Star forming galaxies at z ≈ 6 and reionization

Abstract

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 ≈ 6 galaxies whose flux decrement between the F775W i′-band and F850LP z′-band arises from Lyman-α absorption. We have shown with Keck/DEIMOS and Gemini/GMOS spectroscopy that this technique does indeed select high redshift galaxies, and we discovered Lyman-α 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 10σ limiting magnitude of z'_(AB)=28.5 (equivalent to 1:5 h^(-2)_(70) M_⊙ yr^(-1) at z = 6.1, or 0:1 L^*_(UV) the z ≈ 3 U-drop population). The star formation rate at z ≈ 6 was approximately X6 less than at z ≈ 3. This declining comoving star formation rate (0.005 h_(70) M_⊙ yr^(-1) Mpc^(-3) at z ≈ 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 ≈ 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 ≈ 6. Spitzer images with IRAC at 3.6–4.5 lm show evidence of the age-sensitive Balmer/4000Å, 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 ≈3 X 10^(10) M_⊙ (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 ~ 10.