Star forming galaxies at z ≈ 6 and reionization

We determine the abundance of i'-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 approximate to 6 galaxies whose flux decrement between the F775W i'-band and F850LP z'-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'(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 be the dominant uncertainty. The increased depth of UDF enables us to reach a similar to 10 sigma limiting magnitude of Z'(AB) = 28.5 (equivalent to 1.5 h(70)(-2) M-circle dot yr(-1) at z = 6.1, or 0.1 L-UV(center dot) for the z approximate to 3 U-drop population). The star formation rate at z approximate to 6 was approximately x6 less than at z approximate to 3. This declining comoving star formation rate (0.005 h(70) M-circle dot yr(-1) Mpc(-3) at z approximate to 6 at L-UV > 0.1L* for a Salpeter IMF) poses an interesting challenge for models which suggest that L-UV > 0.1L* star forming galaxies at z similar or equal to 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 >> 6. We have recently discovered evidence of an early epoch of star formation in some of the i'-drops at z approximate to 6. Spitzer images with IRAC at 3.6-4.5 mu m show evidence of the age-sensitive Balmer/4000 angstrom, 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 approximate to 3 x 10(10) M-circle dot (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 similar to 10. (c) 2005 Elsevier B.V. All rights reserved.