Environmental Evidence for Overly Massive Black Holes in Low Mass Galaxies and a Black Hole - Halo Mass Relation at $z \sim 5$

TL;DR

This study uses the deep JWST ALT survey behind Abell 2744 to analyze the environments of faint broad Hα emitters at z~4–5. By comparing their large-scale overdensities to those of a reference star-forming galaxy sample, the authors infer typical host stellar masses around 10^7.7 M⊙, revealing unusually high BH-to-stellar mass ratios (~10–15%) when BH masses are taken at face value. The results hint at a possible BH–halo mass relation at z~5 and suggest that some BL-Hα hosts reside in lower-mass galaxies than implied by SED fits, challenging current hydrodynamical simulations unless short, super-Eddington accretion episodes are invoked. The findings also indicate luminosity-dependent diversity in AGN hosts and emphasize the need for larger, uniformly traced samples to robustly characterize the BH mass–halo mass relation in the early Universe.

Abstract

JWST observations have unveiled faint active galactic nuclei (AGN) at high-redshift that provide insights on the formation of supermassive black holes (SMBHs) and their coevolution with galaxies. However, disentangling stellar from AGN light in these sources is challenging. Here, we use an empirical approach to infer the average stellar mass of 6 faint broad line (BL) Halpha emitters at z = 4 - 5 with BH masses ~ 6 (4 - 15)x10^6 Msun, with a method independent of their spectral energy distribution (SED). We use the deep JWST/NIRcam grism survey ALT to measure the over-densities around BL-Halpha emitters and around a spectroscopic reference sample of ~300 galaxies. In our reference sample, we find that Mpc-scale over-density correlates with stellar mass, while pair counts are flat below ~50 kpc due to satellites. Their large-scale environments suggest that BL-Halpha emitters are hosted by galaxies with stellar masses ~5x10^7 Msun, ~40 times lower than those inferred from galaxy-only SED fits. Adding measurements around more luminous z~6 AGNs, we find tentative correlations between line width, BH mass and the over-density, suggestive of a steep BH to halo mass relation. The main implications are (1) when BH masses are taken at face value, we confirm extremely high BH to stellar mass ratios of ~10 %, (2) the low stellar mass galaxies hosting growing SMBHs are in tension with typical hydrodynamical simulations, except those without feedback, (3) a 1 % duty cycle implied by the host mass hints at super-Eddington accretion, which may imply over-estimated SMBH masses, (4) the masses are at odds with a high stellar density interpretation of the line broadening, (5) our results imply a diversity of galaxy masses, environments and SEDs among AGN samples, depending on their luminosity.

Figures (19)

• Figure 1: The H$\alpha$ profiles of the main sample of BL-H$\alpha$ emitters studied in this paper measured in the NIRCam grism data. Top panels show the 2D continuum-subtracted grism spectra. The middle panels show the optimally extracted 1D spectra. Blue lines show the data, where shaded regions show the errors. The black line shows the combined fit that is composed of a narrow and a broad H$\alpha$ line and narrow [NII]. The red dashed and solid components show the broad and narrow H$\alpha$ component and green shows the best-fit [NII] line, whose wavelength we highlight with dotted green lines. Bottom shows the residuals of the spectral fit. Inset panels show false-color rest-frame optical RGB images constructed from NIRCam F115W/F200W/F356W images, highlighting the point-source morphology of the objects.
• Figure 2: Compilation of H$\alpha$ broad lines measured with JWST at $z=4-6$ from NIRCam Grism (red hexagons this paper; green squares EIGER & FRESCO and ASPIRE in pink) and NIRspec (points, purple JADES, RUBIES and CEERS in blue and UNCOVER in orange) spectroscopy. We highlight how the H$\alpha$ luminosity and width change with SMBH mass and Eddington ratio under the commonly adopted calibrations used. Three of the ALT sources are also in the UNCOVER sample.
• Figure 3: SED fits of the six BL-H$\alpha$ emitters that are the main focus of this paper. Orange curves and shaded regions show the best fit SEDs assuming pure stellar and nebular emission and their uncertainties that ignore an AGN contribution. Purple data points are measurements in broad-band filters, while blue data-points are medium-band filters. Inset stamps are false-color rest-frame UV RGB images of $2.4"\times2.4"$ based on the F070W, F090W and F115W NIRCam imaging data.
• Figure 4: Example SED fits of galaxies in the reference sample of galaxies without broad H$\alpha$ lines in the ALT data. The masses increase from top to bottom -- $10^{7}$, $10^{8.5}$, $10^{10}$ M$_{\odot}$, respectively. Orange curves and shaded regions show the best fits and their uncertainties. Purple data points are measurements in broad-band filters, while blue data-points are medium-band filters. Inset stamps are false-color RGB images of $3"\times3"$ based on the F070W, F090W and F115W NIRCam imaging data, revealing the diverse and resolved morphologies of these galaxies.
• Figure 5: The observed F150W-F444W colors versus the (magnification corrected) F444W magnitude of our galaxy (points colored by their stellar mass) and Broad-Line H$\alpha$ samples (black hexagons). The F150W-F444W colors probe the difference in the continuum level from rest-frame wavelengths $\lambda_0=0.27-0.8 \mu$m, free from emission-line contamination. Open hexagons show sources with suspected broad lines (based on the colors and morphology). The stellar masses of the sample correlate with the F444W magnitude and range from $\approx2\times10^7$ M$_{\odot}$ (blue) to $\approx10^{10}$ M$_{\odot}$ (purple).