EVIDENCE FOR PopIII-LIKE STELLAR POPULATIONS IN THE MOST LUMINOUS LyαEMITTERS AT THE EPOCH OF REIONIZATION: SPECTROSCOPIC CONFIRMATION

Abstract

Faint Lyman-α (Lyα) emitters become increasingly rarer towards the re-ionisation epoch (z ∼ 6−7). However, observations from a very large (∼ 5 deg 2 ) Lyα narrow-band survey at z = 6.6 (Matthee et al. 2015) show that this is not the case for the most luminous emitters, capable of ionising their own local bubbles. Here we present follow-up observations of the two most luminous Lyα candidates in the COSMOS field: 'MASOSA' and 'CR7'. We used X-SHOOTER, SINFONI and FORS2 on the VLT, and DEIMOS on Keck, to confirm both candidates beyond any doubt. We find redshifts of z = 6.541 and z = 6.604 for 'MASOSA' and 'CR7', respectively. MASOSA has a strong detection in Lyα with a line width of 386 ± 30 km s −1 (FWHM) and with very high EW 0 (> 200Å), but undetected in the continuum, implying very low stellar mass and a likely young, metal-poor stellar population. 'CR7', with an observed Lyα luminosity of 10 43.92±0.05 erg s −1 is the most luminous Lyα emitter ever found at z > 6 and is spatially extended (∼ 16 kpc). 'CR7' reveals a narrow Lyα line with 266 ± 15 km s −1 FWHM, being detected in the NIR (rest-frame UV; β = −2.3 ± 0.1) and in IRAC/Spitzer. We detect a narrow HeII1640Å emission line (6σ, FWHM = 130±30 km s −1 ) in CR7 which can explain the clear excess seen in the J band photometry (EW 0 ∼ 80Å). We find no other emission lines from the UV to the NIR in our X-SHOOTER spectra (HeII/OIII]1663Å > 3 and HeII/CIII]1908Å > 2.5). We conclude that CR7 is best explained by a combination of a PopIII-like population which dominates the rest-frame UV and the nebular emission and a more normal stellar population which presumably dominates the mass. HST/WFC3 observations show that the light is indeed spatially separated between a very blue component, coincident with Lyα and HeII emission, and two red components (∼ 5 kpc away), which dominate the mass. Our findings are consistent with theoretical predictions of a PopIII wave, with PopIII star formation migrating away from the original sites of star formation.

Enhanced direct collapse due to Lyman α feedback

“…We note that this is also consistent with recent work suggesting that a massive cluster of Population III stars, an alternative explanation for the origin of CR7 (e.g. Sobral et al 2015;Visbal et al 2016; see also Johnson 2010), is dubious since it is unknown how a sufficiently high mass of Population III stars could be assembled rapidly enough to explain the observed extremely bright emission (e.g. Hartwig et al 2016;Yajima & Khochfar 2016;Xu et al 2016;Visbal et al 2017).…”

“…The extremely bright Lyman α emitter known as CR7 is an intriguing candidate for a high-redshift quasar that may be powered by accretion onto a DCBH (Sobral et al 2015). Recent modeling efforts have demonstrated that a nearby galaxy may well have produced a high enough level of LW radiation to induce the formation of a DCBH in this galaxy and that the nebular emission could be explained by an accreting BH with a mass consistent with formation as a DCBH (e.g.…”

Witnessing galaxy assembly in an extended z≈3 structure

“…First, offsets between the sites of star formation and regions of Lyα emission have been reported in the literature (e.g. Sobral et al 2015). Moreover, density fluctuations (or the presence of sub-halos) in the CGM could account for the observed clumps.…”

An extreme [O III] emitter atz= 3.2: a low metallicity Lyman continuum source

“…The origin of He iiλ1640 emission in galaxies has been a matter of investigation, possibly produced by massive stars both in the local Universe (e.g., Szécsi et al 2015;Gräfener & Vink 2015) and at high-redshift (e.g., Cassata et al 2013;Sobral et al 2015;Pallottini et al 2015). He iiλ1640 detection is generally A51, page 10 of 13 associated with AGN activity, but it has also been reported that He iiλ1640 associated with no C ivλ1550 is a possible feature in star-forming galaxies at z ∼ 3 (Cassata et al 2013).…”