Concurrent Supermassive Black Hole and Galaxy Growth: Linking Environment and Nuclear Activity in z = 2.23 Hα Emitters

We present results from a ≈100 ks Chandra observation of the 2QZ Cluster 1004+00 structure at z = 2.23 (hereafter 2QZ Clus). 2QZ Clus was originally identified as an overdensity of four optically-selected QSOs at z = 2.23 within a 15 × 15 arcmin2 region. Narrow-band imaging in the near-IR (within the K band) revealed that the structure contains an additional overdensity of 22 z = 2.23 Hα-emitting galaxies (HAEs), resulting in 23 unique z = 2.23 HAEs/QSOs (22 within the Chandra field of view). Our Chandra observations reveal that three HAEs in addition to the four QSOs harbor powerfully accreting supermassive black holes (SMBHs), with 2-10 keV luminosities of ≈(8-60) × 1043 erg s-1 and X-ray spectral slopes consistent with unobscured active galactic nucleus (AGN). Using a large comparison sample of 210 z = 2.23 HAEs in the Chandra-COSMOS field (C-COSMOS), we find suggestive evidence that the AGN fraction increases with local HAE galaxy density. The 2QZ Clus HAEs reside in a moderately overdense environment (a factor of ≈2 times over the field), and after excluding optically-selected QSOs, we find that the AGN fraction is a factor of ≈3.5+3.8 -2.2 times higher than C-COSMOS HAEs in similar environments. Using stacking analyses of the Chandra data and Herschel SPIRE observations at 250 μm, we respectively estimate mean SMBH accretion rates (\dot{M}_BH) and star formation rates (SFRs) for the 2QZ Clus and C-COSMOS samples. We find that the mean 2QZ Clus HAE stacked X-ray luminosity is QSO-like (L 2-10 keV ≈ [6-10] × 1043 erg s-1), and the implied \dot{M}_BH/SFR ≈ (1.6-3.2) × 10-3 is broadly consistent with the local M BH/M sstarf relation and z ≈ 2 X-ray selected AGN. In contrast, the C-COSMOS HAEs are on average an order of magnitude less X-ray luminous and have \dot{M}_BH/SFR ≈ (0.2-0.4) × 10-3, somewhat lower than the local M BH/M sstarf relation, but comparable to that found for z ≈ 1-2 star-forming galaxies with similar mean X-ray luminosities. We estimate that a periodic QSO phase with duty cycle ≈2%-8% would be sufficient to bring star-forming galaxies onto the local M BH/M sstarf relation. This duty cycle is broadly consistent with the observed C-COSMOS HAE AGN fraction (≈0.4%-2.3%) for powerful AGN with L X >~ 1044 erg s-1. Future observations of 2QZ Clus will be needed to identify key factors responsible for driving the mutual growth of the SMBHs and galaxies.