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

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  • 12 February 2003
Abstract
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 K_s 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 sigma detection in each of the 3 available ACS epochs). This corresponds to an unobscured star formation rate of ~15 M_sun/yr/h_{70}^2 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< z < 7.0 with an effective comoving volume of ~1.8E5 Mpc^3/h_{70}^3 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} h_{70} M_sun/yr/Mpc^3 at z~6 from rest-frame UV selected star-bursts. This measurement shows that at z~6 the star formation density 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.

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