The case against large intensity fluctuations in the z ∼ 2.5 He舁ii Lyα forest

Previous studies of the 2.2 < z < 2.7 He舁ii舁Lyα forest measured much larger ionizing background fluctuations than are anticipated theoretically. We re-analyse recent Hubble Space Telescope data from the two He舁ii sightlines that have been used to make these measurements, HE 2347−4342 and HS 1700+6416, and find that the vast majority of the absorption is actually consistent with a single, spatially uniform He舁ii photoionization rate. We show that the data constrain the root mean square fractional fluctuation level smoothed over 1 Mpc to be <2 and discuss why other studies had found 10 times larger fluctuations. Our measurement is consistent with models in which quasars dominate the z = 2.5 metagalactic He舁ii-ionizing background (but it can accommodate less compelling models), and it suggests that quasars (rather than stars) are the dominant contributor to the H舁i-ionizing background. We detect a He舁ii transverse proximity effect that is slightly offset in redshift from a known quasar. Its profile and offset may indicate that the quasar turned on 10 Myr ago.