2013ApJ...765...87L

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 15x15 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)x10⁴³ erg/s 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]x10⁴³ erg/s), and the implied {dot}M_BH/SFR ~ (1.6-3.2)x10^–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)x10^–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. Future observations of 2QZ Clus will be needed to identify key factors responsible for driving the mutual growth of the SMBHs and galaxies.