X-ray properties of BzK-selected galaxies in the deepest X-ray fields
  We investigate the X-ray properties of BzK-selected galaxies at z $\sim$ 2
using deep X-ray data in the Chandra Deep Field South and North (CDFS and
CDFN). Of these we directly detect in X-rays 49 sBzKs in CDFS and 32 sBzKs 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 sBzKs.
This is particularly the case when one accounts for the strong correlation
between the IR excess parameter and the bolometric IR luminosity (LIR), e.g.
when controlling for LIR, the IR Non-Excess sBzKs 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 if anything higher than that of sBZKs, 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 microns, underestimated in the UV, or both.
This conclusion reinforces recent results from Herschel which show similar
effects.
