Concurrent supermassive black hole and galaxy growth : linking environment and nuclear activity in Z = 2.23 hα emitters

Lehmer, B. D. and Lucy, A. B. and Alexander, D. M. and Best, P. N. and Geach, J. E. and Harrison, C. M. and Hornschemeier, A. E. and Matsuda, Y. and Mullaney, J. R. and Smail, Ian and Sobral, D. and Swinbank, A. M. (2013) Concurrent supermassive black hole and galaxy growth : linking environment and nuclear activity in Z = 2.23 hα emitters. The Astrophysical Journal, 765 (2): 87. ISSN 0004-637X

Abstract

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 () 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 /SFR ≈ (1.6-3.2) × 10-3 is broadly consistent with the local M BH/M 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 /SFR ≈ (0.2-0.4) × 10-3, somewhat lower than the local M BH/M 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 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.