Gas in the Milky Way halo spans a wide range of physical conditions. Neutral condensations exist which produce strong absorption lines of such species as HI, CII, OI, NI, NII, MgII, SiII, FeII, AlII etc. In addition, lines from more weakly ionized gas are often found including such species as CI and MgI, NaI and CaII. The neutral gas may have temperatures ranging from 102 °K to more than 103 °K and densities ranging up to a few atoms cm-3. The neutral halo clouds likely exist in a much hotter medium (the hot ISM) with a temperature of order 106 °K and density of order 10-3 to 10-4 cm-3. This hot medium which can be studied through its X-ray emission or OVI absorption has only been adequately studied at small z distances. This is because the X-ray emission from high z gas is difficult to discriminate from X-ray emission produced by nearby low z gas (see Section 2.3), and because interstellar OVI absorption studies with the Copernicus satellite by Jenkins (1978) were limited to relatively bright and therefore nearby stars. Highly ionized halo gas as traced by NV, CIV, SiIV, SiIII and AlIII has been extensively studied with the IUE satellite (see Section 2.2). The NV and some of the CIV absorption is probably produced in collisionally ionized cooling gas with temperatures near 105 °K. Photoionization in 104 °K gas may be responsible for the AlIII, SiIII, SiIV and some of the CIV absorption (see Section 6).
For a galactic pole to pole path through the halo and disk perpendicular to the galactic plane at the solar position in the galaxy, the Milky Way would produce the absorption line strengths listed in Table 4. These values were estimated with reference to the IUE UV data for the objects listed at the end of the table. Comments to the table indicate if the equivalent width of a particular feature is dominated by disk absorption (i.e. for |z| < 0.5 kpc) or by halo gas absorption (i.e. for |z| > 0.5 kpc). As can be seen, even within those absorption lines formed by particular ions (e.g. Si II), some lines are mostly produced in the disk (i.e. the weak lines) while others owe their great strength to absorption by low column density high velocity halo clouds. A QSO observer recording a spectrum similar to that listed in Table 4 would refer to the system as `mixed ionization' and would note the strong damped Ly- absorption line. The observer would also remark that the strongest low ionization lines of CII and SiII have equivalent widths which exceed by a factor of 2 to 3 the high ion counterparts (e.g. CIV and SiIV). Most of the lines listed in Table 4 would not be detected because many of the QSO absorption line surveys have minimum detectable equivalent widths in the rest frame which exceed 0.2Å.
The only fine structure excited line listed in Table 4 is CII* 1335. Most of this feature arises in the higher density gas of the galactic disk although for some sight lines through halo gas, moderate density halo clouds would be capable of producing measurable but weak absorption in this excited state. For example, CII* absorption which has W 0.1 Å is seen in the -60 km s-1 cloud toward the halo star HD93521 in the direction l = 183° and b = 62° at a distance r = 0.8 kpc (Caldwell 1979). This cloud is estimated by Danly (1987) to have a z distance exceeding 0.27 kpc.
Two ions of very great importance for tracing hot gas through the Milky Way halo are NV and OVI. The equivalent widths of the NV lines produced in the Milky Way halo are relatively small and would be difficult to detect in most QSO spectra, particularly if the lines are blended with Ly- forest lines. However, theoretical calculations suggest a pole to pole column density of about 1015 atoms cm-2 for OVI (see Section 6). This column density would produce a strongly saturated OVI doublet at 1032 and 1038 Å. For a velocity spread parameter b = 30 km s-1, the two components of the doublet would have equivalent widths of 0.32 and 0.25 Å, respectively. The OVI equivalent widths would be two or three times larger if the gas producing the OVI absorption shares the multicomponent absorbing characteristics of the weakly ionized gas. Even in the presence of the Ly- forest, lines of this strength might be detected if they exist in QSO spectra. Greater efforts should be made to establish equivalent width limits for this very important ion when analyzing QSO absorption line data.