Atomic data for S II - Toward Better Diagnostics of Chemical Evolution
  in High-redshift Galaxies
  Absorption-line spectroscopy is a powerful tool used to estimate element
abundances in the nearby as well as distant universe. The accuracy of the
abundances thus derived is, naturally, limited by the accuracy of the atomic
data assumed for the spectral lines. We have recently started a project to
perform the new extensive atomic data calculations used for optical/UV spectral
lines in the plasma modeling code Cloudy using state-of-the-art quantal
calculations. Here we demonstrate our approach by focussing on S II, an ion
used to estimate metallicities for Milky Way interstellar clouds as well as
distant damped Lyman-alpha (DLA) and sub-DLA absorber galaxies detected in the
spectra of quasars and gamma-ray bursts (GRBs). We report new extensive
calculations of a large number of energy levels of S II, and the line strengths
of the resulting radiative transitions. Our calculations are based on the
configuration interaction approach within a numerical Hartree-Fock framework,
and utilize both non-ralativistic and quasirelativistic one-electron radial
orbitals. The results of these new atomic calculations are then incorporated
into Cloudy and applied to a lab plasma, and a typical DLA, for illustrative
purposes. The new results imply relatively modest changes (~0.04 dex) to the
metallicities estimated from S II in past studies. These results will be
readily applicable to other studies of S II in the Milky Way and other
galaxies.
