The Chemical and Ionization Conditions in Weak Mg II Absorbers

Abstract

We present an analysis of the chemical and ionization conditions in a sample of 100 weak Mg II absorbers identified in the VLT/UVES archive of quasar spectra. Using a host of low ionization lines associated with each absorber in this sample, and on the basis of ionization models, we infer that the metallicity in a significant fraction of weak Mg II clouds is constrained to values of solar or higher, if they are sub-Lyman limit systems. Based on the observed constraints, we present a physical picture in which weak Mg II absorbers are predominantly tracing two different astrophysical processes/structures. A significant population of weak Mg II clouds, those in which N(Fe II) is much less than N(Mg II), identified at both low (z ~ 1) and high (z ~ 2) redshift, are potentially tracing gas in the extended halos of galaxies, analogous to the Galactic high velocity clouds. These absorbers might correspond to alpha-enhanced interstellar gas expelled from star-forming galaxies, in correlated supernova events. On the other hand, N(FeII) approximately equal to N(Mg II) clouds, which are prevalent only at lower redshifts (z < 1.5), must be tracing Type Ia enriched gas in small, high metallicity pockets in dwarf galaxies, tidal debris, or other intergalactic structures.

35 pages (including tables & figures). Accepted for publication in ApJ. A high resolution version of the paper is available at "http://www.astro.wisc.edu/~anand/weakMgII.pdf"

References in corpus (8)

• A large population of metal-rich, compact, intergalactic C IV absorbers - Evidence for poor small-scale metal mixing
• New abundance determinations in z < 1.5 QSO absorbers: seven sub-DLAs and one DLA
• Kinematics of Interstellar Gas in Nearby UV-Selected Galaxies Measured with HST/STIS Spectroscopy
• The O VI Absorbers Toward PG0953+415: High Metallicity, Cosmic-Web Gas Far From Luminous Galaxies
• The Kinematic Evolution of Strong MgII Absorbers
• Super-Solar Metallicity in Weak Mg II Absorption Systems at z ~ 1.7
• Physical Properties of Weak MgII Absorbers at z~2
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