We have determined the abundance of i ^ { \prime } -band drop-outs in the HST /ACS GOODS surveys and the Hubble Ultra Deep Field ( UDF ) .
The majority of these sources are likely to be z \approx 6 galaxies whose flux decrement between the F775W i ^ { \prime } -band and F850LP z ^ { \prime } -band arises from Lyman-alpha absorption .
We have shown with Keck/DEIMOS and Gemini/GMOS spectroscopy that this technique does indeed select high redshift galaxies , and we discovered Lyman- \alpha emission in the expected redshift range for about a third of the galaxies with z ^ { \prime } _ { AB } < 25.6 in the 150 arcmin ^ { 2 } of the GOODS-South field .
The i -drop number counts in the GOODS-North field are consistent , so cosmic variance is possibly not the dominant uncertainty .
The increased depth of UDF enables us to reach a \sim 10 \sigma limiting magnitude of z ^ { \prime } _ { AB } = 28.5 ( equivalent to 1.5 h ^ { -2 } _ { 70 } M _ { \odot } { yr } ^ { -1 } at z = 6.1 , or 0.1 L ^ { * } _ { UV } for the z \approx 3 U -drop population ) .
The star formation rate at z \approx 6 was approximately \times 6 less than at z \approx 3 .
This declining comoving star formation rate ( 0.005 h _ { 70 } M _ { \odot } { yr } ^ { -1 } { Mpc } ^ { -3 } at z \approx 6 at L _ { UV } > 0.1 L ^ { * } for a Salpeter IMF ) poses an interesting challenge for models which suggest that L _ { UV } > 0.1 L ^ { * } star forming galaxies at z \simeq 6 reionized the universe .
The short-fall in ionizing photons might be alleviated by galaxies fainter than our limit , or a radically different IMF .
Alternatively , the bulk of reionization might have occurred at z \gg 6 .
We have recently discovered evidence of an early epoch of star formation in some of the i ^ { \prime } -drops at z \approx 6 . Spitzer images with IRAC at 3.6 - 4.5 \mu m show evidence of the age-sensitive Balmer/4000 Å , dominated by stars older than 100 Myr ( and most probably 400 Myr old ) .
This pushes the formation epoch for these galaxies to z _ { form } = 7.5 - 13.5 .
There are at least some galaxies already assembled with stellar masses \approx 3 \times 10 ^ { 10 } M _ { \odot } ( equivalent to 0.2 M ^ { * } today ) within the first billion years .
The early formation of such systems may have played a key role in reionizing the Universe at z \sim 10 .