Multiple Absorption-Line Spectroscopy of the Intergalactic Medium. I.
  Model
  We present a physically-based absorption-line model for the spectroscopic
study of the intergalactic medium (IGM). This model adopts results from Cloudy
simulations and theoretical calculations by Gnat and Sternberg (2007) to
examine the resulting observational signatures of the absorbing gas with the
following ionization scenarios: collisional ionization equilibrium (CIE),
photoionization equilibrium, hybrid (photo- plus collisional ionization), and
non-equilibrium cooling. As a demonstration, we apply this model to new
observations made with the Cosmic Origins Spectrograph aboard the Hubble Space
Telescope of the IGM absorbers at z~0.1877 along the 1ES 1553+113 sight line.
We identify Ly alpha, C III, O VI, and N V absorption lines with two distinct
velocity components (blue at z_b=0.18757; red at z_r=0.18772) separated by
Delta(cz)/(1+z)~38 km/s. Joint analyses of these lines indicate that none of
the examined ionization scenarios can be applied with confidence to the blue
velocity component, although photoionization seems to play a dominant role. For
the red component, CIE can be ruled out, but pure photoionization and hybrid
scenarios (with T<1.3E5 K) are more acceptable. The constrained ranges of
hydrogen density and metallicity of the absorbing gas are n_H=(1.9-2.3)E-5
cm^-3 and Z=(0.43-0.67)Z_solar. These constraints indicate OVI and HI
ionization fractions, f_OVI=0.10-0.15 and f_HI=(3.2-5.1)E-5, with total
hydrogen column density N_H=(0.7-1.2)E18 cm^-2. This demonstration shows that
joint analysis of multiple absorption lines can constrain the ionization state
of an absorber, and results used to estimate the baryonic matter contained in
the absorber.
