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Paper: Star Forming Galaxies at z ≈ 6 and Reionization
Volume: 380, At the Edge of the Universe: Latest Results from the Deepest Astronomical Surveys
Page: 27
Authors: Bunker, A.; Stanway, E.; Ellis, R.; McMahon, R.; Eyles, L.; Lacy, M.; Stark, D.; Chiu, K.
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 zAB < 25.6 in the 150 arcmin2 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 zAB = 28.5 (equivalent to 1.5 h−270 M yr−1 at z = 6.1, or 0.1L*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 h70M yr−1Mpc−3 at z ≈ 6 at LUV > 0.1L for a Salpeter IMF) poses an interesting challenge for models which suggest that LUV > 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 μm show evidence of the age-sensitive Balmer/4000, dominated by stars older than 100Myr (and most probably 400Myr old). This pushes the formation epoch for these galaxies to zform = 7.5 − 13.5. There are at least some galaxies already assembled with stellar masses ≈ 31010M (equivalent to 0.2M* 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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