We measure the surface density of i ^ { \prime } -band dropout galaxies at z \sim 6 through wide-field HST/ACS imaging and ultra-deep Keck/DEIMOS spectroscopy .
Using deep HST/ACS SDSS- i ^ { \prime } ( F775W ) and SDSS- z ^ { \prime } 
( F850LP ) imaging from the Great Observatories Origins Deep Survey North ( GOODS-N ; 200 arcmin ^ { 2 } ) , we identify 9 i ^ { \prime } -drops satisfying ( i ^ { \prime } - z ^ { \prime } ) _ { AB } > 1.5 to a depth of z ^ { \prime } _ { AB } = 25.6 ( corresponding to L ^ { * } _ { UV } at z \sim 3 ) .
We use HK ^ { \prime } imaging data to improve the fidelity of our sample , discriminating against lower redshift red galaxies and cool Galactic stars .
Three i ^ { \prime } -drops are consistent with M/L/T dwarf stars .
We present ultra-deep Keck/DEIMOS spectroscopy of 10 objects from our combined GOODS-N and GOODS-S i ^ { \prime } -drop sample .
We detect Lyman- \alpha emission at z = 5.83 from one object in the GOODS-S field , which lies only 8arcmin ( i.e . 3 h ^ { -1 } _ { 70 } Mpc ) away from a previously confirmed z = 5.78 object .
One possible Lyman- \alpha 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 6 candidate z \sim 6 galaxies from the GOODS-N field , we determine a lower limit to the unobscured volume-averaged global star formation rate at z \sim 6 of ( 5.4 \pm 2.2 ) \times 10 ^ { -4 } h _ { 70 } M _ { \odot } { yr } ^ { -1 } { Mpc } ^ { -3 } .
We find that the cosmic star formation density in Lyman Break galaxies ( LBGs ) with unobscured star formation rates > 15 M _ { \odot } { yr } ^ { -1 } falls by a factor of 8 between z \sim 3 and z \sim 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 .