IS THE COSMIC ULTRAVIOLET BACKGROUND FLUCTUATING AT REDSHIFT z approx= 6?

摘要

We study the Gunn-Peterson effect of the photoionized intergalactic medium (IGM) in the redshift range 5 < z < 6.4 using semianalytic simulations based on the lognormal model. Assuming a rapidly evolved and spatially uniform ionizing background, the simulation can produce all the observed abnormal statistical features near redshift z approx= 6. They include (1) a rapid increase of absorption depths, (2) large scatter in the optical depths, (3) long-tailed distributions of transmitted flux, and (4) long dark gaps in spectra. These abnormal features are mainly due to rare events, which correspond to the long-tailed probability distribution of the IGM density field, and therefore they may not imply significant spatial fluctuations in the UV ionizing background at z approx= 6.