The case against large intensity fluctuations in the z ~ 2.5 HeII Lyman-alpha forest

Abstract: Previous studies of the 2.2 < z< 2.7 HeII Lyman-alpha forest measured much larger ionizing background fluctuations than are anticipated theoretically. We re-analyze recent Hubble Space Telescope data from the two HeII sightlines that have been used to make these measurements, HE2347-4342 and HS1700+6416, and find that the vast majority of the absorption is actually consistent with a single HeII photoionization rate. We show that the data constrains the RMS fractional fluctuation level smoothed at 1 Mpc to be < 2 and discuss why other studies had found larger fluctuations. Our measurement is consistent with models in which quasars dominate the z=2.5 metagalactic HeII-ionizing background (but it can accommodate less compelling models), and it suggests that quasars (rather than stars) are the dominant contributor to the HI-ionizing background. We detect a HeII 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.