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zigm: UV/Optical attenuation by the intergalactic medium

This multiplicative model computes the mean attenuation of the optical/UV spectrum of an object at redshift z at a random position on the sky due to intergalactic medium (IGM) clouds following either Madau (1995, ApJ 441, 18) or Meiksin (2006, MNRAS, 365, 807). The model calculates the mean expected attenuation due to resonant scattering by Lyman transitions and photoelectric absorption shortward of the Lyman limit. Attenuation by Helium and metals are not included in the Meiksin model, but are expected to be small. The total attenuation is set to zero for wavelengths less than 900 Angstroms. The user chooses whether to include attenuation due to photoelectric absorption.

par1 z: redshift
par2 model: use Madau(0) or Meiksin(1)
par3 include photoelectric absorption(1) or not(0)