Deep rest-frame far-UV spectroscopy of the giant Lyman-alpha emitter 'Himiko'

TL;DR

This study provides deep spectroscopic analysis of the luminous Lyman-alpha emitter 'Himiko' at z=6.595, constraining its ionizing sources and ruling out significant AGN contribution through non-detection of key emission lines.

Contribution

First detailed rest-frame far-UV spectroscopy of 'Himiko' at high redshift, setting upper limits on metal lines and AGN activity, and confirming its spatial extent and redshift.

Findings

No significant HeII or metal line emission detected.

Strong evidence against AGN as primary source of Lya emission.

Confirmed spatial extent and flux of Lya emission.

Abstract

We present deep 10h VLT/XSHOOTER spectroscopy for an extraordinarily luminous and extended Lya emitter at z=6.595 referred to as Himiko and first discussed by Ouchi et al. (2009), with the purpose of constraining the mechanisms powering its strong emission. Complementary to the spectrum, we discuss NIR imaging data from the CANDELS survey. We find neither for HeII nor any metal line a significant excess, with 3 sigma upper limits of 6.8, 3.1, and 5.8x10^{-18} erg/s/cm^2 for CIV $\lambda$1549, HeII $\lambda$1640, CIII] $\lambda$1909, respectively, assuming apertures with 200 km/s widths and offset by -250 km/s w.r.t to the peak Lya redshift. These limits provide strong evidence that an AGN is not a major contribution to Himiko's Lya flux. Strong conclusions about the presence of PopIII star-formation or gravitational cooling radiation are not possible based on the obtained HeII upper limit. Our Lya spectrum confirms both spatial extent and flux (8.8+/-0.5x10^{-17} erg/s/cm^2) of previous measurements. In addition, we can unambiguously exclude any remaining chance of it being a lower redshift interloper by significantly detecting a continuum redwards of Lya, while being undetected bluewards.