  We determine the space density of UV-luminous star-burst galaxies at z~6
using deep HST ACS SDSS-i' (F775W) and SDSS-z' (F850LP) and VLT ISAAC J and K_s
band imaging of the Chandra Deep Field South. We find 8 galaxies and one star
with (i'-z')>1.5 to a depth of z'{AB}= 25.6 (an 8-sigma detection in each of
the 3 available ACS epochs). This corresponds to an unobscured star formation
rate of ~15M_sun/yr/h{70}^2 at z=5.8, equivalent to L* for the Lyman break
population at z =3-4 (Omega_{Lambda}=0.7, Omega_M=0.3). We are sensitive to
star forming galaxies at 5.6<z<7.0 with an effective comoving volume of
\~1.8x10^5 Mpc^3/h{70}^3 after accounting for incompleteness at the higher
redshifts due to luminosity bias. This volume should encompass the primeval
sub-galactic scale fragments of the progenitors of about a thousand L* galaxies
at the current epoch. We determine a volume-averaged global star formation rate
of (6.7 +/- 2.7)x10^{-4}h{70}M_sun/yr/Mpc^3 at z~6 from rest-frame UV selected
star-bursts at the bright end of the luminosity function: this is a lower limit
because of dust obscuration and galaxies below our sensitivity limit. This
measurement shows that at z~6 the star formation density at the bright end is a
factor of ~6 times less than that determined by Steidel et al. (1999) for a
comparable sample of UV selected galaxies at z=3-4, and so extends our
knowledge of the star formation history of the Universe to earlier times than
previous work and into the epoch where reionization may have occurred.
