The Nature of Weak MgiiAbsorbing Structures

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 highionization 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-ionizationonly 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.

The Milky Way halo as a QSO absorption-line system

“…These absorbers are expected to trace neutral and ionized gas in the discs of galaxies and their gaeous halos (including HVCs). The so-called weak Mg ii systems have W 2796 ≤ 0.3 Å; they appear to be less tightly associated with galaxies and are typically found at larger distances from luminous galaxies, in the range 35−100 h −1 kpc (Milutinović et al 2006;Rigby et al 2002). are related to discs, disc-halo interfaces, and galactic winds (e.g., Bouché et al 2012).…”

“…However, compared to strong Mg ii systems, weak Mg ii absorbers probably are located at larger distances from the galaxies, i.e., in their circumgalactic environment (Milutinović et al 2006). For the same line of arguments outlined in the previous section, the contribution of the 21 cm HVCs at d < 60 kpc to the number density of weak Mg ii absorbers must be small.…”

CASTING LIGHT ON THE “ANOMALOUS” STATISTICS OF Mg II ABSORBERS TOWARD GAMMA-RAY BURST AFTERGLOWS: THE INCIDENCE OF WEAK SYSTEMS

“…The QSO Mg II EW distribution shows a clear turnover around W r ∼ 0.3Å, hinting at different populations (e.g., Churchill et al 1999;Nestor et al 2005;Milutinović et al 2006;Narayanan et al 2007, hereafter N07). Here we show for the first time that, contrary to the strong systems, the weak (W r < 0.3Å) and the moderately strong (0.3 ≤ W r < 1Å) Mg II statistics conform to those derived from QSO surveys.…”

A Survey of Weak MgiiAbsorbers at Redshift 〈z〉 = 1.78

“…This thin region that produces Mg ii absorption is most likely surrounded by a lower density region that produces high-ionization C iv absorption, with both regions centered at the same velocity (Rigby et al 2002). Components offset in velocity by tens of km s À1 also often exist in C iv and could represent additional surrounding low-density filaments (Milutinović et al 2005).…”

“…Narayanan et al (2005) used E140M Space Telescope Imaging Spectrograph Hubble Space Telescope archive data to search for Si ii k1260 and C ii k1335 as tracers of Mg ii, and found a number density of absorbers dN /dz ¼ 1:00 AE 0:20. Churchill et al (1999) used the High Resolution Spectrograph on the Keck I Telescope and observed the Mg ii kk2796, 2803 doublet directly, finding dN /dz ¼ 1:74 AE 0:10.…”