Scale-dependent bias in the BAO-scale intergalactic neutral hydrogen
  I discuss fluctuations in the neutral hydrogen (HI) density of the z~2.3
intergalactic medium and show that their relation to cosmic overdensity is
strongly scale-dependent. This behaviour arises from a linearized version of
the well-known "proximity effect", in which bright sources suppress atomic
hydrogen density. Using a novel, systematic and detailed linear-theory
radiative transfer calculation, I demonstrate how HI density consequently
anti-correlates with total matter density when averaged on scales exceeding the
Lyman-limit mean-free-path.
  The radiative transfer thumbprint is highly distinctive and should be
measurable in the Lyman-alpha forest. Effects extend to sufficiently small
scales to generate significant distortion of the correlation function shape
around the baryon acoustic oscillation peak, although the peak location shifts
only by 1.2 percent for a mean source bias of b_j=3. The distortion changes
significantly with b_j and other astrophysical parameters; measuring it should
provide a helpful observational constraint on the nature of ionizing photon
sources in the near future.
