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J.R. Rigby, C.W. Churchill, J.C. Charlton (Penn State)
Weak MgII absorbers [those with W(2796)<0.3Å] outnumber the strong MgII absorbers which are associated with "normal" luminous galaxies (Steidel, 1995, ESO Proc.) However, it is not known what luminous objects produce weak MgII absorption (Churchill et al., 1999, ApJS, 120.) Candidate environments are the outskirts of normal galaxies, dwarf galaxies, stripped material from tidal interaction, and low surface brightness galaxies. Can we distinguish between these cases by investigating the cloud ionization conditions and metallicities and comparing to those of the candidates?
For thirty weak MgII systems, we have inferred the physical conditions in the absorbing gas using CLOUDY (Ferland 1996) photoionization models. Constraints for these models were MgII and FeII cloud column densities derived from HIRES/Keck spectra and lower resolution FOS/HST Lyman \alpha and CIV profiles. We find that the derived metallicities span two orders of magnitude, from 0.01 - 1 times solar. We also find that some absorbers are single phase and others multiphase.
The ionization state of the absorbing gas was constrained by MgII, FeII, and CIV. Some weak MgII clouds must have low ionization conditions because they have relatively strong FeII and stringent CIV limits. In other clouds, the strong CIV cannot arise in the same low ionization phase that produces the MgII and FeII absorption; a separate phase of higher ionization gas is required.
The wide range of metallicities, and the necessity for some absorbers to be single phase and others multiphase, together imply that weak MgII absorption arises in physical environments with different stellar processing histories. We compare the inferred properties of weak absorbers to those of candidate environments studied in the local universe.
The author(s) of this abstract have provided an email address for comments about the abstract: jrigby@astro.psu.edu