Lyman break galaxies and the star formation rate of the Universe at z≈6

We determine the space density of UV-luminous starburst galaxies at z approximate to 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 eight galaxies and one star with (i '-z ') > 1.5 to a depth of z ' (AB) = 25.6 (an 8sigma detection in each of the 3 available ACS epochs). This corresponds to an unobscured star formation rate of approximate to15 h (-2) (70) M-circle dot 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 less than or similar toz less than or similar to 7.0 with an effective comoving volume of approximate to1.8 x 10(5) h (-3) (70) Mpc(3) after accounting for incompleteness at the higher redshifts due to luminosity bias. This volume should encompass the primeval subgalactic-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) x 10(-4) h (70) M-circle dot yr(-1) Mpc(-3) at z similar to 6 from rest-frame UV selected starbursts 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 similar to 6 the star formation density at the bright end is a factor of similar to6 times less than that determined by Steidel et al. 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.