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

Abstract

We present results from a {ap}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 { imes} 15 arcmin$^{2}$ 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 H{$α$}-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 {ap}(8-60) { imes} 10$^{43}$ 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 {ap}2 times over the field), and after excluding optically-selected QSOs, we find that the AGN fraction is a factor of {ap}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 {$μ$}m, we respectively estimate mean SMBH accretion rates ($ackslash$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 $_{2-10 keV}$ {ap} [6-10] { imes} 10$^{43}$ erg s$^{-1}$), and the implied $ackslash$dot${$M$}$_BH/SFR {ap} (1.6-3.2) { imes} 10$^{-3}$ is broadly consistent with the local M $_{BH}$/M $_{sstarf}$ relation and z {ap} 2 X-ray selected AGN. In contrast, the C-COSMOS HAEs are on average an order of magnitude less X-ray luminous and have $ackslash$dot${$M$}$_BH/SFR {ap} (0.2-0.4) { imes} 10$^{-3}$, somewhat lower than the local M $_{BH}$/M $_{sstarf}$ relation, but comparable to that found for z {ap} 1-2 star-forming galaxies with similar mean X-ray luminosities. We estimate that a periodic QSO phase with duty cycle {ap}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 ({ap}0.4%-2.3%) for powerful AGN with L $_X$ {gt}~{} 10$^{44}$ 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.