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

We present deep 10 h VLT/XSHOOTER spectroscopy for an extraordinarily luminous and extended Ly α emitter at z = 6.595 referred to as Himiko and first discussed by Ouchi et al., with the purpose of constraining the mechanisms powering its strong emission. Complementary to the spectrum, we discuss near-infrared imaging data from the CANDELS survey. We find neither for He II nor any metal line a significant excess, with 3σ upper limits of 6.8, 3.1, and 5.8 × 10^-18 erg s^-1 cm^-2 for C IV λ1549, He II λ1640, C III] λ1909, respectively, assuming apertures with 200 km s^-1 widths and offset by -250 km s^-1 w.r.t. to the peak Ly α redshift. These limits provide strong evidence that an AGN is not a major contribution to Himiko's Ly α flux. Strong conclusions about the presence of Pop III star formation or gravitational cooling radiation are not possible based on the obtained He II upper limit. Our Ly α spectrum confirms both spatial extent and flux (8.8 ± 0.5 × 10^-17 erg s^-1 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 Ly α, while being undetected bluewards.