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