The Compton-thick AGN in the CDF-N

TL;DR

This study identifies and analyzes Compton-thick AGN in the Chandra Deep Field North using X-ray spectral data, revealing their properties, obscuration levels, and consistency with background models.

Contribution

First direct X-ray spectral identification of Compton-thick AGN in CDF-N, confirming their characteristics and relation to obscured galaxy populations.

Findings

10 Compton-thick AGN candidates identified, 9 reflection dominated.

Most candidates show high IR flux ratios indicating dust obscuration.

Candidate sources align with X-ray background synthesis model predictions.

Abstract

We present X-ray spectral analysis of the brightest sources (f_{2-10 keV}>10^{-15}$ cgs) in the Chandra Deep Field North. Our sample consists of 222 sources; for the vast majority (171) either a spectroscopic or a photometric redshift is available. Our goal is to discover the Compton-thick AGN in a direct way i.e. through their X-ray spectra. Compton-thick AGN give away their presence in X-rays either directly through the absorption turnover redshifted in the Chandra passband, or through a flat, reflection-dominated, spectrum. The above selection criteria yield 10 Compton-thick AGN candidates of which the nine are reflection dominated. The IR or sub-mm data where available, corroborate the presence of a heavily obscured nucleus in most cases. All the five candidate Compton-thick sources with available 24 micron data present very high values of the f_{24}/f_R flux ratio suggesting that they are dust obscured galaxies. The low f_x/f_{IR} ratio also suggest the presence of obscured nuclei in many cases. Four of the candidate Compton-thick sources are associated with sub-mm galaxies at high redshifts z$\sim2$. The number count vs. flux distribution of the candidate Compton-thick AGN as well as their distribution with redshift agree reasonably well with the predictions of the X-ray background synthesis models of Gilli et al.