On the nature and physical conditions of the luminous Lya emitter CR7 and its rest-frame UV components

TL;DR

This study uses new HST and VLT data to analyze the UV components and physical conditions of the luminous Lyα emitter CR7 at z=6.6, revealing spatially offset HeII emission, UV continuum properties, and star formation activity without clear AGN signatures.

Contribution

It provides the first spatially resolved analysis of CR7's UV components, constraining metallicity and ionization sources, and highlights the importance of spatial resolution in early galaxy formation studies.

Findings

HeII emission is spatially offset from Lya peak, towards clump B.

CR7's UV continuum indicates active star formation with low metallicity.

No significant variability detected in UV clumps over five years.

Abstract

We present new HST/WFC3 observations and re-analyse VLT data to unveil the continuum, variability and rest-frame UV lines of the multiple UV clumps of the most luminous Ly$\alpha$ emitter at z=6.6, CR7. Our re-reduced, flux calibrated X-SHOOTER spectra of CR7 reveal a HeII emission line in observations obtained along the major axis of Lyman-alpha (Lya) emission with the best seeing conditions. HeII is spatially offset by +0.8'' from the peak of Lya emission, and it is found towards clump B. Our WFC3 grism spectra detects the UV continuum of CR7's clump A, yielding a power law with $\beta=-2.5^{+0.6}_{-0.7}$ and $M_{UV}=-21.87^{+0.25}_{-0.20}$. No significant variability is found for any of the UV clumps on their own, but there is tentative (~2.2$\sigma$) brightening of CR7 in F110W as a whole from 2012 to 2017. HST grism data fail to robustly detect rest-frame UV lines in any of the clumps, implying fluxes <2x10$^{-17}$ erg s$^{-1}$ cm$^{-2}$ (3 $\sigma$). We perform CLOUDY modelling to constrain the metallicity and the ionising nature of CR7. CR7 seems to be actively forming stars without any clear AGN activity in clump A, consistent with a metallicity of ~0.05-0.2 Z$_{\odot}$. Component C or an inter-clump component between B and C may host a high ionisation source. Our results highlight the need for spatially resolved information to study the formation and assembly of early galaxies.