The ALPINE-CRISTAL-JWST Survey: Revealing Less Massive Black Holes in High-Redshift Galaxies

TL;DR

This work probes high-redshift black hole–host galaxy co-evolution by systematically searching for faint broad-line AGN in 18 massive star-forming galaxies at $z=4.4$–$5.7$ using JWST/NIRSpec IFU. A rigorous H$ alpha$ spectral-fitting pipeline, complemented by outflow diagnostics from [OIII], identifies 7 broad-line AGN candidates across 33 central apertures, including one highly robust case with $FWHM_{ m H\alpha} sim2800$ km s$^{-1}$. The inferred BH masses span $10^{6}$–$10^{7.5} M_ $ with host stellar masses $ $ $M_ star oughly 10^{9.5}$–$10^{10.5} M_ $, placing them near or below the local $M_{ m BH}-M_ star$ relation and highlighting selection biases that have favored overmassive BHs in previous high-$z$ studies. Simulations reveal that broad-line detectability is strongly dependent on host mass and Eddington ratio, implying a substantial population of undermassive BHs remains difficult to detect in lower-mass systems. The results also uncover a strong association between AGN activity and galaxy mergers, and underscore the limitations of emission-line diagnostics in definitively identifying faint high-redshift AGN, motivating future deeper high-ionization line measurements.

Abstract

We present a systematic search for broad-line active galactic nuclei (AGNs) in the ALPINE-CRISTAL-JWST sample of 18 star-forming galaxies ($M_\star>10^{9.5}~M_{\odot}$) at redshifts $z=4.4-5.7$. Using JWST/NIRSpec IFU, we identify 7 AGN candidates through the detection of broad \Ha\ emission lines from 33 aperture spectra centred on photometric peaks. These candidates include one highly robust AGN detection with FWHM $\sim$ 2800 \kms\ and six showing broad components with FWHM $\sim 600-1600$ \kms, with two in a merger system. We highlight that only broad-line detection is effective since these candidates uniformly lie within narrow emission-line ratio diagnostic diagrams where star-forming galaxies and AGNs overlap. The broad-line AGN fraction ranges from 5.9\% to 33\%, depending on the robustness of the candidates. Assuming that the majority are AGNs, the relatively high AGN fraction is likely due to targeting high-mass galaxies, where simulations demonstrate that broad-line detection is more feasible. Their black hole masses range from $10^6$ to $10^{7.5}~M_{\odot}$ with $0.1 \lesssim L_{\rm bol}/L_{\rm Edd}\lesssim 1$. Counter to previous JWST studies at high redshift that found overmassive black holes relative to their host galaxies, our candidates lie close to or below the local $M_{\rm BH}-M_\star$ scaling relations, thus demonstrating the effect of selection biases. This study provides new insights into AGN-host galaxy co-evolution at high redshift by identifying faint broad-line AGNs in galaxy samples, highlighting the importance of considering mass-dependent selection biases and the likelihood of a large population of AGNs being undermassive and just now being tapped by JWST.

Figures (14)

• Figure 1: Demonstration of our aperture spectral extraction for DC_536534. Left: NIRCam F444W image (top) and the H$\alpha$ emission line flux map (bottom). For better visualization, we normalize the F444W image using an Asinh stretch and reproject the H$\alpha$ map to the world coordinate system (WCS) of the F444W image. The spectral extraction aperture is indicated by the green circle. Right: The extracted NIRSpec IFU spectrum from the marked aperture, showing key emission line features in both the G235M and G395M gratings.
• Figure 2: Flowchart of the spectral fitting pipeline for identifying broad-line AGNs. Rectangular boxes describe the model fits to the H$\alpha$ region with the three final models shown at the bottom. Diamond boxes represent decision criteria for model selection, with red numbers indicating the count of spectra passing or failing each criterion.
• Figure 3: Simulated NIRSpec IFU spectra showing the H$\alpha$ region for AGNs of varying properties. Each panel shows spectra for three different host galaxy masses ($\log{M_\star/M_\odot} = 8.0, 9.0, 10.0$), with Eddington ratio increasing from left to right ($\log{\lambda_{\rm Edd}} = -1.0, -0.5, 0.0$). Black hole masses and broad H$\alpha$ line widths are derived using the local scaling relations from Reines2013Reines2015. The spectra include instrumental effects and noise levels matching our ALPINE-CRISTAL-JWST observations.
• Figure 4: Detection limits for broad-line AGNs in the $M_\star$-$M_{\rm BH}$ plane. Solid and dashed lines show the 50% detection probability thresholds for different Eddington ratios ($\log{\lambda_{\rm Edd}}$), using central aperture (IFU centre) and integrated galaxy spectra (Full galaxy), respectively. Note that for $\log{\lambda_{\rm Edd}}=0.5$, the 'Full galaxy' line overlaps with the 'IFU center' line. Black dash-dotted and dotted lines represent local $M_{\rm BH}-M_\star$ relations from Reines2015 and Greene2020. AGN systems below the local relation are unlikely to be detected, especially in low-mass galaxies. Our IFU aperture spectra method provides less biased detection due to reduced contamination from extended galaxy emission. High-redshift AGN detections from recent JWST studies Ding2023Harikane2023Maiolino2024Stone2024aYue2024b are shown for comparison. All previously detected AGNs lie well above our predicted detection limits and have Eddington ratios of $\log{\lambda_{\rm Edd}}\sim-0.5$, consistent with our most favorable detection scenario.
• Figure 5: Spectral analysis of our most robust AGN candidate, DC_536534 (HZ1/CRISTAL-03). Panels show fits to the H$\alpha$$+$[N ii] (left), H$\beta$ (middle), and [O iii] (right) emission lines. Top row: fits incorporating narrow$+$broad$+$outflow components; middle row: fits using only narrow components. Reduced $\chi^2$ and measured FWHM values are indicated for each fit (note that these FWHM values are not corrected for instrumental broadening). Bottom panels show fit residuals, demonstrating the necessity of the broad H$\alpha$ component. We show the $\Delta$BIC on the top left. The $\Delta$BIC$_{na-br}$ compares the BIC value of narrow model and narrow$+$broad model for H$\alpha$ and the $\Delta$BIC$_{na-br}$ tests between narrow$+$broad and narrow$+$broad$+$outflow models. The instrumental dispersion corrected FWHM of broad H$\alpha$ is provided in Table \ref{['tab:AGN_prop']}. Blue and orange horizontal line segments in the top left panel mark wavelength ranges used for spatial analysis in Fig. \ref{['fig:QA4HabMap']}.