We determine the space density of UV-luminous star-burst galaxies at z \sim 6 using deep HST ACS SDSS- i ^ { \prime } ( F775W ) and SDSS- z ^ { \prime } ( 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 ^ { \prime } - z ^ { \prime } ) > 1.5 to a depth of z ^ { \prime } _ { AB } = 25.6 ( an 8 \sigma detection in each of the 3 available ACS epochs ) .
This corresponds to an unobscured star formation rate of \approx 15 h _ { 70 } ^ { -2 } M _ { \odot } { yr } ^ { -1 } at z = 5.9 , 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 \la z \la 7.0 with an effective comoving volume of \approx 1.8 \times 10 ^ { 5 } h _ { 70 } ^ { -3 } { Mpc } ^ { 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 \pm 2.7 ) \times 10 ^ { -4 } h _ { 70 } M _ { \odot } { yr } ^ { -1 } { Mpc } ^ { -3 } at z \sim 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 \sim 6 the star formation density at the bright end is a factor of \sim 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 .