Optical Spectroscopy of X-ray sources in the Extended Chandra Deep Field South

TL;DR

This study uses optical spectroscopy to identify and analyze X-ray sources in the Extended Chandra Deep Field South, revealing properties of AGN host galaxies, obscuration dependence, and flux ratio differences related to dust effects.

Contribution

First spectroscopic redshifts for X-ray sources in the field, showing AGN host galaxy colors, obscuration trends, and flux ratio differences based on dust absorption.

Findings

AGN host galaxies are mostly in the green valley.

Obscured AGN fraction depends on luminosity and redshift.

Obscured and unobscured AGN differ in mid-IR to X-ray flux ratios.

Abstract

We present the first results of our optical spectroscopy program aimed to provide redshifts and identifications for the X-ray sources in the Extended Chandra Deep Field South. A total of 339 sources were targeted using the IMACS spectrograph at the Magellan telescopes and the VIMOS spectrograph at the VLT. We measured redshifts for 186 X-ray sources, including archival data and a literature search. We find that the AGN host galaxies have on average redder rest-frame optical colors than non-active galaxies, and that they live mostly in the "green valley". The dependence of the fraction of AGN that are obscured on both luminosity and redshift is confirmed at high significance and the observed AGN space density is compared with the expectations from existing luminosity functions. These AGN show a significant difference in the mid-IR to X-ray flux ratio for obscured and unobscured AGN, which can be explained by the effects of dust self-absorption on the former. This difference is larger for lower luminosity sources, which is consistent with the dust opening angle depending on AGN luminosity.