The Properties of Galaxies at z>5

Abstract

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.