A couple of weak Mg II doublets are observed per unit redshift along the LOS toward a typical quasar. These are produced by numerous metal-enriched gas pockets, some of them quite small (tens of pc), most of which are not close to luminous galaxies. A new STIS/HST spectrum of PG1634+706, combined with a HIRES/Keck spectrum, provides high resolution coverage of Lyα , Mg II, Fe II, Si II, Si III, Si IV, C II, and C IV for three single cloud, weak Mg II absorbers. We compare and contrast their phase structures, metallicities, and ionization conditions. Two of the three weak Mg II absorbers are constrained by the Lyα profiles to have at least Solar metallicity. Each of these two systems has a broader C IV phase (effective Doppler parameter b ~ 10 km/s) centered on a narrower Mg II cloud (b ~ 2 km/s), however one also requires an additional broader component to fit the C IV, offset by 15 km/s from the first. The presence or absence of this additional "cloud" may be the structural difference between the classes of weak Mg II absorbers with strong or with weak C IV. The third absorber along the line of sight is unusual in that its Lyα and C IV are quite strong despite a weaker Mg II doublet. This implies that it can have considerably lower metallicity, and the complex structure of the C IV implies a series of clouds with varying densities (ionization parameters), only one of which is high enough to give rise to observable Mg II. We conclude that there is more than one type of environment traced by the presence of weak Mg II absorbers. Studies of multiple phases in larger samples and searches for star-forming counterparts are needed to identify candidate environments such as supernova shell fragments, in dwarf galaxies or intergalactic, high velocity clouds enriched by sweeping up group debris, or interstellar gas in low surface brightness galaxies. This work was supported by NSF AST 96-17185 and NASA NAG5-6399.
American Astronomical Society Meeting Abstracts
- Pub Date:
- December 2000