Mapping the Lyα Emission around a z ∼ 6.6 QSO with MUSE: Extended Emission and a Companion at a Close Separation

Abstract

We utilize the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope to search for extended Ly α emission around the z  ∼ 6.6 QSO J0305−3150. After carefully subtracting the point spread function, we reach a nominal 5 σ surface-brightness limit of SB 5 σ  = 1.9 × 10 −18  erg s −1 cm −2  arcsec −2 over a 1 arcsec 2 aperture, collapsing five wavelength slices centered at the expected location of the redshifted Ly α emission (i.e., at 9256 Å). Current data suggest the presence (5 σ accounting for systematics) of a Ly α nebula that extends for 9 kpc around the QSO. This emission is displaced and redshifted by 155 km s −1 with respect to the location of the QSO host galaxy traced by the [ ] 158 μ m emission line. The total luminosity is  = (3.0 ± 0.4) × 10 42  erg s −1 . Our analysis suggests that this emission is unlikely to rise from optically thick clouds illuminated by the ionizing radiation of the QSO. It is more plausible that the Ly α emission is due to the fluorescence of the highly ionized optically thin gas. This scenario implies a high hydrogen volume density of cm −3 . In addition, we detect a Ly α emitter (LAE) in the immediate vicinity of the QSO, i.e., with a projected separation of ∼12.5 kpc and a line-of-sight velocity difference of 560 km s −1 . The luminosity of the LAE is  = (2.1 ± 0.2) × 10 42  erg s −1 and its inferred star-formation rate is SFR ∼ 1.3 M ⊙ yr −1 . The probability of finding such a close LAE is one order of magnitude above the expectations based on the QSO–galaxy cross-correlation function. This discovery is in agreement with a scenario where dissipative interactions favor the rapid build-up of supermassive black holes at early cosmic times.