Aligned metal absorbers and the ultraviolet background at the end of reionization

We use observations of spatially aligned C II, C IV, Si II, Si IV, and OI absorbers to probe the slope and intensity of the ultravioletbackground (UVB) at z ˜ 6. We accomplish this by comparingobservations with predictions from a cosmological hydrodynamicsimulation using three trial UVBs applied in post-processing: aspectrally soft, fluctuating UVB calculated using multifrequencyradiative transfer; a soft, spatially uniform UVB; and a hard, spatiallyuniform `quasars-only' model. When considering our pairedhigh-ionization absorbers (C IV/Si IV), the observed statistics stronglyprefer the hard, spatially uniform UVB. This echoes recent findings thatcosmological simulations generically underproduce strong C IV absorbersat z > 5. A single low/high ionization pair (Si II/Si IV), bycontrast, shows a preference for the HM12 UVB, whereas two more (C II/CIV and O I/C IV) show no preference for any of the three UVBs. Despitethis, future observations of specific absorbers, particularly Si IV/CIV, with next-generation telescopes probing to lower column densitiesshould yield tighter constraints on the UVB.