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Lyman Break Galaxies and the Star Formation Rate of the Universe at z ∼ 6

By Elizabeth R. Stanway, Andrew J. Bunker and Richard G. Mcmahon


We determine the space density of UV-luminous star-burst 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 8 galaxies and one star with (i ′ − z ′)> 1.5 to a depth of z ′ AB = 25.6 (an 8 σ detection in each of the 3 available ACS epochs). This corresponds to an unobscured star formation rate of ≈ 15 h −2 70 M ⊙ yr−1 at z = 5.9, equivalent to L ∗ for the Lyman break population at z = 3 − 4 (ΩΛ = 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 × 10 5 h −3 70 Mpc3 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 ±2.7) ×10 −4 h70 M ⊙ yr −1 Mpc −3 at z ∼ 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 ∼ 6 the star formation density at the bright end is a factor of ∼ 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

Year: 2003
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