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 zAB'<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 ∼10 σ limiting magnitude of zAB'=28.5 (equivalent to 1.5h70-2M_⊙yr^-1 at z = 6.1, or 0.1LUV∗ for the z ≈ 3 U-drop population). The star formation rate at z ≈ 6 was approximately ×6 less than at z ≈ 3. This declining comoving star formation rate (0.005 h₇₀ M_⊙ yr ^-1 Mpc ^-3 at z ≈ 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 ≃ 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 μm 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 ≈3 × 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.