Anticipating High-Resolution STIS Spectra of Four Multiphase Mg II Absorbers: A Test of Photoionization Models^{* **}

In this paper we propose a test of the validity of a photoionization modeling technique that is applicable when a combination of high- and low-resolution spectra are available for various chemical transitions. We apply this technique to the four Mg II systems along the line of sight toward the z_em = 1.335 quasar PG 1634+706 to infer the physical conditions in the multiple phases of their absorbing gas. We apply constraints from (1) High Resolution Spectrograph (HIRES/Keck I) profiles (R = 45,000 and FWHM = 6.6 km s^-1) of low-ionization species Mg II and Fe II and (2) a Hubble Space Telescope (HST) archival, low-resolution Faint Object Spectrograph (FOS) spectrum (FWHM = 230 km s^-1) covering Lyα, Lyβ, Si II, Si III, Si IV, C II, C III, C IV, and N V. For this bright quasar, very high signal-to-noise ratio (S/N) Space Telescope Imaging Spectrograph (STIS) spectra at high resolution (FWHM = 10 km s^-1) were recently obtained (1999 May and June), covering 1850-3100 Å. However, at the time of this writing, these data are still proprietary in the HST archive, and we deliberately present this paper without knowledge of the spectra. Since, in the near term, it is only plausible to obtain high-resolution UV spectra for a handful of the brightest quasars, it is important to determine, without any "after the fact" biases, what can be learned from low-resolution FOS data. Photoionization models of the four Mg II systems are constrained by the existing FOS and HIRES absorption profiles. In general, it is possible to constrain the low-ionization Mg II phase and to infer the presence of an additional high-ionization phase, but not to infer the detailed properties of the latter. Using some examples of consistent models, we simulate STIS high-resolution profiles of the key transitions that will be covered by the new observations, at the expected S/N. This study will serve as a fair test of the applicability of the photoionization modeling of a combination of low- and high-resolution profiles and also as an unbiased guide for extracting the detailed gaseous conditions from the forthcoming, high-resolution STIS observations.

We find that the four Mg II absorbers along the PG 1634+706 line of sight exhibit a variety of Mg II kinematic structures and higher ionization phases. The z = 0.8182 system is a single-cloud, weak Mg II absorber without detected C IV in the FOS spectrum [W_r(C ) < 0.07 Å at 3 σ]. The new STIS observations could detect C IV to a higher sensitivity and distinguish between a broad C IV phase and C IV associated with the narrow Mg II cloud. In contrast, the z = 0.9056 absorber is a weak, single-cloud system with strong C IV absorption in the FOS spectrum; we infer that it must have supersolar metallicity, depletion of Fe relative to Mg, or an α-group-enhanced abundance pattern. In this system, three phases were required to match the FOS profiles: the narrow b ~ 3 km s^-1 Mg II cloud phase, a broader b ~ 20 km s^-1 phase required to produce the observed C IV, and an effective b ~ 400 km s^-1 phase needed to fit the wings of the Lyα line. The z = 0.9902 system is a strong Mg II absorber with five blended components; it has a relatively low metallicity, Z < -1, constrained by a strong Lyman limit break; and it also requires a broad b ~ 40 km s^-1 phase to fit the observed C IV. The z = 1.0414 system has four, very weak, blended Mg II clouds and is most unusual in that two components of C IV absorption are apparent, even in the low-resolution FOS spectrum. The redward component is centered on the Mg II, but the stronger component is ~200 km s^-1 to the blue of the Mg II clouds and thus requires an offset, broad, high-ionization phase.