The Nature of Weak Mg II Absorbing Structures

April 2006 • 2006ApJ...641..190M

Authors • Milutinović, Nikola • Rigby, Jane R. • Masiero, Joseph R. • Lynch, Ryan S. • Palma, Chris • Charlton, Jane C.

Abstract • We consider geometries and physical models for weak low-ionization absorbers based on the relative incidence of low- and high-ionization systems. We present a survey of weak low-ionization systems in 35 high-resolution HST STIS quasar spectra, sometimes supplemented by Keck HIRES and HST FOS data. We found 16 metal-line systems, with low- and/or high-ionization absorption detected. Weak low-ionization absorbers trace an abundant population of metal-enriched regions. Models show that these systems have a ~10 pc region of higher density gas and a ~1 kpc region that represents a lower density, higher ionization phase. The goal of our survey was to compare systems detected in low- and/or high-ionization gas. All but 1 of the 10 weak low-ionization systems have a related high-ionization phase. In three cases the high-ionization gas has only a single component, kinematically centered on the low-ionization absorption, and the other six cases have additional high-ionization components offset in velocity. The high-ionization absorption in weak low-ionization systems has similar kinematic structure to that in high-ionization-only systems. There are just six systems with only a high-ionization phase, as compared to the nine systems with both low- and high-ionization phases. We conclude that filamentary and sheetlike geometries are favored, due to the relatively small observed cross section of high-ionization-only systems. Our statistical arguments suggest that although low-ionization absorbers are not closely associated with luminous galaxies, they arise in their immediate environments within the cosmic web.

Based in part on observations obtained with the NASA/ESA Hubble Space Telescope, which is operated by the Space Telescope Science Institute (STScI) for the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.

Based in part on observations obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

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Joe Masiero

Associate Scientist