  There has been great progress in recent years in discovering star forming
galaxies at high redshifts (z>5), close to the epoch of reionization of the
intergalactic medium (IGM). The WFC3 and ACS cameras on the Hubble Space
Telescope have enabled Lyman break galaxies to be robustly identified, but the
UV luminosity function and star formation rate density of this population at
z=6-8 seems to be much lower than at z=2-4. High escape fractions and a large
contribution from faint galaxies below our current detection limits would be
required for star-forming galaxies to reionize the Universe. We have also found
that these galaxies have blue rest-frame UV colours, which might indicate lower
dust extinction at z>5. There has been some spectroscopic confirmation of these
Lyman break galaxies through Lyman-alpha emission, but the fraction of galaxies
where we see this line drops at z>7, perhaps due to the onset of the
Gunn-Peterson effect (where the IGM is opaque to Lyman-alpha).
