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Session 84 - IGM.
Display session, Friday, January 09
Exhibit Hall,

[84.04] The Population of Weak Mg II Absorbers

J. R. Rigby, C. W. Churchill, J. C. Charlton (Penn State)

We have found 30 weak (0.02 < W_r(2796) < 0.3 ÅMg\kern 0.1em\sc ii absorption systems in an unbiased survey of 26 quasars over the redshift range 0.4 = 0.9 is dN/dz= 1.7\pm 0.1; thus, they comprise the bulk (\sim 65%) of all Mg\kern 0.1em\sc ii absorbers. Fe\kern 0.1em\sc ii was detected in 11 of the systems. The Fe\kern 0.1em\sc ii to Mg\kern 0.1em\sc ii column density ratios range from super--solar to below 0.1 solar, indicating that the \alpha--group to iron--group enrichment processes are varied among weak Mg\kern 0.1em\sc ii systems. These weak systems represent a new population of Mg\kern 0.1em\sc ii absorbers in that the associated luminous objects they select are unknown. In contrast, strong Mg\kern 0.1em\sc ii absorbers (W_r > 0.3 Åare known to select the population of normal galaxies. Weak Mg\kern 0.1em\sc ii systems may select some combination of normal galaxy outskirts, ejected or tidally stripped material, and low surface brightness galaxies. In particular, those systems with high iron--group abundances could select out the population of ``giant'' low surface brightness galaxies.

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