  In a recent paper Lehnert & Bremer have photometrically selected a sample of
galaxies at z>4.8 from a single VLT/FORS2 pointing and spectroscopically
confirmed half of them to be at 4.8<z<5.8. To study the properties of such
galaxies further, we have photometrically selected a similar sample (V(AB)>28,
i(AB)<26.3, i(AB)-z(AB)>0) from the HST ACS images of the Chandra Deep Field
South. This selection results in a sample of 44 sources from ~150 sq. arcmin.
We find that such galaxies are often barely resolved in the ACS images, having
half-light radii of 0.1-0.3 arcsec (<2 kpc). They show no difference in spatial
clustering from sources selected by i(AB)<26.3, i(AB)-z(AB)>0, which are
generally galaxies of lower redshift. However, their distribution over the
field is not uniform and their surface density varies considerably over areas
comparable to a single 8m or HST pointing. The reliable determination of the
surface and volume densities of such galaxies requires a sky area considerably
larger than the current ACS imaging of this field. No individual z>5 candidate
was detected to a 3-sigma limit of 6 x 10^-17 erg s^-1 cm^-2 at 0.5-5 keV by
Chandra (a limiting luminosity of below 2 x 10^43 erg s^-1 at z~5.3). By
summing over all positions, we find that the mean source must be undetected at
a level at least a factor 4 times fainter than this. This rules out anything
other than a weak AGN contribution to the emission from these objects and thus
luminous AGN made little contribution to the final stages of re-ionization of
the Universe.
