Star Forming Galaxies at z≈ 6 and Reionization

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Title:		Star Forming Galaxies at z≈ 6 and Reionization
Authors:		Bunker, A.; Stanway, E.; Ellis, R.; McMahon, R.; Eyles, L.; Lacy, M.; Stark, D.; Chiu, K.
Publication:		At the Edge of the Universe: Latest Results from the Deepest Astronomical Surveys ASP Conference Series, Vol. 380, proceedings of the conference held 9-13 October, 2006 at Sintra, Portugal. Edited by José Afonso, Henry C. Ferguson, Bahram Mobasher, and Ray Norris., p.27
Publication Date:		12/2007
Origin:		ASPC
Bibliographic Code:		2007ASPC..380...27B

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-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-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 the dominant uncertainty. The increased depth of UDF enables us to reach a ˜ 10 σ limiting magnitude of z'_AB=28.5 (equivalent to 1.5 h^-2₇₀ M_ȯ yr^-1 at z=6.1, or 0.1 L^*_UV 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_ȯ 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≃q 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× 10¹⁰ M_ȯ (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.

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