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.