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~ 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 8σ detection in each of the 3 available ACS epochs). This corresponds to an unobscured star formation rate of ~15 h^-2₇₀ M_solar yr^-1 at z= 5.9, equivalent to L* for the Lyman-break population at z= 3-4 (Ω_Λ= 0.7, Ω_M= 0.3). We are sensitive to star-forming galaxies at 5.6 <~z<~ 7.0 with an effective comoving volume of ~1.8 × 10⁵h^-3₇₀ Mpc³ 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) × 10^-4h₇₀ M_solar yr^-1 Mpc^-3 at z~ 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~ 6 the star formation density at the bright end is a factor of ~6 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.