Lyman break galaxies and the star formation rate of the Universe at <MATH>z approx. 6</MATH>.

We determine the space density of UV-luminous starburst galaxies at z ≈ 6 using deep HST ACS SDSS-i&#39; (F775W) and SDSS-z&#39; (F850LP) and VLT ISAAC J and Ks band imaging of the Chandra Deep Field South. We find eight galaxies and one star with (i&#39; - z&#39;) &gt; 1.5 to a depth of z&#39;[sub 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[sup -2, sub 70] M⊙ yr[sup -1] at z = 5.9, equivalent to L[sup *] for the Lymanbreak population at z = 3-4 (Ω[sub Λ] = 0.7, Ω[sub M] = 0.3). We are sensitive to star-forming galaxies at 5.6 ... z ... 7.0 with an effective comoving volume of ≈1.8 &#215; 10[sup 5] h[sup -3, sub 70] Mpc&#179; 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[sup *] galaxies at the current epoch. We determine a volume-averaged global star formation rate of (6.7 &#177; 2.7) &#215; 10[sup -4] h[sub 70] M⊙ yr[sup -1] Mpc[sup -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. [ABSTRACT FROM AUTHOR]