Hubble Space Telescope Imaging and Keck Spectroscopy of z ≈ 6 i-Band Dropout Galaxies in the Advanced Camera for Surveys GOODS Fields

We measure the surface density of i'-band dropout galaxies at z ~ 6 through wide-field Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging and ultradeep Keck DEIMOS spectroscopy. Using deep HST ACS SDSS i' (F775W) and SDSS z' (F850LP) imaging from the Great Observatories Origins Deep Survey-North (GOODS-N; 200 arcmin2), we identify nine i'-drops satisfying (i'-z')AB > 1.5 to a depth of zimg1.gif = 25.6 (corresponding to Limg2.gif at z ~ 3). We use HK' imaging data to improve the fidelity of our sample, discriminating against lower redshift red galaxies and cool Galactic stars. Three i'-drops are consistent with M/L/T dwarf stars. We present ultradeep Keck DEIMOS spectroscopy of 10 objects from our combined GOODS-N and GOODS-S i'-drop sample. We detect Lyα emission at z = 5.83 from one object in the GOODS-S field, which lies only 8' (i.e., 3 himg3.gif Mpc) away from a previously confirmed z = 5.78 object. One possible Lyα emitter at z = 6.24 is found in the GOODS-N field (although identification of this spatially offset emission line is ambiguous). Using the rest-frame UV continuum from our six candidate z ~ 6 galaxies from the GOODS-N field, we determine a lower limit to the unobscured volume-averaged global star formation rate at z ~ 6 of (5.4 ± 2.2) × 10-4 h70 M☉ yr-1 Mpc-3. We find that the cosmic star formation density in Lyman break galaxies (LBGs) with unobscured star formation rates greater than 15 M☉ yr-1 falls by a factor of 8 between z ~ 3 and z ~ 6. Hence, the luminosity function of LBGs must evolve in this redshift interval: a constant integrated star formation density at z > 3 requires a much steeper faint-end slope, or a brighter characteristic luminosity. This result is in agreement with our previous measurement from the GOODS-S field, indicating that cosmic variance is not a dominant source of uncertainty.