Strong environmental active galactic nuclei enhancement among dusty star-forming galaxies in z > 2 protoclusters

Context . Galaxy protoclusters (PCs) at z ≳ 2 are dense regions extending up to a few megaparsec in which the availability of cold gas and the elevated rates of galaxy interactions trigger intense often dust-obscured star formation in the member galaxies. These same mechanisms are also expected to promote supermassive black hole (SMBH) growth, but this possible effect remains unclear, largely due to the heterogeneous galaxy selections and active galactic nucleus (AGN) identification methods employed in previous studies. Aims . We quantitatively assessed the effect of the PC environment on SMBH growth by measuring the incidence of X-ray AGN among dusty star-forming galaxies (DSFGs) in PCs and in a homogeneously selected control sample of field galaxies. We also investigated the physical mechanisms that drive any difference between the two environments. Methods . We considered the DSFG population of ALMA-detected galaxies in the submillimeter/millimeter continuum of seven PCs at 2 < z < 4.5 and constructed a selection-matched control field sample from the COSMOS survey. We statistically compared the incidence of X-ray selected AGN in the PC and control samples and the physical properties of the host galaxies as obtained through uniform spectral energy distribution fitting. Results . We found a significantly higher X-ray AGN fraction in PCs by ≈2.7×, which is different from the field expectation with a Poisson significance p = 3×10 −4 . Similar values were obtained by splitting the sample into two redshift bins, ≈2.7× at z = 2–3 and ≈2.6× at z = 3–4.5, with significances of p = 0.003 and 0.03, respectively. The field and PC DSFG samples are well matched in stellar mass, star-formation rate, and dust mass, indicating that the observed enhancement is not driven by selection effects or systematically higher host masses. In particular, AGN in PCs are hosted by galaxies with similar stellar masses and star formation rates as field AGN. Conclusions . Our results provide quantitative and unbiased evidence that the dense PC environment enhances the AGN incidence and in turn SMBH growth among DSFGs beyond what is expected from host galaxy properties alone, likely through an increased gas availability and interaction-driven fueling. At higher redshift, current statistics limit firm conclusions, but the observed trends suggest that environmental triggering of AGN can operate already at z ≳ 4. This work represents a first step toward a homogeneous assessment of the overall environmental effects on SMBH growth across cosmic time.