Star formation at z ∼ 6:: i-dropouts in the advanced camera for surveys guaranteed time observation fields

Using an i - z dropout criterion, we determine the space density of z similar to 6 galaxies from two deep ACS GTO fields with deep optical-IR imaging. A total of 23 objects are found over 46 arcmin(2), or similar to0.5 +/- 0.1 objects arcmin(-2) down to z(AB) similar to 27.3 (6 sigma), or a completeness-corrected similar to0.5 +/- 0.2 objects arcmin(-2) down to z(AB) similar to 26.5 (including one probable z similar to 6 active galactic nucleus). Combining deep ISAAC data for our RDCS 1252-2927 field (J(AB) similar to 25.7 and K-s;AB similar to 25.0; 5 sigma) and NICMOS data for the Hubble Deep Field North (J(110;AB) and H-160; AB similar to 27.3, 5 sigma), we verify that these dropouts have relatively. at spectral slopes, as one would expect for star-forming objects at z similar to 6. Compared with the average-color (beta = -1.3) U-dropout in the Steidel et al. z similar to 3 sample, i-dropouts in our sample range in luminosity from similar to1.5L(*) (z(AB) similar to 25.6) to similar to0.3L(*) (z(AB) similar to 27.3) with the exception of one very bright candidate at z(850; AB) similar to 24.2. The half-light radii vary from 0."09 to 0."21, or 0.5 kpc to 1.3 kpc. We derive the z similar to 6 rest-frame UV luminosity density (or star formation rate density) by using three different procedures. All three procedures use simulations based on a slightly lower redshift (z similar to 5) V-606-dropout sample from Chandra Deep Field-South ACS images. First, we make a direct comparison of our findings with a no-evolution projection of this V-dropout sample, allowing us to automatically correct for the light lost at faint magnitudes or lower surface brightnesses. We find 23% +/- 25% more i-dropouts than we predict, consistent with no strong evolution over this redshift range. Adopting previous results to z similar to 5, this works out to a mere 20% +/- 29% drop in the luminosity density from z similar to 3 to z similar to 6. Second, we use the same V-dropout simulations to derive a detailed selection function for our i-dropout sample and compute the UV-luminosity density [7.2 +/- 2.5) _x 10(25) ergs s(-1) Hz(-1) Mpc(-3) down to z(AB) similar to 27]. We find a 39% +/- 21% drop over the same redshift range (z similar to 3-6), consistent with the first estimate. This is our preferred value and suggests a star formation rate of 0.0090 +/- 0.0031 M-. yr(-1) Mpc(-3) to z(AB) similar to 27, or similar to 0.036 +/- 0.012 M-. yr(-1) Mpc(-3) by extrapolating the luminosity function to the faint limit, assuming alpha = - 1.6. Third, we follow a very similar procedure, except that we assume no incompleteness, and find a rest-frame continuum luminosity that is similar to2-3 times lower than our other two determinations. This final estimate is to be taken as a lower limit and is important if there are modest changes in the colors or surface brightnesses from z similar to 5 to z similar to 6 (the other estimates assume no large changes in the intrinsic selectability of objects). We note that all three estimates are well within the canonical range of luminosity densities necessary for reionization of the universe at this epoch by star-forming galaxies.