T1AGN) and the reference samples (SDSS dr17, shen2016rest S16, matthews2021placing M21, deconto2023high DM23), where the SDSS dr17 AGN are shown with contours. We are also including the three broad line AGN in metal-poor dwarf galaxies (DG-T1AGN). The dashed line marks the threshold luminosity dividing the high- and the low-luminosity sub-samples. The actual $\log(\rm L_{H\beta,br})$ of SBS_0335-052E (black thick edge) is 38.3, but it was shifted to 40.0 for the sake of a tighter image layout. The azure and black rectangles mark the regions adopted to define the control samples (Sec.\ref{['sec:results']}).
• Figure 2: Examples of the spectral fits of a low-luminosity (left) and a high-luminosity (right) AGN of the sample. The different components are colour-coded as stated in the legend. The uncertainty on the data is shown as a shaded area. The @series Fe ii Fe ii Fe ii pseudo-continuum is highlighted in green. The vertical grey dashed lines mark the most prominent emission lines. All the spectral fits are shown in \ref{['fig:fits_atlas']}.
• Figure 3: Continuum-subtracted spectral composites of the low-luminosity (left) and high-luminosity (right) subsamples. The $\mathrm{H}\beta$ and [@series O iii O iii O iii] narrow components have been cut for visualisation purposes. In the right panel we also show the composite quasar spectrum described in shen2016rest, made of 74 luminous quasars at $1.5<z<3.5$. The iron bump between 4434--4684 Å (green shaded area) is evidently weaker in the JWST low-luminosity sample than in the local AGN. However, the strength of the @series Fe ii Fe ii Fe ii bump in the high-luminosity sample is comparable to the lower redshift analogue samples.
• Figure 4: $R_{\rm Fe}$ -$\log(\rm FWHM_{H\beta})-\log(\rm L_{H\beta,br})$ diagram; specifically: $\log(\rm FWHM_{H\beta})$ versus $\log(\rm L_{H\beta,br})$ colour-coded by $R_{\rm Fe}$ . Objects marked with a circle represent $R_{\rm Fe}$ upper limits as defined in the text. The background 2D histogram is obtained by binning the control sample (with a minimum of 30 objects per bin); individual objects from JWST are colour-coded with the same scale in $R_{\rm Fe}$ . Due to the large number of upper limits in the low-luminosity region, we also include the values derived from the composite spectra as symbols with thicker black edges. The coloured rectangles mark the regions where the control samples were drawn. The black solid arrow marks the direction of the PCA, as described in Sec. \ref{['sec:acc_pars']}. SBS_0335-052E (black thick edge pentagon) has been shifted for a tighter image layout.
• Figure 5: Orthogonal sections of the $\log(\rm FWHM_{H\beta})-\log(L_{H\beta})-$$R_{\rm Fe}$ parameter space; specifically (left) $R_{\rm Fe}$ versus $\log(L_{H\beta})$ in (colour-coded) bins of $\rm FWHM_{H\beta}$, and (right) $R_{\rm Fe}$ versus $\rm FWHM_{H\beta}$ in (colour-coded) bins of $\log(L_{\rm H\beta})$. The thick-edged diamond and star represent the values derived from the composite spectra. The colour-coded lines represent the third quantity evaluated on the binned sources of the control samples. SBS_0335-052E (black thick edge pentagon) has been shifted for a tighter image layout. It is clear that, while the high-luminosity objects of the sample reside -albeit with some scatter- in the expected locus of the parameter space according to the control sample, the low-luminosity objects exhibit far lower $R_{\rm Fe}$ . Interestingly, two low-luminosity objects (JADES-029777 and XID-2028) detach from the bulk of the faint sample, being close to the $R_{\rm Fe}$ expected. As we discuss further in Sec.\ref{['sec:Xray_perspective']}, these are the two only faint AGN which are X-ray detected.