Table 4 lists the approximate absorption line strengths expected for a sight line through the Milky Way disk and halo at the position of the sun. The measurements are appropriate for a sight line which is inclined at 60° from the galactic disk and which passes through halo and disk gas with positive and negative z. The resulting absorption line system would be called a damped Ly- system with mixed ionization. The HI Ly- line would mostly arise in disk gas while the highly ionized lines of NV, CIV and SiIV would mostly arise in halo gas. The lines of lower ionization would be produced in disk and halo gas. The absorption line system would exhibit multiple components and have a velocity spread as recorded in the strongest lines of CII, SiII and MgII of about 140 km s-1. As shown by Savage and Jeske (1981) absorption line systems with the basic characteristics of the system listed in Table 4 are sometimes found in the spectra of QSOs. The frequency of occurrence of the subset of QSO systems having the characteristics of the Milky Way `solar region' system listed in Table 4 is small because of the presence of the strong damped Ly- line from disk gas having a HI column density of about 6 x 1020 cm-2. Even if only the halo part of the absorption is considered, the fact that the strong low ionization lines of SiII and CII are about 2x stronger than the CIV and SiIV absorption appears to place the absorption system listed in Table 4 in a category which is different from the most common QSO mixed ionization systems having absorption redshifts between approximately 1 and 2 (Wolfe 1986; Danly, Blades and Norman 1987).
|species||(Å) b||log [f] c||W(Å) d||species||(Å) b||log [f] c||W(Å) d|
a For a galactic sight line at the position of the sun
extending through halo and disk gas to each side
of the galactic disk at an inclination of approximately 60° from
the disk. The list is incomplete for
(rest) < 1200 Å, which
is near the short limit of
the IUE spectrograph.
There are many reasons why we might expect the character of the Milky Way halo absorption line system found for gas in the solar neighborhood and those systems most commonly found in QSO spectra to appear different. First, the frequency of occurrence of the most common systems implies that if they are associated with galaxy halos then the absorption likely occurs many Holmberg radii from the center of the absorbing galaxy. Thus we need to consider how the appearance of the Milky Way halo system might change as the sight line passes further and further from the galactic center. Second, the character of the extragalactic background radiation which illuminates and ionizes the galaxy halo gas will change with redshift. Third, the character of the halo gas will change as the vigor of the fountain changes. Thus the halo gas characteristics may be sensitive to galactic evolution and in particular to the massive star production rate.
The appearance of a galaxy disk-halo absorption line system will change with galactocentric distance because of a number of factors. Some of these include:
It would be interesting to consider the consequences of the six effects listed above and then also evaluate the consequences of changing the extragalactic background and the stellar content of the underlying galaxy. Unfortunately such a task is full of uncertainties.
The best way to establish what the Milky Way looks like at large galactocentic distances is to look in those directions where galactic rotation permits a velocity separation of distant matter from local matter, i.e. in or near the directions l 90° and l 270° and at low enough galactic latitudes to permit the viewing of very distant gas at large |z| distances. Some objects which have been observed at optical and/or ultraviolet wavelengths which may meet these criteria are:
I am grateful to my colleagues at the University of Wisconsin-Madison for their many helpful discussions relating to the properties of Milky Way halo gas. I'm appreciative of support for this work through NASA grant NAG 5-186.