The ALPINE–CRISTAL– JWST survey: revealing less massive black holes in high-redshift galaxies

We present a systematic search for broad-line active galactic nuclei (AGNs) in the ALPINE–CRISTAL–JWST sample of 18 star-forming galaxies ($M_\star >10^{9.5}~{\rm M}_{\odot }$) at redshifts $z=4.4\!-\!5.7$. Using James Webb Space Telescope (JWST)/NIRSpec IFU, we identify seven AGN candidates through the detection of broad H $\alpha$ emission lines from 33 aperture spectra centred on photometric peaks. These candidates include one highly robust AGN detection with full width at half-maximum (FWHM) $\sim$2800 km s$^{-1}$ and six showing broad components with FWHM $\sim 600\!-\!1600$ km s$^{-1}$, 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 per cent to 33 per cent, 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 $10^{7.5}~{\rm M}_{\odot }$ 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.