Spectroscopic properties of luminous Ly α emitters at z ≈ 6-7 and comparison to the Lyman-break population
Abstract
We present spectroscopic follow-up of candidate luminous Ly α emitters (LAEs) at z = 5.7-6.6 in the SA22 field with VLT/X-SHOOTER. We confirm two new luminous LAEs at z = 5.676 (SR6) and z = 6.532 (VR7), and also present HST follow-up of both sources. These sources have luminosities LLy α ≈ 3 × 1043 erg s-1, very high rest-frame equivalent widths of EW0 ≳ 200 Å and narrow Ly α lines (200-340 km s-1). VR7 is the most UV-luminous LAE at z > 6.5, with M1500 = -22.5, even brighter in the UV than CR7. Besides Ly α, we do not detect any other rest-frame UV lines in the spectra of SR6 and VR7, and argue that rest-frame UV lines are easier to observe in bright galaxies with low Ly α equivalent widths. We confirm that Ly α line widths increase with Ly α luminosity at z = 5.7, while there are indications that Ly α lines of faint LAEs become broader at z = 6.6, potentially due to reionization. We find a large spread of up to 3 dex in UV luminosity for >L⋆ LAEs, but find that the Ly α luminosity of the brightest LAEs is strongly related to UV luminosity at z = 6.6. Under basic assumptions, we find that several LAEs at z ≈ 6-7 have Ly α escape fractions ≳ 100 per cent, indicating bursty star formation histories, alternative Ly α production mechanisms, or dust attenuating Ly α emission differently than UV emission. Finally, we present a method to compute ξion, the production efficiency of ionizing photons, and find that LAEs at z ≈ 6-7 have high values of log10(ξion/Hz erg-1) ≈ 25.51 ± 0.09 that may alleviate the need for high Lyman-Continuum escape fractions required for reionization.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- November 2017
- DOI:
- 10.1093/mnras/stx2061
- arXiv:
- arXiv:1706.06591
- Bibcode:
- 2017MNRAS.472..772M
- Keywords:
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- galaxies: evolution;
- galaxies: high-redshift;
- dark ages;
- reionization;
- first stars;
- cosmology: observations;
- galaxies: evolution - galaxies: high-redshift;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 18 pages, 8 figures. Accepted for publication by MNRAS. Added new HST observations of VR7