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

Abstract

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 Ks 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 h70-2 M⊙ yr-1 at z = 5.9, equivalent to L* for the Lyman-break population at z = 3-4 (Ω Λ = 0-7, ΩM = 0.3). We are sensitive to star-forming galaxies at 5.6 ≲ z ≲ 7.0 with an effective comoving volume of ≈ 1.8 × 105 h70-3 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) × 10-4 h70 M⊙ 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.