The systematic search for $z\gtrsim 5$ active galactic nuclei in the $Chandra$ Deep Field South

Abstract

We investigate early black hole (BH) growth through the methodical search for $z\gtrsim 5$ 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 $z\sim 5$ this corresponds to luminosities as low as $\sim 10^{42}$ ($\sim 10^{43}$) $\mathrm{erg}{\mathrm{s}}^{-1}$ 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 $z\sim 5$ Lyman Break Galaxies. Based on lower redshift relations we would expect $\sim 20$ 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 $z\gtrsim 5$ 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 $z\gtrsim 5$ AGN we are setting the foundation for constraining early BH growth and seed formation scenarios.

Accepted for publication in MNRAS, 22 pages, 15 figures, for an overview of our analysis see figure 2

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