Evidence for PopIII-like stellar populations in the most luminous Lyman-$α$ emitters at the epoch of re-ionisation: spectroscopic confirmation (1504.01734v2)

Abstract: Faint Lyman-$\alpha$ (Ly$\alpha$) emitters become increasingly rarer towards the re-ionisation epoch (z~6-7). However, observations from a very large (~5deg$^2$) Ly$\alpha$ survey at z=6.6 (Matthee et al. 2015) show that this is not the case for the most luminous emitters. Here we present follow-up observations of the two most luminous z~6.6 Ly$\alpha$ candidates in the COSMOS field: MASOSA' andCR7'. We used X-SHOOTER, SINFONI and FORS2 (VLT), and DEIMOS (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 Ly$\alpha$ with a line width of $386\pm30$ km/s (FWHM) and with high EW$_0$ (>200 \AA), but it is undetected in the continuum. CR7, with an observed Ly$\alpha$ luminosity of $10^{43.93\pm0.05}$erg/s is the most luminous Ly$\alpha$ emitter ever found at z>6. CR7 reveals a narrow Ly$\alpha$ line with $266\pm15$ km/s FWHM, being detected in the NIR (rest-frame UV, with $\beta=-2.3\pm0.1$) with an excess in $J$, and also strongly detected in IRAC/Spitzer. We detect a narrow HeII1640$\AA$ emission line ($6\sigma$) which explains the excess seen in the $J$ band photometry (EW$_0$~80 \AA). We find no other emission lines from the UV to the NIR in our X-SHOOTER spectra (HeII/OIII]1663A>3 and HeII/CIII]1908A>2.5). We find 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 dominates the mass. HST/WFC3 observations show that the light is indeed spatially separated between a very blue component, coincident with Ly$\alpha$ 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.

• The paper confirms luminous Lyα emitters MASOSA (z=6.541) and CR7 (z=6.604) with PopIII-like stellar features through detailed spectroscopy.
• Methodologies using X-SHOOTER, FORS2, SINFONI, and DEIMOS provided high-resolution insights into the Lyα profiles and the narrow He⁺ line indicative of metal-free stars.
• These findings support active PopIII-like star formation as a critical process during re-ionisation and early galaxy assembly.

Evidence for PopIII-like Stellar Populations in Luminous Lyman-α Emitters

The paper authored by David Sobral et al. aims to identify and characterize some of the most luminous Lyman-α (Lyα) emitters detected during the epoch of re-ionisation at approximately redshift $z \sim 6.6$. The team conducted spectroscopic observations to confirm candidates from a Lyα narrow-band survey over a substantial 5 square degrees, focusing specifically on two of the brightest candidates, designated 'MASOSA' and 'CR7', in the COSMOS field.

The research utilizes observations from X-SHOOTER, FORS2, SINFONI on the VLT, and DEIMOS on Keck to confirm the high-redshift nature of these candidates. The confirmed redshifts for the two emitters are $z = 6.541$ for MASOSA and $z = 6.604$ for CR7. MASOSA is characterized by a prominent Lyα detection with a line width of $386 \pm 30$ km/s and an exceedingly high equivalent width (EW$_0$) of greater than 200 Å. This suggests it is composed of a young, metal-poor stellar population with a low stellar mass. CR7, on the other hand, has the highest recorded Lyα luminosity at post-re-ionisation ($10^{43.92 \pm 0.05}$ erg/s) and separates into three components in high-resolution imaging, a morphological detail captured in HST/WFC3 observations.

A key finding is the detection of a narrow He$^+$ line at $1640$ Å in CR7, indicative of hot, metal-free stars akin to Population III stars. The He$^+$/O$^2^+$ and He$^+$/C$^2^+$ ratios, alongside a lack of detection of other metal lines apart from He$^+$, anchor the explanation that CR7 could be a living laboratory of PopIII-like star formation which dominates its rest-frame UV nebular emission.

This paper's implications are significant both practically and theoretically. CR7 stands as the best candidate yet for a system with active PopIII star formation, providing an opportunity to paper early stellar populations responsible for re-ionisation, as well as the formation of the first galaxies. The conclusion suggests scenarios where PopIII stars could form in regions perhaps previously enriched by earlier star generations, with the prior generation of stars insufficiently spreading metals to inhibit their formation.

Future developments are likely to involve more detailed spectroscopic analysis with current and upcoming observational facilities. This includes the potential use of the James Webb Space Telescope (JWST) for its ability to resolve faint features in the infrared, delving deeper into the chemical composition and formation conditions of these ancient stellar systems. Understanding how such massive stellar assemblies affect their environment provides direct insight into conditions in the early universe, star formation processes, and the progression of cosmic re-ionisation.