X-ray properties of BzK-selected galaxies in the deepest X-ray fields

• Published in: Monthly notices of the Royal Astronomical Society Vol. 428; no. 4; pp. 3089 - 3103
• Main Authors: Rangel, C., Nandra, K., Laird, E. S., Orange, P.
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 01-02-2013
• Subjects: Astrophysics; Correlation analysis; Star & galaxy formation; Ultraviolet radiation; X-ray astronomy; infrared: galaxies; ultraviolet: galaxies; galaxies: active; X-rays: galaxies; galaxies: star formation; catalogues; galaxies: nuclei
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/sts256

We investigate the X-ray properties of BzK-selected galaxies at z ∼ 2 using deep X-ray data in the Chandra Deep Field-South (CDFS) and Chandra Deep Field-North (CDFN). A subset of these BzK galaxies have been proposed as Compton-thick active galactic nucleus (AGN) candidates based on a high ratio of infrared (IR) to ultraviolet (UV) star formation rates (SFRs). With the benefit of deep 24 μm observations, our sample of these IR-excess galaxies is larger than previous studies and combined with the deepest X-ray data yield new insights into the population. We identify 701 and 534 star-forming BzK galaxies (sBzK galaxies) in the range z = 1.2-3.0 in CDFS and CDFN, respectively. Of these we directly detect in X-rays 49 sBzK galaxies in CDFS and 32 sBzK galaxies in CDFN. Stacking the undetected sources also reveals a significant X-ray signal. Investigating the X-ray detection rate and stacked flux versus the IR-excess parameter (i.e. SFRtotal/SFRUV, corr), we find no strong evidence for an increased X-ray detection rate, or a harder X-ray spectrum in IR-excess sBzK galaxies. This is particularly the case when one accounts for the strong correlation between the IR-excess parameter and the bolometric IR luminosity (L
IR); when controlling for L
IR, the IR-non-excess sBzK galaxies show a detection rate at least as high. While both direct detections and stacking suggest that the AGN fraction in sBzK galaxies is high, there is no clear evidence for widespread Compton-thick activity in either the sBzK population generally or the IR-excess sBzK subsample. The very hard X-ray signal obtained for the latter in earlier work was most likely contaminated by a few hard X-ray sources now directly detected in deeper X-ray data. The X-ray detection fraction of passive BzK galaxies in our sample is anything higher than that of sBZK galaxies, so there is no evidence for coeval black hole growth and star formation from X-ray analysis of the BzK populations. Because increased AGN activity in the IR-excess population is not indicated by our X-ray analysis, it appears that the bulk of the IR-excess sBzK population are luminous star-forming galaxies whose SFRs are either overestimated at 24 μm, underestimated in the UV, or both. This conclusion reinforces recent results from Herschel which show similar effects.