Confirmation of double peaked Lyα emission at z = 6.593

Abstract

Distant luminous Lyman-α emitters (LAEs) are excellent targets for spectroscopic observations of galaxies in the epoch of reionisation (EoR). We present deep high-resolution (R = 5000) VLT/X-shooter observations, along with an extensive collection of photometric data of COLA1, a proposed double peaked LAE at z = 6.6. We rule out the possibility that COLA1’s emission line is an [OII] doublet at z = 1.475 on the basis of i) the asymmetric red line-profile and flux ratio of the peaks (blue/red=0.31 ± 0.03) and ii) an unphysical [OII]/Hα ratio ([OII]/Hα > 22). We show that COLA1’s observed B-band flux is explained by a faint extended foreground LAE, for which we detect Lyα and [OIII] at z = 2.142. We thus conclude that COLA1 is a real double-peaked LAE at z = 6.593, the first discovered at z > 6. COLA1 is UV luminous (M1500 = −21.6 ± 0.3), has a high equivalent width (EW0,Lyα = 120−40+50 Å) and very compact Lyα emission (r50,Lyα = 0.33−0.04+0.07 kpc). Relatively weak inferred Hβ+[OIII] line-emission from Spitzer/IRAC indicates an extremely low metallicity of Z < 1/20 Z⊙ or reduced strength of nebular lines due to high escape of ionising photons. The small Lyα peak separation of 220 ± 20 km s−1 implies a low HI column density and an ionising photon escape fraction of ≈15 − 30%, providing the first direct evidence that such galaxies contribute actively to the reionisation of the Universe at z > 6. Based on simple estimates, we find that COLA1 could have provided just enough photons to reionise its own ≈0.3 pMpc (2.3 cMpc) bubble, allowing the blue Lyα line to be observed. However, we also discuss alternative scenarios explaining the detected double peaked nature of COLA1. Our results show that future high-resolution observations of statistical samples of double peaked LAEs at z > 5 are a promising probe of the occurrence of ionised regions around galaxies in the EoR.

The role of galaxies and AGN in reionizing the IGM – III. IGM–galaxy cross-correlations at z ∼ 6 from eight quasar fields with DEIMOS and MUSE

“…In both cases, the minimum UV luminosity of the contributing sources is M min UV < −20.0 (2σ). In practice however, a model where only galaxies brighter than M UV = −20.0 is implausible because it would require an overhelmingly high luminosity-averaged escape fraction of ≈ 1, contradicting existing z ∼ 6 measurements (Matthee et al 2018) and marking a stark departure from measurements at lower-redshift (e.g. Vanzella et al 2016;Izotov et al 2016Izotov et al , 2018Tanvir et al 2019;Fletcher et al 2019).…”

“…Direct measurements of the ionising parameters of galaxies have proven challenging and offer a fractured picture. Spectroscopy of the afterglow of gamma ray bursts indicate extremely low escape fractions 2 % for their hosts (Tanvir et al 2019), whereas the peak separation of Lyman-α in a bright LAE (COLA1) results in an indirect measurement of f esc ∼ 15 − 30 % (Matthee et al 2018). Individual galaxies at high redshift have been found to have hard radiation spectra (e.g.…”

Rapid Reionization by the Oligarchs: The Case for Massive, UV-bright, Star-forming Galaxies with High Escape Fractions

“…The luminous Lyα emitter, COLA1 at z = 6.6 (f esc ∼ 30%, M UV = −21.5, Σ SFR = 100 M /yr/kpc 2 ) is a poster-child oligarch (Hu et al 2016;Matthee et al 2018). It displays double-peaked Lyα with a low-peak separation reminiscent of local LyC leakers (Verhamme et al 2017).…”

The MUSE-Wide Survey: A determination of the Lymanαemitter luminosity function at 3 <z< 6

“…A few of the most-luminous LAES at z 6 have already received observational attention (e.g. Ouchi et al 2009;Lidman et al 2012;Hu et al 2016;Matthee et al 2018), with one object being suggested to either host metal-free stars (Sobral et al 2015) or a direct-collapse black hole (Pallottini et al 2015). At z ∼ 2−3 Sobral et al (2018b) presented recently spectroscopic results on 20 bright LAEs (log L Lyα [erg s −1 ] > 42.7).…”