Multiple Absorption-Line Spectroscopy of the Intergalactic Medium. I. Model

Abstract

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.