The Faintest X-ray Sources from z=0-8

TL;DR

This study uses deep Chandra X-ray data to analyze the faintest X-ray sources across redshifts 0 to 8, finding that high-redshift sources show no significant flux and low-redshift sources are consistent with star formation-driven X-ray emission.

Contribution

It introduces an optimized averaging method to measure mean X-ray fluxes of optically selected sources over a wide redshift range, setting new upper limits and confirming the star formation-X-ray relation.

Findings

High-redshift (z>5) sources show no significant X-ray flux.

X-ray emission at lower redshifts is consistent with star formation activity.

The star formation-X-ray relation remains invariant from z=0 to 4.

Abstract

We use the new 4 Ms exposure of the CDF-S field obtained with the Chandra X-ray satellite to investigate the properties of the faintest X-ray sources over a wide range of redshifts. We use an optimized averaging procedure to investigate the weighted mean X-ray fluxes of optically selected sources in the CDF-S over the redshift range z=0-8 and down to 0.5-2 keV fluxes as low as 5e-19 erg/cm^2/s. None of the samples of sources at high redshifts (z>5) show any significant flux, and at z=6.5 we place an upper limit on the X-ray luminosity of 4e41 erg/s in the rest-frame 3.75-15 keV band for the sample of Bouwens et al. (2006). This is consistent with any X-ray production in the galaxies being solely due to star formation. At lower redshifts we find significant weighted mean X-ray fluxes in many samples of sources over the redshift range z=0-4. We use these to argue that (1) the relation between star formation and X-ray production remains invariant over this redshift range, (2) X-ray sources below the direct detection threshold in the CDF-S are primarily star-forming, and (3) there is full consistency between UV and X-ray estimations of the star formation history.