The systematic search for $z\gtrsim5$ active galactic nuclei in the $Chandra$ Deep Field South

TL;DR

This study conducted a thorough search for high-redshift ($z extgreater{}5$) active galactic nuclei in the Chandra Deep Field South using deep X-ray and multiwavelength data, finding no convincing candidates and thus providing constraints on early black hole growth.

Contribution

It presents a systematic, multiwavelength approach to identify $z extgreater{}5$ AGN and sets constraints on their abundance and growth modes based on null detection.

Findings

No convincing $z extgreater{}5$ AGN detected in the field.

Constraints on black hole occupation and growth at high redshift.

Potential explanations include low occupation fraction and short growth phases.

Abstract

We investigate early black hole (BH) growth through the methodical search for $z\gtrsim5$ AGN in the $Chandra$ Deep Field South. We base our search on the $Chandra$ 4-Ms data with flux limits of $9.1\times\ 10^{-18}$ (soft, 0.5 - 2 keV) and $5.5\times\ 10^{-17}\ \mathrm{erg}\ \mathrm{s}^{-1}\ \mathrm{cm}^{-2}$ (hard, 2 - 8 keV). At $z\sim5$ this corresponds to luminosities as low as $\sim10^{42}$ ($\sim10^{43}$) $\mathrm{erg}\ \mathrm{s}^{-1}$ in the soft (hard) band and should allow us to detect Compton-thin AGN with $M_\mathrm{BH}>10^7 M_{\odot}$ and Eddington ratios > 0.1. Our field ($0.03~\mathrm{deg}^2$) contains over 600 $z\sim5$ Lyman Break Galaxies. Based on lower redshift relations we would expect $\sim20$ of them to host AGN. After combining the $Chandra$ data with GOODS/ACS, CANDELS/WFC3 and $Spitzer$/IRAC data, the sample consists of 58 high-redshift candidates. We run a photometric redshift code, stack the GOODS/ACS data, apply colour criteria and the Lyman Break Technique and use the X-ray Hardness Ratio. We combine our tests and using additional data find that all sources are most likely at low redshift. We also find five X-ray sources without a counterpart in the optical or infrared which might be spurious detections. We conclude that our field does not contain any convincing $z\gtrsim5$ AGN. Explanations for this result include a low BH occupation fraction, a low AGN fraction, short, super-Eddington growth modes, BH growth through BH-BH mergers or in optically faint galaxies. By searching for $z\gtrsim5$ AGN we are setting the foundation for constraining early BH growth and seed formation scenarios.