  We have carried out model calculations of the photoionized intergalactic
medium (IGM) to determine the effects on the predicted ionic column densities
due to uncertainties in the published dielectronic recombination (DR) rate
coefficients. Based on our previous experimental work and a comparison of
published theoretical DR rates, we estimate there is in general a factor of 2
uncertainty in existing DR rates used for modeling the IGM. We demonstrate that
this uncertainty results in factors of ~1.9 uncertainty in the predicted N V
and Si IV column densities, ~1.6 for O VI, and ~1.7 for C IV. We show that
these systematic uncertainties translate into a systematic uncertainty of up to
a factor of ~3.1 in the Si/C abundance ratio inferred from observations. The
inferred IGM abundance ratio could thus be less than the solar Si/C ratio or
greater than 3 times the solar ratio. If the latter is true, then it suggests
the metagalactic radiation field is not due purely to active galactic nuclei,
but includes a significant stellar component. Lastly, column density ratios of
Si IV to C IV versus C II to C IV are often used to constrain the decrement in
the metagalactic radiation field at the He II absorption edge. We show that the
variation in the predicted Si IV to C IV ratio due to a factor of 2 uncertainty
in the DR rates is almost as large as that due to a factor of 10 change in the
decrement. Laboratory measurements of the relevant DR resonance strengths and
energies are the only unambiguous method to remove the effects of these atomic
physics uncertainties from models of the IGM.

Astrophysics