Anticipating high-resolution STIS spectra of four multiphase MgII absorbers: A test of photoionization models

In this paper we propose a test of the validity of a photoionization modeli ng technique that is applicable when a combination of high- and low-resolut ion spectra are available for various chemical transitions. We apply this t echnique 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 multipl e phases of their absorbing gas. We apply constraints from (1) High Resolut ion 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 Telesco pe (HST) archival, low-resolution Faint Object Spectrograph (FOS) spectrum (FWHM = 230 km s(-1)) covering Ly alpha, Ly beta, Si II, Si III, Si IV, C I I, 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 res olution (FWHM = 10 km s(-1)) were recently obtained (1999 May and June), co vering 1850-3100 Angstrom. 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 onl y plausible to obtain high-resolution UV spectra for a handful of the brigh test quasars, it is important to determine, without any "after the fact" bi ases, what can be learned from low-resolution FOS data. Photoionization mod els 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-ioniz ation Mg II phase and to infer the presence of an additional high-ionizatio n phase, but not to infer the detailed properties of the latter. Using some examples of consistent models, we simulate STIS high-resolution profiles o f 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 p rofiles and also as an unbiased guide for extracting the detailed gaseous c onditions from the forthcoming, high-resolution STIS observations. We find that the four Mg II absorbers along the PG 1634+706 line of sight e xhibit 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 dete cted C IV in the FOS spectrum [W-r(C IV) < 0.07 <Angstrom> at 3 sigma]. The new STIS observations could detect C IV to a higher sensitivity and distin guish 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 alpha -group -enhanced abundance pattern. In this system, three phases were required to match the FOS profiles : the narrow b similar to 3 km s(-1) Mg II cloud pha se, a broader b similar to 20 km s(-1) phase required to produce the observ ed C IV, and an effective b similar to 400 km s(-1) phase needed to Dt the wings of the Ly alpha line. The z = 0.9902 system is a strong Mg II absorbe r 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 <similar to> 40 km s(-1) phase to fit the observed C IV. The z = 1.0414 sy stem 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 stron ger component is similar to 200 km s(-1) to the blue of the Mg II clouds an d thus requires an offset, broad, high-ionization phase.