A SPectroscopic survey of biased halos In the Reionization Era (ASPIRE): Broad-line AGN at $z=4-5$ revealed by JWST/NIRCam WFSS

TL;DR

This study leverages JWST/NIRCam WFSS data from the ASPIRE survey to identify 16 broad Hα emitters at z~4–5, probing low-mass black hole growth in the early universe. By measuring broad Hα kinematics, estimating BH masses (~$10^7$–$10^8 M_0$) and bolometric luminosities (~$10^{44.5}$–$10^{45.5}$ erg s^-1) with Eddington ratios around 0.07–0.47, the work reveals active, moderately accreting AGN embedded in host galaxies whose rest-frame UV–optical continua and morphologies suggest substantial stellar components. The study constructs Hα and bolometric luminosity functions, finds a rising AGN fraction with higher Hα luminosity, and presents early evidence for clustering and Balmer-absorption features that hint at dense circumnuclear gas and environmental effects. It also highlights selection biases between broad-line and photometrically selected low-luminosity AGNs, underscoring the need for inclusive, multiwavelength approaches to map faint AGN populations during the reionization era.

Abstract

Low-luminosity AGNs with low-mass black holes (BHs) in the early universe are fundamental to understanding the BH growth and their co-evolution with the host galaxies. Utilizing JWST NIRCam Wide Field Slitless Spectroscopy (WFSS), we perform a systematic search for broad-line ${\rm Hα}$ emitters (BHAEs) at $z\approx 4-5$ in 25 fields of the ASPIRE (A SPectroscopic survey of biased halos In the Reionization Era) project, covering a total area of 275 arcmin$^2$. We identify 16 BHAEs with FWHM of the broad components spanning from $\sim$ 1000 km s$^{-1}$ to 3000 km s$^{-1}$. Assuming the broad linewidths arise due to Doppler broadening around BHs, the implied BH masses range from $10^7$ to $10^{8}~M_\odot$, with broad ${\rm Hα}$-converted bolometric luminosity of $10^{44.5}-10^{45.5}$ erg s$^{-1}$ and Eddington ratios of $0.07-0.47$. The spatially extended structure of the F200W stacked image may trace the stellar light from the host galaxies. The ${\rm Hα}$ luminosity function indicates an increasing AGN fraction towards the higher ${\rm Hα}$ luminosities. We find possible evidence for clustering of BHAEs: two sources are at the same redshift with a projected separation of 519 kpc; one BHAE appears as a composite system residing in an overdense region with three close companion ${\rm Hα}$ emitters. Three BHAEs exhibit blueshifted absorption troughs indicative of the presence of high-column-density gas. We find the broad-line and photometrically selected BHAE samples exhibit different distributions in the optical continuum slopes, which can be attributed to their different selection methods. The ASPIRE broad-line ${\rm Hα}$ sample provides a good database for future studies of faint AGN populations at high redshift.

Figures (13)

• Figure 1: The selection criteria of ASPIRE BHAE (red stars) among all photometric sources in the 25 ASPIRE fields (gray dots). We label the applied color cut and size cut as red lines.
• Figure 2: The broad-H$\alpha$ emitters identified in 25 ASPIRE fields. For each source, the upper left panel shows a 2$^{\prime\prime}$$\times$2$^{\prime\prime}$ RGB thumbnail composed of JWST/NIRCam F356W, F200W, and F115W images. The top panel shows the 2D grism spectrum with continuum and background removed. The bottom panel is the optimally extracted 1D spectrum (black) and the corresponding error spectrum (gray-shaded region). We show the total best-fit line profile as the red solid line, the narrow component as the orange dashed line, and the broad component as the green dashed line. For J0923P0402-BHAE-1, J1526M2050-BHAE-1, and J1526M2050-BHAE-3, we also show the H$\alpha$ profiles without the absorption features as the red dashed lines. J0923P0402-BHAE-1 and J0430M1445-BHAE-1 present two broad H$\alpha$ components with FWHM$>$1000 km s$^{-1}$. We label the FWHMs of their total H$\alpha$ emission, which are used to estimate the BH masses.
• Figure 3: Same as Figure \ref{['fig:individual_Fig1']}. For J2232P2930-BHAE-1, we also show the [N$\;$]$\lambda\lambda6548,6583$ emission lines in blue and [S$\;$]$\lambda\lambda6718,6732$ in cyan. In the 2D spectrum of J2232P2930-BHAE-1, the emission on its top right originates from a companion HAE at the same redshift (companion-2, see §\ref{['sec:J2232P2930-BHAE-1']}).
• Figure 4: The properties of broad H$\alpha$ emission lines of BHAEs. In the left panel, we show the relation between the FWHM and the luminosity of the broad H$\alpha$ components. We show the fraction of H$\alpha$ luminosity in the broad components as a function of the broad H$\alpha$ FWHMs in the middle panel, and as a function of the broad H$\alpha$ luminosity in the right panel. The ASPIRE BHAEs are denoted as the red stars. We present the measurements of literature BHAEs in Greene2023 ($4.5<z<8.5$, orange circle), Harikane2023 ($4<z<7$, orange square), Maiolino2023 ($4<z<7$, orange diamond) and Matthee2023 ($4.2<z<5.5$, orange triangle). The blue-shaded region denotes the distribution of $z<0.6$ quasars with H$\alpha$ emission lines in SDSS spectra Wu2022.
• Figure 5: Black hole mass ($M_{\rm BH}$) versus bolometric luminosity ($L_{\rm bol}$). The red stars show the estimates in this work. The lengths of arrows indicate the changes of $M_{\rm BH}$ and $L_{\rm bol}$ when $A_V$ varies from 0 to 2. The orange circles, diamonds, triangles, and squares show the estimates from the literature BHAEs in Kocevski2023Greene2023Maiolino2023Matthee2023Harikane2023. Note that the $L_{\rm bol}$ estimator used in Harikane2023 is different as discussed in §\ref{['sec:BH_Lbol']}. We present the distribution of low-redshift ($z<0.6$) quasars from Wu2022 as the blue-shaded region, with the depth demonstrating their number density. The blue-edged circles are measurements for $z>6.5$ quasars, deriving $M_{\rm BH}$ from $\rm H\beta$ emission lines and $L_{\rm bol}$ from $L_{\rm 5100}$. We draw equal Eddington ratio lines (gray dashed) of $\lambda_{\rm Edd}=0.01, 0.1, 1, 10$.