2015MNRAS.451.2050Z

We present deep 10 h VLT/XSHOOTER spectroscopy for an extraordinarily luminous and extended Lyα 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 HeII nor any metal line a significant excess, with 3σ upper limits of 6.8, 3.1, and 5.8x10^–18erg/s/cm2 for CIVλ1549, HeIIλ1640, CIII]λ1909, respectively, assuming apertures with 200 km/s widths and offset by -250 km/s w.r.t. to the peak Lyα redshift. These limits provide strong evidence that an AGN is not a major contribution to Himiko's Lyα flux. Strong conclusions about the presence of Pop III star formation or gravitational cooling radiation are not possible based on the obtained HeII upper limit. Our Lyα spectrum confirms both spatial extent and flux (8.8±0.5x10-¹⁷erg/s/cm2) 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 Lyα, while being undetected bluewards.