A GLIMPSE of Intermediate Mass Black holes in the epoch of reionization: Witnessing the Descendants of Direct Collapse?

TL;DR

This work targets IMBHs during the epoch of reionization by searching for broad-line AGN in faint, high-$z$ galaxies, leveraging JWST/NIRSpec spectroscopy plus strong gravitational lensing in Abell S1063 to reach $M_{ m BH}\sim 10^{5.5}\,M_\odot$. The authors develop a template-based emission-line fitting pipeline that uses the [O III] narrow-line profile as a kinematic template to decompose H$\alpha$+[N II] and identify broad H$\alpha$ emission, supported by global optimization and MCMC to robustly estimate parameters. They report 10 broad-line AGN in the redshift range $4.5<z<7.0$ (8 secure, 2 tentative), with BH masses down to $\sim 10^{5.5}\,M_\odot$ and BH-to-stellar-mass ratios spanning down to $\lesssim 0.1\%$, many lying on the local $M_{\rm BH}-M_*$ relation. The inferred BH mass function shows an enhanced abundance at the low-mass end around $M_{\rm BH}\sim 10^{5.5}\,M_\odot$, consistent with direct-collapse BH formation and supporting DCBHs as a significant seed channel for the earliest SMBHs.

Abstract

JWST has revealed an abundance of supermassive black holes (BHs) in the early Universe, and yet the lowest mass seed black holes that gave rise to these populations remain elusive. Here we present a systematic search for broad-line Active Galactic Nuclei (AGNs) in some of the faintest high-$z$ galaxies surveyed yet by combining ultra-deep JWST/NIRSpec G395M spectroscopy with the strong lensing aid in Abell S1063. By employing the profile of the [OIII]$λ5007$ emission lines as a template for narrow-line components and carefully cross-validating with mock observations, we identify a sample of ten broad-line AGNs at $4.5<z<7.0$ (eight secure, two tentative). The inferred BH masses from the broad H$α$ line explore the intermediate BH mass regime down to $\sim 10^{5.5}\,M_\odot$. The stellar mass ($M_*$) is estimated with a galaxy+AGN composite model, and we find the BH to stellar mass ratio spans down to $M_{\rm BH}/M_*\lesssim 0.1\%$, unveiling populations on the empirical $M_{\rm BH}-M*$ relation observed in the local universe. We also derive the black hole mass function and investigate its low-mass end at this epoch. While we confirm the agreement of our results with previous studies at $M_{\rm BH}\gtrsim10^{6.5}M_{\odot}$, we find the mass range of $\sim 10^{5.5}\,M_\odot$ features an enhanced abundance with respect to the extrapolated best-fit Schechter function. Comparison with theoretical models suggests that a possible origin for this enhanced abundance is the direct-collapse BH formation, supporting the scenario that the direct collapse of massive gas clouds is a significant pathway for the earliest supermassive BHs.

Figures (2)

• Figure 1: Location of broad-line AGNs in the survey field. The NIRSpec footprint is indicated by the red-shaded regions, which are overlaid on the NIRCam F444W image. The lensing magnification map for a source at $z_{\mathrm{s}}=6$ is denoted as the viridis contours. Red circles mark the positions of our identified broad-line AGNs, filled circles for robust objects, and open circles for tentative candidates.
• Figure 2: The spectra of our broad line AGN and the best-fit result. For each source, the top panel displays the two-dimensional spectrum. The middle panel presents the corresponding one-dimensional spectrum, obtained through optimal extraction (black histogram). The uncertainty is represented by the gray shaded region. Overplotted on the 1D spectrum is the best-fit model (blue solid line), which is decomposed into narrow components from [O iii] line template (green dotted line) and the broad H$\alpha$ emission (red dashed line). The bottom panel presents the residual. The inset image presented in the middle panel shows the RGB image generated with JWST/NIRCam F150W, F277W, and F444W filters, and the corresponding NIRSpec/G395M shutters. The final two sub-panels are dedicated to presenting the spectra and corresponding model fits for two tentative detections, ID 38548 and ID 7404. Basic fitting parameters and properties of sources are listed in the middle panel. $\Delta \rm BIC$ shows the difference between the BIC for the best-fit model and the narrow-only model, and $\chi_\nu^2$ is the reduced chi-squared value corresponding to the best-fit model. $\log (M_{\rm BH}/M_\odot)$ is the black hole mass, estimated from the properties of the broad H$\alpha$ emission. $\rm FWHM_{\rm H\alpha}$ and $\rm SNR_{H\alpha}$ present the FWHM and the signal-to-noise ratio of the broad H$\alpha$ emission. $\mu$ is the gravitational lensing magnification factor for the source, and $z$ is the redshift.