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

TL;DR

This study examines the X-ray characteristics of BzK-selected galaxies at z~2 in deep fields, revealing high AGN fractions but no widespread Compton thick activity, and suggests IR excess may be due to star formation rate estimation issues.

Contribution

It provides the first detailed X-ray analysis of BzK galaxies in the deepest fields, clarifying the nature of IR excess and AGN activity in these high-redshift galaxies.

Findings

High X-ray detection rate among BzK galaxies.

No strong link between IR excess and AGN activity.

IR excess likely due to SFR estimation errors.

Abstract

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.